PROSEA, Introduction to Oils and fats

Introduction to Oils and fats

Choice of species

Oils and fats are water insoluble substances, called lipids, consisting of a mixture of triglycerides and containing also small amounts of other lipid and lipid-soluble compounds. Oils are liquid and fats are (semi )solid at temperatures of 18-24°C. Oils and fats are of vital importance in human nutrition, but have also several technical applications. About 80% of the present world supply is of vegetable and the remainder of animal origin. This Prosea volume reviews plant resources for edible and nonfood oils and fats, collectively called vegetable oils in the introductory chapter.

The 14 major oil crops (17 species) that produce most of the world vegetable oils are presented in Table 1. Only 9 of these will be included in Chapter 2, because 5 have been treated already in other Prosea volumes on account of their primary use. This applies to the pulses (Prosea 1) groundnut (Arachis hypogaea L.) and soya bean (Glycine max (L.) Merr.), the cereal (Prosea 10) maize (Zea mays L.) and the fibre plants (Prosea 17) cotton (Gossypium spp.) and flax/linseed (Linum usitatissimum L.). The dominant oil crops in South East Asia are coconut (Cocos nucifera L.) and oil palm (Elaeis guineensis Jacq.). The remaining major oil crops included in Chapter 2 are rapeseed (Brassica juncea (L.) Czern., B. napus L., B. rapa L.), safflower (Carthamus tinctorius L.), sunflower (Helianthus annuus L.), olive (Olea europaea L.), sesame (Sesamum orientale L.) castor (Ricinus communis L.) and tung-oil tree and wood-oil tree (Vernicia fordii Hemsl. and V. montana Lour., respectively). Rapeseed may have little potential as a crop for South East Asia, but the high degree of interchangeability of major oils makes it a likely component of edible oils available in local markets. Outside of the Mediterranean region, olive oil and table olives are speciality or gourmet items consumed world wide by more affluent people.

Another 8 oil crops including 11 species (Table 2), which are of local importance only or have potential for future cultivation in South East Asia, are dealt with in Chapter 2. Rice (Oryza sativa L.), from which rice bran oil is derived as a by product, appeared earlier in the volume on cereals (Prosea 10). Niger seed (Guizotia abyssinica (L.f.) Cass.) and jojoba (Simmondsia chinensis (Link) C.K. Schneider) would have potential for low rainfall regions in South East Asia because of their considerable drought resistance. Jojoba produces a liquid wax of high technical value.

Brief descriptions of 34 minor oil producing plant species are given in Chapter 3. Other plant species with vegetable oil or fat as a secondary product besides their primary use are listed in Chapter 4.

Origin and geographic distribution

Of all the major oil crops, only the coconut has a South East Asian Pacific origin. Since its first domestication some 3000 years ago by Malesian peoples, the coconut has become the "tree of life" for all coastal tropical regions, on account of the many useful products derived from the palm. Coconut was the most important world source of vegetable oil until overtaken by soya bean in the early 1960s.

Coconut oil is generally preferred for cooking purposes in South East Asia, but the oil palm is now by far the most important oil crop in the region. The oil palm was taken to South East Asia from its West African origin more than 150 years ago and a plantation industry started to develop gradually in the 1920s. The oil palm gained prominence as the dominant oil crop in Malaysia and Indonesia in the 1980s and a decade later also in Papua New Guinea and Thailand. The oil palm continues to be a major oil crop in West Africa and a few countries in tropical America. The American oil palm (Elaeis oleifera (Kunth) Cortes) is of no economic importance in South East Asia, but germplasm has been introduced for experimental plantings. Early European travellers to the Guinean coast of West Africa compared palm oil favourably to olive oil and this may have inspired the botanist Jacquin in 1763 to name the palm Elaeis guineensis. Olive oil is extracted from the fruits of the Mediterranean olive tree, which is one of the oldest (4000 BC) cultivated oil crops in the world. Despite its superb quality, olive oil production continues to be mostly restricted to the Mediterranean region due to the specific ecological requirements of the tree. There are some pockets of olive cultivation in North and South America, Australia, South Africa and even in China and Japan.

Soya bean cultivation gradually spread all over Asia from its region of origin in north east China during the first millennium AD and was introduced into North America in the 18th Century. Groundnut has a South American origin and was taken to Africa and Asia by Portuguese, Spanish and Dutch seafarers in the 16th Century. Soya bean and groundnut occupy 1st and 5th place respectively in the world production of vegetable oils, but in South East Asia they are mostly cultivated as pulses. The seeds are used for the preparation of fresh, roasted, fermented and dried food products and only a minor portion is crushed for oil extraction.

Brassica rapa and B. juncea are two ancient (2000 BC) oil crops of West and South Asia, while B. napus has a fairly recent West European origin. The three species together are important producers of vegetable oils in temperate climates, but a recently developed day length neutral B. napus may provide opportunities for rapeseed cultivation in the highlands of South East Asia. Ethiopian mustard (B. carinata A. Braun) is an oil crop restricted to East Africa. Safflower is also an old crop of West Asian origin (2000 BC), cultivated initially for the orange dye obtained from the florets in the Mediterranean and Asian regions, and reaching China and Japan in 200-400 AD. Safflower is now a source of high quality edible oil with considerable areas of cultivation in India, the Americas and Australia. It is grown on a small scale in South East Asia, particularly in Thailand.

Sunflower was domesticated by North American people more than 5000 years ago and was taken to Europe in the 16th Century. It became an important oil crop in the 19th Century in Russia and subsequently in Central Europe, the Balkan Peninsula, North and South America, West Europe, China and India. In South-East Asia sunflower is grown in Burma (Myanmar) and to a small extent also in Thailand. Sesame has been cultivated for its seeds and edible oil for at least 4000 years in South and West Asia, but its region of origin may have been Ethiopia. It is now produced in many countries in Africa, tropical America and Asia, including Burma (Myanmar), Thailand and Vietnam.

Cotton was domesticated for its seed fibre more than 6000 years ago in South Asia (Gossypium arboreum L. and G. herbaceum L.) and in South and Central America (G. barbadense L. and G. hirsutum L.). The American (tetraploid) cotton species dominate present cotton cultivation in all major production areas. In addition to being the most important fibre plant, cotton is also the 4th world oil crop. In South East Asia, Burma (Myanmar) is the largest producer of cottonseed and oil, followed by Thailand, Indonesia, the Philippines, Vietnam and Laos. Linseed has a West or South Indian origin and was first cultivated for its fibre at about 1000 BC. This is another oil-producing fibre plant with specialized fibre (flax) and seed (linseed) cultivars, the former being grown in temperate and the latter in subtropical regions. However, linseed has no potential for cultivation in the South East Asia.

Of the two major cereals yielding a vegetable oil as secondary product, maize is a major oil crop worldwide but of little significance in South East Asia. Rice bran oil is only a minor commodity due to predominantly traditional methods of processing of most rice in Asia, where more than 60% of the rice is grown. China, Japan, Burma (Myanmar), India and Vietnam produce sizable quantities of rice bran oil in nonfood and edible grades.

The two remaining major oil crops mentioned in Table 1 both produce oils for industrial applications. Castor originated in Ethiopia and was cultivated already 6000 years ago in Egypt for lamp oil and medicinal purposes. It spread throughout the Mediterranean basin, to West and South Asia, reached China in the 9th and Europe in the 15th Centuries and is now grown world wide in warmer climates. The tung-oil tree and wood-oil tree originate in China, where the drying oil has been used for the past 2000 years as wood preservative, for oiling of paper and waterproofing of fabrics. Outside China the tung-oil tree is grown commercially in a few countries including Argentina, Paraguay and Malawi. Efforts to introduce the wood-oil tree in Indonesia around 1930 did not lead to large plantings.

An overview of origin and distribution of 8 other oil crops, including the earlier mentioned rice, is presented in Table 2. Only tengkawang (Shorea spp.) and Philippine tung (Reutealis trisperma (Blanco) Airy Shaw) are of South East Asian origin. Jojoba and chia (Salvia hispanica L.) are indigenous to South or Central America; rice and kokam butter tree (Garcinia indica (Thouars) Choisy) are from South Asia; Chinese tallow tree (Triadica sebiferum (L.) Small) is from China; niger seed has its origin in Ethiopia. The majority of the 35 minor oil-bearing species treated in Chapter 3 have an Asian origin (24 species); 10 species are from South America and 1 from West Africa.

Edible uses of vegetable oils

Vegetable oils are essential components of the daily diet as a source of energy, essential fatty acids and some vitamins. They have also several important functions in food preparation and processing, such as tenderizing, lubricating and adding flavour during cooking and frying and providing structure in bakery products (Stauffer, 1996).

In many rural areas of developing countries large quantities of unrefined vegetable oils are still used directly for cooking and frying purposes, e.g. palm oil in West Africa, coconut oil in South-East Asia and various seed oils (rapeseed, sesame, niger seed) in South Asia and East Africa. Much of the olive oil consumed as salad and cooking oil in the Mediterranean region is also virtually unrefined apart from some filtration.

Otherwise, all vegetable oils are subjected to various processes following extraction to adapt them to a large diversity of uses. These include salad oils and dressings, margarines, vanaspati, mayonnaises, cooking and frying oils for home consumption. Large quantities of commercial grade frying oils, shortenings, fats and emulsifiers are applied in the food industry for potato crisp, pastry, bakery, biscuit, confectionery, ice cream, coffee creamer and other products. Vegetable oils are also used in the fish and canning industry (Hatje, 1989).

Technical uses of vegetable oils

Soap and lamp oil are two of the traditional nonfood applications of vegetable oils, notably coconut, oil palm, rapeseed and olive, since time immemorial.

About 15% of present day vegetable oils are converted into soaps and oleochemicals, such as fatty acids, methyl esters, fatty alcohols, fatty acid amides, fatty amines and epoxidized oils (Helme et al., 1995; Pryde & Rothfus, 1989; Pantzaris, 1997). These are intermediate materials for the manufacture of a wide range of industrial and technical products including toilet soaps, shampoos, cosmetics, candles, waxes and polishes, detergents, surfactants, paints and varnishes, waterproofing of textiles, printing inks, lubricating greases, corrosion inhibitors, plasticizers and stabilizers in (PVC) plastics and even diesel fuels. Refined vegetable oils, particularly palm oil, find application as non toxic lubricants of food-processing machinery, in the sheet steel and tinning industries to prevent oxidation and also as lubricating oil of textile fibres during spinning and weaving. Castor oil has many technical and pharmaceutical applications.

Seeds, fruits and meals for food and feed

Direct consumption of seed and fruits

A considerable proportion of the seeds or fruits harvested from some oil crops is not used for oil extraction, but consumed directly as a snack or processed food products.

Oilseeds

About 85% of the world soya bean crop undergoes extraction for oil and protein, but in Asia much of it is used in preparing fresh, fermented and dried food products such as sprouted beans, roasted seeds, soya milk, tofu, tempeh and soya sauce. More than 40% of the world groundnut crop and about 80% in South East Asia is consumed as roasted whole seeds or peanut butter for various food preparations. Non oilseed sunflower cultivars (10% of the total crop) produce seeds for direct consumption as (roasted) snacks, in confectionery and baking and also as birdseed. Large quantities of whole sesame seeds are used to prepare sweets and in baking or as roasted snacks in South Asia. A paste from ground hulled sesame seeds forms a favourite food in North Africa and the Middle East.

Fruits

It is estimated that 45% of all coconuts produced worldwide are used for edible purposes other than oil extraction: coconut water from immature coconuts, coconut milk from freshly grated endosperm, shredded and desiccated fresh coconut as a side dish and as an ingredient of bakery and confectionery products and ball copra, an Indian speciality. In West Africa, the mesocarp pulp of freshly harvested oil-palm fruits, after boiling and removal of fibres, forms the basis of a nutritious soup. About 8% of the olive crop is processed into green and black table olives.

Meals

The residual cake after oil extraction is ground into a meal, which is a valuable source of protein to supplement livestock feeds and sometimes also human food (Bell, 1989). More than half of the total meal production is from soya bean. While in most oilseeds the meal is a by product of oil extraction, it is in fact the main product of the soya bean crop, being much larger in quantity and value than the oil. Other important producers of meals are canola rapeseed, cottonseed, sunflower and groundnut, while sesame, safflower and linseed also produce high protein meals. Maize (corn germ), copra and palm-kernel meals have a lower protein and higher fibre content than soya bean and other meals. About 10% of the meals of soya bean, sunflower and groundnut in particular, are processed into flour and protein concentrates for use in bakery and other food products, such as bread and pies, fortified breakfast cereals, meat and milk extenders, meat like products and infant food formulations.

Low-grade oil cakes and meals or those containing anti nutritional and toxic substances (e.g. from traditional rapeseed and castor) are used as organic fertilizers. The fibre mass of extracted oil-palm fruits provides fuel for the mill boilers.

Production and international trade

Table 3 gives an overview of world area and mean annual production during the last four years (1997-2000) for each of 14 major edible and nonfood oil crops, in decreasing order of importance based on oil output. Production figures include amounts of "seed" (kernels for the oil palm and copra for the coconut), meal and oil. Export percentages for meals and oils show the relative magnitude of international trade and local consumption for each commodity. The degree of global cultivation and regional concentration of these oil crops are reflected in the total number of countries and in the five most important ones, which together account for a large proportion of total production (indicated in brackets).

Soya bean is by far the largest oil crop, producing 62% of the world's oilseed meals (169 million t) and 28% of all vegetable oils (86 million t). The United States, Brazil and Argentina together account for at least 60% of world soya bean oil production. About one-third of soya bean meal and oil is traded internationally. The oil palm is a close second with its palm and palm-kernel oils forming 25% of world oil production. However, palm oil is by far the largest vegetable oil in the international market, mainly as a result of the high proportion of export from Malaysia and Indonesia, which together produce about 81% of world palm oil. Rapeseed is, with 13% of total meal and 15% of world oil the third most important oil crop. Sunflower, groundnut, cottonseed and coconut (copra) are 4th to 7th in the ranking order, producing together another 21% of all meals and 25% of the world vegetable oils. The remaining 7 crops produce only 7% of the world vegetable oils, 1.5% of it of nonfood type. But olive and sesame oils are highly appreciated for their quality and maize (germs and bran) is a substantial contributor of meals for livestock feeds.

World average productivity of each oil crop can be estimated from the annual production divided by the area of cultivation in Table 3. It is between 0.3 and 0.5 t oil per ha for soya bean, rapeseed, sunflower, groundnut, coconut, olive and castor, while still lower for cottonseed, sesame, safflower and linseed. With 3.2 t palm oil per ha, the oil palm is 6-10 times more productive than any of the other oil crops and to this 0.4 t palm-kernel oil should be added as well. Maximum oil yields vary from 1.0 t/ha for soya bean and 1.6 t/ha for rapeseed to 2.7 t/ha for sunflower and 3.6 t/ha for coconut. However, for the oil palm 7 t/ha has been achieved already with advanced planting material and under optimum ecological conditions. Clearly, oil palm is by far the most efficient producer among all oil crops.

Total world production of oils and fats, of both animal and vegetable origin, has increased approximately by a factor of 9 since the beginning of the 20th Century, with the share of vegetable oils going up from 49% in 1910 to 81% in 2000 (Table 4). Exponential growth in production has taken place over the last 25 years in oil palm, soya bean, rapeseed and sunflower.

The total value of the annual production of all oil crops (Table 5) is estimated at US$ 40-62 billion for vegetable oils and another US$ 42-63 billion for the meals and unprocessed seeds. On average about one-third is exported, which means that vegetable oils and meals collectively represent some of the most important commodities in world trade. Total value of soya bean oil and palm oil are comparable, but soya bean also produces an enormous quantity of meal with a total value more than double of that of the oil. Olive and groundnut oils in particular, but also coconut and palm-kernel oils, usually fetch premium prices of 50-100% more than those paid for the main vegetable oils.

South East Asia produces more than 66% of all coconut and 81% of all palm oils in the world, but otherwise plays only a minor role in oil crops at global level (Table 6). Burma (Myanmar) has hardly any oil palm or coconut production, but data show a great diversity in other oil crops including soya bean, sunflower, groundnut, cottonseed and sesame. Indonesia has, in addition to oil palm and coconut, significant areas of soya bean and groundnut in cultivation. Malaysia and Papua New Guinea depend almost entirely on oil palm and coconut, while in addition to these, Thailand has also groundnut, sesame and sunflower. Vietnam production data include soya bean, groundnut, coconut and sesame.

Oil content

The oil content in the harvested and dried seeds of annual oil crops, such as rapeseed, sunflower, groundnut, sesame, safflower and linseed, usually varies between 40% and 50% of seed weight. However, the seeds of soya bean and cotton only contain about 20% oil. The mesocarp of ripe oil-palm fruits has 45-50% oil, but it is common to express palm oil content on a fresh fruit bunch basis, which is 20-25% for modern cultivars. The kernel of the oil palm and copra of the coconut have similar water content (6%) and oil composition, but oil content is considerably higher in copra (60-65%) than in palm kernels (50%). In olive, 5-6 kg ripe fruits will produce about 1 kg oil (16-20% oil to fresh fruit).

Chemical aspects of vegetable oils

Triglycerides

Vegetable oils consist for more than 95% of triglycerides (triacylglycerols), which are esters of glycerol and fatty acids. The basic chemical structure of a triglyceride is given in Figure 1b. The fatty acids have a carboxylic acid group (COOH) on one end of an aliphatic chain of 6 to 24 hydrocarbons, almost always in even numbers (Stauffer, 1996). The fatty acids are called saturated, mono unsaturated or polyunsaturated according to the number of double bonds present in the carbon chain. There are several fatty acids, but in the major edible vegetable oils only few fatty acids predominate, mainly palmitic (C16:0), oleic (C18:1) and linoleic (C18:2) acids. The chemical structure of these three fatty acids with the position of double bonds is given in Figure 1a. Most naturally occurring unsaturated fatty acids are in the cis configuration, which means that the two hydrogen atoms at the double bonds are on the same side. Isomers in the trans configuration are rare in natural vegetable oils, but are formed during the hardening process by hydrogenation.

The fatty acid composition of major edible vegetable oils is presented in Table 7. These data averages are only for demonstrative purposes, as fatty acid composition may vary considerably under the influence of climate and cultivar. The following categories can be distinguished:

• lauric oils from coconut and palm kernel (and also the babassu and cohune palm) with high contents of short chain lauric acid (C12:0);
• palm oil with a mixture of palmitic and oleic acids; in palm olein the oleic oil content is raised by fractionation during processing;
• oleic oils from the olive, canola, high oleic sunflower (and also the pataua palm);
• linoleic oils from soya bean, sunflower, cottonseed, maize and safflower;
• oleic linoleic oils from groundnut and sesame (and also niger seed);
• traditional rapeseed oils with a high content of erucic acid (C22:1).

Simple triglycerides have three identical fatty acids, such as tripalmitin (glycerol with 3 molecules of palmitic acid) and triolein (glycerol with 3 oleic acid molecules), the latter being a major triglyceride in olive oil. Mixed triglycerides have saturated and unsaturated fatty acids. In most natural vegetable oils the unsaturated fatty acids are placed on the central 2 position and the saturated fatty acids on the outer 1 and 3 positions of the glycerol molecule. The overall composition of fatty acids and their arrangement on the three positions of the triglycerides determine to a large extent the chemical and physical characteristics of an oil (Åppelqvist, 1989).

Unsaturated oils are subject to autoxidation at the site of double bonds initiated by (hydro)peroxides and leading to oxidative rancidity. This may cause a characteristic "cardboard" flavour. The relative rate of oxidation of linolenic (C18:3) and linoleic (C18:2) fatty-acid chains is 25 and 12 times higher respectively than that of oleic (C18:1) acid. Oxidation is slowed down by anti oxidants (e.g. tocopherols), which occur naturally in many vegetable oils, or by adding such organic compounds in low concentrations during manufacturing. Processing and storage under oxygen free and dark conditions are important measures to prevent oil rancidity.

Rapeseed and soya bean oils contain some α-linolenic acid, but this should not affect the quality as an edible oil when properly processed and stored. However, linseed oil is unsuitable for consumption, as it contains more than 50% linolenic acid. When exposed to air, the oil polymerizes into a flexible film as a result of rapid oxidation at the double bonds. It finds wide application as drying oil in paints and industrial coatings. Edible linseed oil with only a few percent linolenic acid and a much higher linoleic acid content is now being produced from recently developed "Linola" linseed cultivars (Dribnenki et al., 1999).

The fatty acid composition of the castor and tung oils deviates from that of the edible oils (Åppelqvist, 1989). Ricinoleic acid, which predominates in castor oil, has an aliphatic chain of 20 carbons with a single double bond and a hydroxyl group. The α-eleostearic acid (C18:3) of tung oil has one double bond in the cis and two in the trans configuration. Both oils have many applications in the chemical industry. There are a number of other chemical reactions, in addition to oxidation, with important consequences for the properties of vegetable oils (Stauffer, 1996). One that occurs naturally is the cleaving of the triglyceride esters into glycerol and free fatty acids (FFA) by hydrolysis causing hydrolytic rancidity. The reaction is catalyzed in the presence of water by metals or by the action of the enzyme lipase. The fat splitting lipase is found in oil-palm fruits (Hartley, 1988) and is also produced by fungi and other micro organisms contaminating vegetable oils. Low content in free fatty acids is an important quality requirement of crude and refined vegetable oils. Hydrolysis with the base potassium (or sodium) hydroxide is called saponification, as it neutralizes the fatty acids to soap. The esters can also be split reductively, with sodium and potassium metals, into glycerol and fatty alcohols, which are used to manufacture detergents and lubricants. Glycerolysis is a reaction of glycerol and fatty acids, with potassium carbonate as basic catalyst, to form monoglyceride emulsifiers. Hydrogenation, by adding hydrogen gas to unsaturated oils in the presence of a nickel catalyst, converts double bonds into single bonds in the carbon chains. This improves the oxidative stability of oils and may also change liquid oils into solid fats. The properties of the fats can be improved further by inter esterification, which involves heating in the presence of sodium methoxide or metallic sodium. It produces a more plastic fat as a result of changes in the position of fatty acids in the triglycerides (Stauffer, 1996; Young et al., 1986).

The chemical properties of oils are determined by a number of analytical tests (Gunstone et al., 1986). One is the iodine value (IV) to measure oil unsaturation (number of double bonds) and is expressed as grams of iodine absorbed by 100 g of oil. Examples of IV are presented in Table 7. The saponification value (number of mg potassium hydroxide to saponify 1 g oil) indicates carbon chain length of the fatty acids, the higher the value the shorter the chain lengths, e.g. 260 for coconut and 190 for soya bean oil. Several tests have been developed to measure oxidative stability (e.g. the active oxygen method and oil stability index), oxidative degradation (e.g. peroxide value, anisidine value and total oxidation number) and free fatty acid content of vegetable oils (Gunstone et al., 1986; Stauffer, 1996).

Other components

Unrefined vegetable oils contain 0.1-3% lecithins, including phospholipids, and small amounts of waxes, sterols, and other hydrocarbons such as carotenoids (pro vitamin A), tocopherols (vitamin E) and tocotrienols (Åppelqvist, 1989; Stauffer, 1996).

Phospholipids are triglycerides with one of the three apolar fatty acid chains replaced by a polar head consisting of an alcohol group (e.g. choline, ethanolamine, serine) linked to the glycerol group by phosphate (Figure 1c). This amphiphilic characteristic makes them very useful as emulsifiers. Phospholipids also form the main component of cell membranes in animals. Waxes are apolar lipids of long chain fatty acids linked to fatty alcohols. Sterols are lipid compounds with ring structures including cholesterol (rare in vegetable oils) and phytosterols. The waxes, sterols and other hydrocarbons form part of the so-called unsaponifiable matter content, up to 2% in crude oils but usually less than 0.5% in refined oils (Stauffer, 1996).

The oil extracted from jojoba seeds consists almost entirely of unique liquid waxes composed of mono-unsaturated fatty acids (20-24 carbons) esterified to long chain alcohols. Sulphurized jojoba oil is an excellent replacement for sulphurized oil of the sperm whale, which has traditionally been the extreme-pressure lubricant (Pryde & Rothfus, 1989).

Physical aspects of vegetable oils

Solubility

Triglycerides are soluble in the solvents hexane, benzene or acetone. The small amounts of hydrophilic substances present in vegetable oils, such as the phospholipids, monoglycerides, free fatty acids and oxidation products, are removed during refining by washing with water (Stauffer, 1996).

Melting and solidification

Pure triglycerides have a fairly exact melting point, i.e. the temperature at which they change from a solid into a liquid substance when slowly heated. The melting point becomes higher with increasing carbon chain length and degree of saturation of the fatty acids and also by a change from the cis to trans isomer (e.g. during hydrogenation). Examples of melting points are: trilaurin 47°C, tripalmitin 66°C, palmitoleopalmitin 37°C, triolein 5°C and trilinolein -13°C (Gunstone et al., 1986; Stauffer, 1996).

These are melting points of triglycerides having the stable β-crystal structure in the solid form, which occurs under conditions of slow cooling. Unstable α crystals are formed under rapid and β-crystals under intermediate cooling rates. Melting points vary according to crystal structure, e.g. for tripalmitin in the α form the melting point is 45°C and 64°C in the β-form. Triglycerides solidified in the α crystal structure have a waxy appearance, in the β form they are grainy and hard, while in the β'-form they have a smooth and creamy texture. The β'-form, which reverts to the β-form fairly quickly for a single triglyceride, is maintained much longer in complex mixtures such as shortenings and margarines.

Natural vegetable oils are a mixture of several triglycerides, with the result that they melt gradually over a range of temperatures. Various analytical methods have been developed to specify melting and solidification characteristics of natural and manufactured oils and fats. Melting point (mp) tests include the complete mp, Wiley mp, dropping mp and slip point. The latter is measured by slowly warming a sample of solid fat in a capillary tube and observing the temperature at which it moves. For instance, the slip point of crude palm oil from Malaysia was 31-38°C, of palm olein 19-23°C and super olein 13-16°C (Pantzaris, 1997). Another test quoted in tables specifying vegetable oils is the titre, which is the temperature at which a liquid oil starts to solidify under controlled conditions (Gunstone et al., 1986). Examples of titres are: palm oil 40-42°C, coconut oil 20-24°C, soya bean oil 15-18°C, sunflower oil 16-20°C, rapeseed oil 6-10°C and linseed oil 19-21°C. The solid fat index (SFI) and content (SFC) measure the proportion of the solids in an oil at various temperatures. Specifications of oils and fats usually include a solid fat profile, which is a curve relating temperatures to SFI or SFC.

Other thermal properties

The smoke point, which is the temperature at which a heated oil begins to give off smoke, is an important index to assess the quality of frying oils. It should be about 240°C for industrial deep frying oils, but may be some 10 degrees lower for oils used for domestic purposes. The flashpoint (flashes of burning when exposed to a flame) and fire point (full burning when ignited) are important temperature indicators for safety. The flashpoint is generally 90-140°C higher than the smoke point and the fire point another 50-60°C above the flashpoint. The build up of free fatty acids in particular, but also other contaminants, have a lowering effect on the smoke and other temperature points, rapidly making oils unsuitable and also dangerous for frying purposes. Refined sunflower, groundnut, sesame and palm olein are among the vegetable oils with the highest smoke points.

Nutritional and health aspects of vegetable oils

Source of energy

A major nutritional function of edible oils and fats is the supply of energy, 38 kJ (9 kcal) per g of oil compared to about 19 kJ (4.5 kcal) per g for carbohydrates and proteins. The average daily energy requirement for Asian adult persons (55 kg mean body weight) according to the FAO and WHO recommendations (FAO, 1972) is a minimum of 8500 kJ (2000 kcal), to be provided by a balanced diet containing 50% carbohydrates, 30% lipids (oils and fats) and 15% proteins. This would mean a daily intake of 70 g (2500 kJ) or 25 kg edible oils and fats per year, but in reality average consumption is often less than half the recommended quantity, e.g. in the Philippines 8 kg, India 11 kg and in Indonesia 15 kg. This situation is in stark contrast with the 47 kg (European Union) and 51 kg (United States) of edible oils and fats consumed annually per person in some developed countries (Mielke & Mielke, 2001). Inadequate dietary energy is an important cause of malnutrition in Asia and increasing the availability of vegetable oils is a most efficient way of alleviating this problem. Especially young children have difficulty in digesting large quantities of unrefined carbohydrate rich food. Generally, oils and fats with lower melting characteristics have a better digestibility.

Triglycerides in the food are partly hydrolyzed by pancreatic lipase and micelles (colloidal solutions) of 2 monoglycerides and free fatty acids are absorbed by the intestinal wall. Here they are esterified into new triglycerides, which are then transported through the lymphatic system and the bloodstream as chylomicra (tiny globules of emulsified fat) to the various body tissues to provide energy by oxidative combustion or to be deposited in adipose (fat storage) tissue (Vles & Gottenbos, 1989).

Membranes’ phospholipids and metabolic regulators

About half the normal (70 g) daily oil and fat intake is converted into energy. The remainder provides fatty acids for incorporation into phospholipids, which are synthesized in the body and form the main components of cell membranes and other bi layer lipid structures in animals and humans, e.g. myelin nerve sheath and eye retina (Graille & Pina, 1999; Stauffer, 1996). The diglyceride component of most phospholipids (see Figure 1c) contains one saturated and one unsaturated fatty acid, about 20% have two unsaturated and a small fraction two saturated fatty acids. Although most of the fatty acids, except linoleic acid, can be synthesized in the body, phospholipid synthesis is much more efficient when these are readily available in the dietary oils and fats in a composition of 32% saturated, 45% mono unsaturated and 23% polyunsaturated. Linoleic acid is an essential fatty acid, because complete absence in the diet will eventually lead to various deficiency symptoms (e.g. retarded growth and skin lesions). It is partly converted in the liver to γ-linolenic acid (C18:3) and subsequently to arachidonic acid (C20:4), which are both essential fatty acids. In addition to being important components of the membrane phospholipids, the essential fatty acids are also substrates for the biosynthesis of various metabolic regulators (eicosanoids), which influence amongst others the central nervous system, blood pressure, the aggregation of blood platelets and cardiac function. α Linolenic acid, which is present in linseed, brassica oilseed and soya bean oil (Table 7), can partly replace the functions of essential fatty acids, but this may lead to inferior membrane phospholipids and metabolic regulators. High concentrations of this polyunsaturated fatty acid in the diet should therefore be avoided (Vles & Gottenbos, 1998).

Cholesterol

In contrast to animal fats, vegetable oils contain very little cholesterol. It is mostly synthesized in the body from lipid compounds into a steroid alcohol with four rings attached to a side chain of 8 hydrocarbons (Figure 1d) and it is often esterified to a fatty acid. Cholesterol plays a role in the proper functioning of cell membranes and many hormones are derived from it. In the liver, cholesterol is converted into cholic (bile) acids, which are excreted in the intestines and work as emulsifiers facilitating the digestion of fat (Stauffer, 1996). Cholesterol is transported in the bloodstream in lipoprotein particles, which are composed of cholesterol, cholesterol esters, triglycerides, phospholipids and highly specific proteins (Vles & Gottenbos, 1998). High density lipoprotein (HDL) transports excess cholesterol from the cells to the liver and other organs for conversion into bile or hormones. Low density lipoprotein (LDL) delivers cholesterol to various body tissues, but it may also become the cause of atherosclerosis by forming plaques on injured arterial walls.

There is ample evidence from epidemiological, clinical and biochemical research that high levels of LDL in the blood increase the risks of cardiovascular diseases. The composition of dietary fatty acids influences blood cholesterol levels. Saturated fatty acids (C14:0 and longer chains) increase LDL levels, while unsaturated fatty acids have a lowering effect. On the other hand, the short chain saturated fatty acids (C8:0-C12:0) present in the lauric oils (coconut and palm kernel) are shown to have no adverse effects on LDL levels. Their high digestibility makes them very suitable for infant foods and in therapeutic diets for patients with fat maldigestion problems (Vles & Gottenbos, 1989). Trans isomers of unsaturated fatty acids, as a result of hydrogenation, also appear to raise the level of LDL in the blood. On the other hand, the effects of type and quantity of dietary fatty acids on LDL levels are confounded with other risk factors including lifestyle, such as stress and lack of physical exercise, smoking, overweight, diabetes as well as genetic predisposition.

Generally, there are good reasons for recommending a lowering of saturated and increasing unsaturated oils and fats in the diet, particularly in countries where total fat consumption is excessive. However, there is a difference between saturated animal and vegetable fats. For instance, palm oil contains about 50% saturated fatty acids, but nevertheless behaves nutritionally like an unsaturated oil and does not increase LDL levels in the blood. This could be explained by the predominant composition of the triglycerides in vegetable oils, with saturated fatty acids on the outer 1 and 3 positions and an unsaturated fatty acid on the inner 2 position. Hydrolysis during pancreatic digestion leads to 2 monoglycerides with unsaturated fatty acids, which are easily absorbed by the intestinal wall. Many of the longer chain saturated fatty acids give rise to insoluble calcium salts that cannot be absorbed and are excreted (Graille & Pina, 1999).

Vitamins

Oils and fats are carriers of the lipophylic vitamins A, D, E and K and facilitate their absorption. Vitamin A is formed in the body by oxidative cleavage of carotenoids, β-carotene in particular, which are available from vegetables and fruits and also from crude palm oil. Deficiency in vitamin A causes loss of night vision and eventually total blindness. Vitamin D, which is needed in particular for proper bone development in children, is produced by conversion of cholesterol in skin exposed to sunshine (ultraviolet irradiation). Most vegetable oils are good sources of vitamin E or tocopherol, which has the properties of an anti oxidant inhibiting autoxidation of polyunsaturated fatty acids. The main symptom of vitamin E deficiency in animals and humans is loss of fertility. Vitamin K is produced by the bacterial flora in the large intestine. Inadequate level of bile acids and antibiotics treatments that affect the intestinal bacteria may lead to vitamin K deficiency symptoms, especially prolonged bleeding after injury, due to poor blood clotting (Stauffer, 1996; Vles & Gottenbos, 1989).

Nutritional aspects of oilseed meals

The average chemical composition of 11 oilseed meals is presented in Table 8. These data are useful as indicators of major variation in composition between meal types, but of course they cannot reflect the considerable variation encountered within the same commodity due to differences in cultivars, agronomic and processing conditions and also in analytical methods. The cake after oil extraction is ground to a meal and dried to 7-11% moisture content according to type of oil crop. Residual oil content depends on method of extraction, being much lower after solvent (1-5%) than after mechanical extraction (7-9%). The higher the oil content the higher the energy value of the meal.

Crude protein contents vary from 20-50%. Soya bean, sunflower (hulled seeds), groundnut (shelled) and sesame meals have particularly high crude protein contents (≥44%). Rapeseed, cottonseed, safflower and linseed meals contain 36-42%, but palm kernel, copra and maize germ meals have only half as much (21%). The protein quality depends on composition and concentration of amino acids, especially the essential amino acids. In oilseed meals, lysine, methionine and cysteine content are emphasized, because these are the amino acids frequently in limited supply in the grain based feeds used for pig and poultry (Bell, 1989). Soya bean and rapeseed meals are high in lysine (>6% of total protein), the others contain only half of that or less. On the other hand, soya bean meal contains less methionine and cysteine than other meals. Sesame meal is particularly high in methionine (>3%). Excessive heating during oilseed processing can have a negative effect on lysine and other essential amino acid contents.

Carbohydrates are of two types: (a) structural polysaccharides from cell walls, which are unavailable to monogastric animals, usually called crude fibre and (b) energy reserve sugars and starch like polysaccharides, which are digestible by all types of animals (Vohra, 1989). Carbohydrate and protein contents are inversely related, but the fraction of crude fibre over total carbohydrates varies with the type of meal. Soya bean, sesame and safflower meals are particularly low in crude fibre content. The content of available carbohydrates varies from 26-34% in most meals. However, in palm kernel, copra and maize-germ meals the available carbohydrate (41-46%) contents are considerably higher, as are the crude fibre contents (15-17%) in palm-kernel and maize germ meals. The seed hulls are high in crude fibre and hulling reduces this fraction in the meal considerably. Seeds may be hulled or not before crushing for oil extraction, or hulls may be added to the meals afterwards, depending on the purpose of the meal. Ruminants require a higher crude fibre content in the feed than monogastric animals. Generally, oilseed meals are supplements for use in animal feeds (Bell, 1989). Choice of type of oilseed meal and exact formulations in the feed will depend on content and quality of protein and carbohydrates, but also on palatability, digestibility and presence of toxic or antinutritional factors.

Sunflower and groundnut meals are also good sources of the water soluble vitamins of the B-complex.

The ash content includes minerals. Some oilseed meals are rich in dietary minerals, but availability is often lower than contents would suggest, due to the presence of phytates and other insoluble complex compounds. Sunflower meals in particular are high in P, Ca, K and Fe minerals.

Toxic and antinutritional substances

Some oilseeds contain toxic or antinutritional factors, which may still be present in the oil but particularly in the presscake after processing. Certain factors can be eliminated or reduced by heat and other pretreatments of the seeds. Proper formulations with different meal types are applied to avoid toxic concentrations in the feeds. The oils and meals from oil palm, sunflower, coconut, maize (germ) and safflower are free of significant amounts of such compounds.

Soya bean seeds contain a number of antinutritional substances, such as trypsin inhibitors (causing poor digestibility of proteins), haemagglutinins (inducing clotting of red blood cells), an oligo- or glycopeptide with goitrogenic action (interfering with iodine uptake) and phytates (causing reduced uptake of phosphorus and other minerals). All three factors are largely inactivated by heating through boiling, steaming or toasting of the seeds. Soya beans also contain small amounts of heat-stable factors, including saponins (glucosides inhibiting activity of proteolytic enzymes), isoflavone glucosides (estrogenic effects) and oligosaccharides (raffinose and stachyose) causing flatulence. Fermentation reduces many of these negative effects (Salunkhe et al., 1992).

Traditional rapeseed contains high concentrations of erucic acid (30-50%) and glucosinolates (1-4%). Erucic acid absorbed in large quantities increases the risks of cardiovascular diseases. Glucosinolates produce (iso)thiocyanates and nitriles (pungency in oil and meal) upon hydrolysis by the enzyme myrosinase. These are goitrogenic factors interfering with the iodine uptake and thyroxine synthesis in the thyroid gland (Salunkhe et al., 1992). Cooking of the flaked seed inactivates the myrosinase. Canola is a type of rapeseed developed by breeding with almost no erucic acid (see Table 7) and very low glucosinolate content (0.1-0.4%). Minor antinutritional factors in rapeseed are tannins (forming complexes with proteins, minerals and vitamins and so reducing bio availability) and sinapine causing some chicken to produce eggs with a fishy flavour (Bell, 1989).

Groundnut is a legume like soya bean. Its seeds also contain lectins (haemagglutinins), trypsin inhibitors, goitrogens, phytates and flatulence factors, and methods of inactivation are similar. Cottonseeds contain highly toxic gossypol, a complex phenolic compound which induces depressed appetite and weight loss. It also causes discolouration of egg yolks. Gossypol can be inactivated by adding ferrous salts to the meal. Linseed meal for feeding purposes needs further processing to remove mucilage, to hydrolyze the cyanogenic glucoside linamarin (by the endogenous enzyme linase), and to inactivate vitamin B6 antagonists (by heating). Heating also removes the cyanide resulting from the hydrolysis of linamarin (Bell, 1989).

Prolonged storage of poorly dried seeds can lead to aflatoxin contamination of oils and especially meals. Aflatoxins are secondary metabolites of the moulds Aspergillus flavus and A. parasiticus and already at very low concentrations (>1 ppm) highly toxic and carcinogenic to most animals (Bell, 1989). Allowable maximum safety levels of aflatoxin contamination in foods for human consumption vary from 5-30 parts per billion (Weiss, 2000). Poorly stored groundnuts and maize in particular are most susceptible to contamination with aflatoxins, but also cottonseed and copra are associated with this problem. Of particular importance in maize are fumosins. These are mycotoxins produced by Fusarium moniliforme and associated with oesophageal cancer. Methods of control include first of all prevention by ensuing proper conditions during seed (or copra) storage and transport, and monitoring before processing. Contaminated seedlots should be destroyed at all times. Aflatoxin contamination in oils is removed during the lye treatment (at pH>8) as part of the refining process. It is possible to detoxify meals to some extent by heating and various chemical treatments (Ranasinghe, 1999; Weiss, 2000).

The 63 species listed in Table 9 represent only a minor fraction of those plant resources from which vegetable oils and fats can be extracted for edible or nonfood purposes. It includes all major oil crops of the world and those of local importance or which could potentially be cultivated in South-East Asia. They belong to 27 families (25 dicotyledons and 2 monocotyledons). For the Palmae family 6 oil-bearing species are treated including the two most important oil crops of South-East Asia, Cocos nucifera and Elaeis guineensis. The Euphorbiaceae is another family with 6 oil-bearing species indicated in Table 9, but all produce non-edible oils including Ricinus communis and Vernicia spp. Other families with major oil-bearing species are the Compositae (Carthamus tinctorius, Helianthus annuus), Cruciferae (Brassica juncea, B. napus and B. rapa), Gramineae (Zea mays), Leguminosae (Arachis hypogaea, Glycine max), Linaceae (Linum usitatissimum), Malvaceae (Gossypium barbadense and G. hirsutum), Pedaliaceae (Sesamum orientale) and Oleaceae (Olea europaea).

Some 14 species are annual or biennial herbs belonging to the families Compositae (3), Cruciferae (3), Gramineae (2), Labiatae (2), Leguminosae (2), Linaceae and Pedaliaceae. The other 49 oil-bearing species are perennial shrubs, trees or palms. Seeds are the main oil containing plant parts in almost all mentioned species, except for the fruit mesocarp of Elaeis guineensis and Olea europaea (Table 9). The oil (storage lipids) in the cells of seed or mesocarp is contained in bodies called oleosomes (Napier et al., 1996). In some seed, such as rapeseed, soya bean and groundnut, the oil is concentrated in the cotyledons, in many others in the endosperm (Vaughan, 1970).

Climatic factors

There is great variation in the ecological requirements of oil crops, which include plant species belonging to tropical, subtropical or temperate climates. An overview of ecological data for the major oil crops of South-East Asia is given in Table 10.

Much of the South-East Asian region is situated within the humid tropics. The climate is characterized by mean daily temperatures of >24°C in the lowlands with little seasonal variation, relative humidity of >70% at midday, annual rainfall of >1800 mm well-distributed over the year (only 2-3 months with less than 100 mm) and >2000 hours of sunshine per year (Anonymous, 1993). About 20% of the area is mountainous (> 400 m altitude) with cooler climates. Mean temperatures decrease by about 0.6°C for each 100 m of elevation in equatorial regions and the amplitudes of day and night temperatures widen (Braak, 1946).

Oil palm and coconut are crops typical of the lowland humid tropics. The climatic conditions in South-East Asia are more suited to the oil palm than those of West Africa, particularly the better rainfall distribution throughout the year and higher number of sunshine hours. These are the main factors contributing to the much higher oil yields obtained in countries such as Malaysia, Indonesia and Papua New Guinea. Altitude and latitude are more restrictive for oil palm than coconut, because lower temperatures are detrimental to growth and production of the former at an earlier stage. Oil-palm yields become economically unattractive above 400 m altitude near the equator and beyond 10-12° latitudes, while the coconut can still be productive in coastal regions at 20° latitudes and in equatorial uplands up to 1000 m altitude. However, cold-tolerant oil-palm accessions have been collected in the Bamenda Highlands (Cameroon) and above 1000 m altitude near Lake Tanganyika (Tanzania) (Blaak & Sterling, 1996).

Soya bean and groundnut are basically short-day annual crops of the lowland tropics and subtropics. However, both have been successfully adapted by selection and breeding to the summer season of temperate climates or the cooler climates of the tropical highlands. In South-East Asia groundnut grows best in the lowlands and soya bean in the somewhat cooler climate of submontane tropical areas. Sesame, sunflower and cotton are mostly grown further from the equator, e.g. in Burma (Myanmar), northern Thailand and Vietnam. Sesame can be grown under hot and humid conditions, but sunflower requires a relatively cool and dry climate, which is only found at higher altitudes in equatorial regions. Cotton is best cultivated in drier areas under irrigation. The remaining major oil crops listed in Table 1 are not grown to any large extent in South-East Asia, except maize, but this is only grown as a cereal. Rapeseed requires relatively cool climates, such as the winter season in subtropical India and China, or the summer season in temperate climates of China, Europe, Canada and Australia. Safflower is a crop of the semi-arid subtropics, castor and linseed are cultivated in the subtropics and warm temperate regions, while olive needs the seasonal and relatively dry climate typical of the Mediterranean region for satisfactory yields. The wood-oil tree (Vernicia montana) is well adapted to the lower montane climate of the tropics, but it is of limited economic significance in South-East Asia.

Soils

Soil is a dynamic medium comprising disintegrated rock particles, water, air, organic matter and living organisms. Soil formation involves physical, chemical and biological processes that are accelerated by high temperatures and rainfall. Fertility of soils depends on their chemical and structural properties such as acidity, organic matter content, texture and ability to retain nutrients and water. Many tropical soils are strongly weathered due to rapid formation and degradation and are low in plant nutrients. The process of degradation is aggravated by imprudent soil management. Less than one-fifth of the tropical soils have a fair to high level of fertility. These are mainly soils formed on recent alluvial or volcanic sediments (Anonymous, 1995).

The general distribution of major soil groups in the humid tropics of Asia is as follows (Anonymous, 1993; Mohr et al., 1972):

• latosols developed over igneous or sedimentary rocks, including oxysols (4%) and ultisols (35%), which are generally deep and well-drained red to yellowish acidic soils with good physical properties and low to moderate nutrient reserves;
• inceptisols (24%), which are mostly deep, well-drained and fertile sedentary soils of volcanic origin (andepts) or non-volcanic origin (tropepts); some are poorly drained but often highly fertile soils (aquepts);
• entisols (24%), which include young well-drained alluvial soils (fluvents), infertile sandy soils (psamments and spodosols) and shallow soils on steep hillsides (lithics);
• histosols (organic) and other minor soil types (13%).

Oil palm and coconut can be grown on many of these soils, the main requirements being soil depth of at least 1.50 m, free drainage and good water-holding capacity. Soils may vary from sandy loams to heavy clays for oil palm, but coconut grows better on the lighter soil types. Coconut is salt-tolerant, in contrast to oil palm, and thrives well in sandy coastal soils. The best soils for soya bean are well-drained loamy clays, while lighter soils are preferred for groundnut production. Acid soils (pH < 5.5) will reduce the activity of N-fixing bacteria in these legume crops (Weiss, 2000). Sesame, sunflower and cotton can be grown on a wide range of well-drained soils, but only cotton is fairly salt-tolerant. Ranges of preferred soil pH for the 7 oil crops of South-East Asia are indicated in Table 10.

Production systems

Basically, three production systems can be distinguished in oil crops. These are: collection of oil-bearing fruits or seeds from wild and semi-wild palms and forest trees, cultivation in small landholdings and commercial production in large-scale plantations.

Collection

In tropical West Africa and Brazil, the oil palm exists in semi-wild groves as a result of shifting cultivation and spontaneous establishment in cleared areas (Hartley, 1988). The fruit bunches are harvested for the extraction and local consumption of palm oil, while the kernels are sold. The palms are also frequently tapped for palm wine. The fruits, nuts or seeds of several tree species of the humid tropical forests are collected by indigenous people for their oils and fats. For instance the nuts of Shorea spp. (tengkawang) growing in the forests of Kalimantan (Borneo) and other areas of South-East Asia are collected for the edible fat. The seeds of Reutealis trisperma (Philippine tung), which grows naturally in forests of the Philippines, yield a drying oil for various nonfood uses.

Smallholdings

The size of smallholdings in South-East Asia is generally 0.5-4 ha. Capital inputs are usually low, few improved practices are applied and tasks are done manually. Oil palm started as an estate crop in South-East Asia and large-scale plantations dominate in Malaysia and most other countries. However, in Indonesia about 32% of the present area consists of smallholdings, usually organized around a nucleus estate with a large oil mill (Jacquemard & Jannot, 1999). More than 90% of all coconuts in South-East Asia are grown in small landholdings (Ohler, 1999). Coconuts are traditionally planted in garden plots and often grown with fruit trees. The coconut is so common in the region that almost all backyard gardens have one or a few palms, which supply the household with coconut meat, milk and oil. The major annual oil crops of South-East Asia, such as soya bean, groundnut, sesame, cotton and sunflower, are almost entirely grown in small landholdings, usually in rotation with rice and other crops and sometimes in mixed cropping systems.

Commercial plantations

Large-scale production of oil crops such as soya bean, rapeseed, groundnut and sunflower, is common in the Americas and Europe. In South-East Asia only oil palm and to a small extent coconut are produced in large plantations. For oil palm, individual estate size varies from a few hundred to 10 000 ha. Several estates may belong to one private or public company and there are a few very large companies in Indonesia owning more than 150 000 ha of oil-palm plantations (Jacquemard & Jannot, 1999). Costs of establishing an oil-palm plantation together with the necessary infrastructure and oil extraction mill are high. Nevertheless, the return on investment in oil-palm plantations in South-East Asia has been much higher generally during the past decade than for any of the other plantation crops, e.g. cocoa or rubber. This has been a major factor in the tremendous expansion of oil-palm production in the region.

Propagation

The major oil crops of South-East Asia are propagated by seed. In addition to careful cleaning, drying and storage to retain high viability, seeds for planting require additional treatment to break seed dormancy in the case of oil palm, some groundnut types and sunflower.

Seed-nuts of oil palm are subjected to a heat treatment for 60-80 days at about 39°C followed by cooling and rehydration to induce a flush of rapid germination. This is followed by 10-14 months in a polybag nursery to grow the newly germinated seeds into seedlings ready for transplanting into the field (Hartley, 1988). In contrast, mature coconuts start germinating soon after harvesting. Coconuts for seed are usually stored in a cool place for a few weeks before placing them in a germination bed. Well-germinated coconuts are raised in nurseries for 3-8 months to obtain transplantable seedlings (Ohler, 1999).

The dormancy of fresh sunflower and groundnut seeds (Virginia type) can be overcome by exposure to ethylene before sowing, but it disappears naturally after a few months storage. Annual oil crops are sown directly in the field, where seedling emergence will occur within 5-15 days for soya bean, cotton and non-dormant seed of groundnut and sunflower, depending on depth of planting, soil moisture and temperature. Seedling emergence of sesame may be a few days slower (Weiss, 2000).

The development of methods of clonal propagation by in vitro embryogenesis is well advanced in the oil palm, but the persistent and unpredictable problems of abnormal flowering in some clones still remain unsolved (Corley & Stratford, 1998). Similar methods of vegetative multiplication of the coconut are still in the experimental stage (Bourdeix, 1999).

Field preparation and planting

Oil palm and coconut

In the case of new plantations a proper topographical and soil survey to determine suitability for oil-palm or coconut cultivation should be carried out first. This is followed by planning the layout of plantation blocks, roads and sites for oil mill and various buildings. Clearing forest land includes underbrushing, tree felling and controlled burning, lining of plant rows, digging and refilling of planting holes. In non-forest areas, disc ploughing followed by several harrowings can clear the land of strong-growing weeds and other vegetation. When replanting old plantations the stumps and stems of old palms should be totally removed to avoid basal stem rot disease (Ganoderma spp.), as well as infestations of the rhinoceros beetle (Oryctes rhinoceros). Coconut stems provide useful timber. Oil-palm and coconut plantations are usually established on flat or gently undulating land. Where soil permeability is poor, the construction of a drainage system may be necessary. Planting on steep hills will require terracing or construction of individual platforms (Ohler, 1999; Piggot, 1990). A leguminous cover crop is often sown after land preparation or soon after planting to protect the soil from erosion, suppress weeds and add to the soil humus and nitrogen supply. The main cover crop species used are Calopogonium muconoides Desv., Centrosema pubescens Benth. and Pueraria phaseoloides (Roxb.) Benth., often in a mixture. Except in regions with no distinct dry season, the best time for transplanting into the field is at the beginning of the main rainy season. This gives the young palm time to form a good root system before the next dry season arrives. Oil palms are usually planted 9 m apart in a triangular pattern, which gives 143 plants/ha. Tall coconuts are planted at 8-9 m distance and dwarf palms at 7 m spacing in a triangular or square configuration. Such plant densities are a compromise to balance competition for light, water and nutrients of young and mature palms aimed at maximizing economic returns over the total duration of a plantation.

Oil-palm development schemes for smallholders follow similar procedures for field preparation and planting, usually with technical and financial support from the nearby nucleus plantation. Coconut smallholdings are generally not organized in formal schemes, mainly because there is no need for immediate oil extraction and the main products (green coconuts and copra) can be sold directly at the farm gate or on the local market.

Annual oil crops

In South-East Asia the farmers often grow soya bean and groundnut directly after paddy-rice by planting seeds 20-25 cm apart between the rice stubble without tillage, or even broadcast soya bean in the standing rice, and so utilize residual soil moisture. With sufficient rainfall or supplementary irrigation available during the full crop cycle, field preparation prior to sowing will produce higher yields. Cultivation involves ploughing to break up the soil for improved aeration and water infiltration, and harrowing to kill weeds and prepare seedbeds. Similar preparations are adequate for sunflower and cottonseed, but sesame with its smaller seeds requires a finer seedbed. Seeds may be broadcast, but are more often sown by hand in rows spaced at 40-50 cm for soya bean and groundnut, 50-70 cm for sesame and sunflower and 80-100 cm for cotton. Rows on ridges facilitate drainage after heavy rainfall or surface irrigation under dry conditions. Farmers usually sow 2-5 seeds per plant hill to compensate for suboptimal seed quality and if necessary thin out to normal plant densities after seedling emergence. Seed rates for sole-crop smallholder plots are therefore higher than those for mechanized production systems (see Table 14).

In larger commercial farms, land is prepared by tractor-drawn implements and precision seed drills ensure regular plant spacing and correct planting depth. Plant densities for each crop vary with soil and climatic conditions: 250 000-400 000 plants/ha for soya bean and sesame, 80 000-150 000 plants/ha for groundnut and 30 000-60 000 plants/ha for sunflower and cotton (Weiss, 2000).

Intercropping

Oil palm is best grown as a sole crop, because the dense canopy of mature palms allows insufficient light transmission to the undergrowth. Leguminous cover crops established at field planting make place for shade-tolerant plants such as ferns after the canopy has closed some 5-6 years later. Intercropping with annual food crops during the first 2-3 years after planting is possible, but may increase risks of structural and nutritional degradation of the soil. For that reason it is usually discouraged in smallholder schemes. Combinations of oil palm at conventional or wider spacings with cocoa or coffee have been tried on Malaysian plantations without much success in regard to additional economic returns (Hartley, 1988).

Coconut, on the other hand, is commonly intercropped with food and cash crops in coconut-based farming systems, which are more profitable than sole-cropped coconut (Das, 1999; Opio, 1999). The canopy of coconuts lets through 20% of the total solar energy to the crops underneath in 10-year old plantations, and this gradually increases to 50% in 40-year old plantations at conventional spacings. The interplanted crops also do not compete too strongly for water or nutrients with the more deep-rooted coconut palms. Smallholders in Asia intercrop coconuts with food crops, including root and tuber crops, cereals, pulses such as groundnut and soya bean, vegetables, bananas and other fruits. Mature coconuts provide optimum shade for cocoa and coffee, while their stems make good supports for black pepper. Cocoa under coconut is an important combination in Malaysia and Papua New Guinea (Wood, 1989). Multiple-storey cropping systems with cocoa, coffee or black pepper forming the middle storey below the coconut canopy and pineapple or annual crops cultivated at ground level, can be successful under optimum conditions of soil, rainfall and crop management (Nair, 1979, 1983).

Soya bean and groundnut are intercropped or strip-cropped with other crops such as maize, sorghum, cassava, banana, sugar cane and fruit trees. Yields are lower mainly due to shading, but the combination of crops is often more profitable to the smallholder. Groundnut is generally more tolerant of shade than soya bean (Weiss, 2000).

Plant nutrition

Nutrient removal

Estimates of macro-nutrient removal by the harvested products of some major oil crops are presented in Table 11. In the perennial oil palm and coconut large amounts of nutrients are also immobilized in the trunks and crown. More nutrients will also be removed in the annual oil crops, if plant stover is not left on the field and ploughed back into the soil. Oil palm appears to be very efficient in the use of nutrients per t of oil. Soya bean and groundnut require particularly high amounts of N, but that is partly explained by the high protein content of the seeds. In the case of coconut, Cl is absorbed in quantities comparable to a macro-nutrient.

Fertilizer use

For sustainable crop production it is necessary to replenish the nutrients lost during cultivation and removed by the harvested crop. This can be achieved by applying organic and mineral fertilizers, but also by cultivating legumes as an intercrop, in crop rotation or as a cover crop.

Organic fertilizers include farmyard manures and composts prepared from agricultural and domestic waste materials. They supply not only nutrients through mineralization of the organic matter, but also improve the soil structure and its ability to absorb nutrients by increasing organic matter content. Disadvantages are the large quantities needed for effective results and therefore high costs of transportation, as well as the rather unbalanced nutrient composition.

Mineral or inorganic fertilizers are available as single-nutrient or compound (mainly N, P and K) products.

In coconut, N and K are the major nutrients needed and it is the only known crop requiring Cl in quantities comparable to a macronutrient (Ohler, 1999). Generally, N and K are also the most important nutrients for optimum oil-palm growth and production; significant responses to P and Mg are found in some soils (Hartley, 1988). Fairly reliable methods of foliar diagnosis have been developed for oil palm and coconut to determine the nutrient status of the palms. In conjunction with the results of fertilizer trials this provides the means to harmonize fertilizer application with cropping levels and thus to avoid excessive use of mineral fertilizers and their potentially harmful effects to the environment. While the leguminous cover crop provides some nutrients to young oil palms, coconuts may obtain permanent nutritional benefits from the interplanted crops in coconut-based farming systems.

In South-East Asia mineral fertilizers are seldom applied in smallholdings of oil crops in spite of evidence showing that even low rates may increase yields. High costs and poor access to sources of supply are reasons for low utilization in crops like soya bean and groundnut by subsistence farmers. They also tend to give priority to spending available resources on fertilizers for cereal food crop production. Actually, groundnut and soya bean need few additional nutrients when grown after highly fertilized crops. It is estimated also that these legumes obtain about half of their N requirements from symbiotic fixation by Rhizobium bacteria in the root nodules. Soya bean and groundnut respond positively to a dose of 20 kg/ha of N fertilizer at the early stage of growth. Positive yield responses to P, K, Ca and S fertilizers are obtained in soya bean and groundnut depending on soil type. Sesame responds positively to N (higher oil content) and sunflower to K and Ca fertilizers.

Deficiencies in micronutrients are often linked to specific soil conditions and can be rectified by low doses (1-4 kg/ha of the element) applied to the soil or as foliar sprays. Boron is among the most frequently reported deficient micronutrients in oil crops.

Crop protection

Diseases

Oil palm is relatively free from major diseases in South-East Asia (Turner, 1976). Basal stem rot (Ganoderma sp.) is often related to replanting old coconut or oil-palm plantations, and crown disease is a transient physiological disorder in young palms. Some fungal diseases of nursery seedlings can be effectively controlled by fungicides and cultural methods. Plant quarantine measures should prevent the inadvertent introduction of Fusarium wilt and Cercospora leaf spot from Africa and oil-palm diseases from tropical America.

Coconut, on the other hand, is affected by several serious diseases (Ohler, 1999). Most devastating are the yellowing and wilt diseases that are caused by mycoplasm-like organisms and are present in all important coconut areas in South-East Asia. They include Malaysian, Natuna and Socorro wilts. They resemble the disastrous lethal yellowing diseases of tropical America and Africa. No effective control measures exist except that some dwarf coconut types appear more resistant than tall varieties. Cadang-cadang is an important coconut disease in the Philippines and is caused by a viroid. Fungal diseases include basal stem rot (Ganoderma sp.), stem bleeding (Thielaviopsis sp.), leaf blight (Pestalotia sp.) and leaf spot or rot (Drechslera sp.).

The annual oil crops all harbour several diseases. Serious diseases in soya bean and groundnut include fungal and bacterial leaf spots, rusts and virus infections (Weiss, 2000). In groundnut there are also soil-borne problems of bacterial wilt (Pseudomonas solanacearum) and aflatoxin in the seeds caused by Aspergillus flavus infection. The most important diseases in sunflower include Sclerotinia wilt, rust and downy mildew. Sesame is affected by bacterial and fungal leaf spots and so-called phyllody, a disease probably caused by a mycoplasma and important in Burma (Myanmar). Crop rotation helps to reduce disease incidence and there is also host resistance to some of these diseases.

Pests

All oil crops are affected by numerous insect pests. Methods of integrated pest management (IPM) of most oil-palm insect pests were developed in Malaysia (Wood, 1976) and have been adopted generally in South-East Asia. By applying biological methods of control and well-timed applications of narrow-spectrum insecticides in combination with close monitoring it has been possible to prevent major outbreaks of important oil-palm pests and to reduce economic damage to very low levels. Implementation of IPM is effective in oil palm in South-East Asia, particularly the monitoring and biological aspects of control, because it is a plantation crop and even smallholder schemes are usually concentrated in large blocks. The situation is less ideal in coconut, where more than 90% of the palms are on smallholder plots although often concentrated in coastal areas. Nevertheless, relatively simple preventive measures to control important insect pests like the rhinoceros beetle (Oryctes rhinoceros) and weevils (Rhynchophorus spp.) can also be effective in coconut (Mariau, 1999).

In annual oil crops the damage due to leaf-eating insects, aphids (vectors of virus diseases), pod and seed borers and storage insects is often large. Intercropping can significantly reduce infestations of many insect pests (Weiss, 2000). IPM is generally impractical in the scattered small plots of annual oil crops like soya bean and groundnut. Farmers apply insecticides, but do not always wait until certain thresholds of crop damage have been reached, or may not apply the recommended types and rates of insecticides. This can lead to residues from pesticides in the seed. In sunflower insecticides that are toxic to pollinating insects such as bees may affect seed yields.

Other pests include nematodes (in groundnut and soya bean), birds (in sunflower) and rats (in oil palm and coconut).

Annual oil crops

Crops of soya bean, rapeseed, sunflower, groundnut, sesame, safflower and linseed are usually mature within 3-4.5 months after sowing. Season and cultivar (early and late-maturing types) are main factors of variation in the length of the cropping period. In Asia these crops are grown predominantly by smallholders and manual harvesting is the common practice.

Mature whole plants are pulled up or cut, or only the head is removed in the case of sunflower, and taken to a mud or concrete floor for drying in the sun. About 4-6 days later the "seeds" are removed from the plants by threshing with sticks or cattle, winnowed, cleaned and further dried in the sun or sheltered place. Rapeseed and sesame in particular are harvested before full maturity to avoid considerable yield losses caused by early seed shattering in the field. Storage of oilseeds for prolonged periods under humid tropical conditions requires well dried seeds (6-8% moisture content for most oilseeds) and cool and dry conditions to prevent early deterioration. Rapeseed is one of the easiest types of seed to store when properly dried. In the case of groundnut, the pods are picked by hand from the dried plants and seeds are often stored in the shell. Groundnut seeds stored under humid conditions and with a moisture content higher than 10% are prone to aflatoxin contamination. Soya bean seeds are also very difficult to store for longer periods in tropical climates, except in air conditioned warehouses.

Cotton has a growth period of 4.5-6 months including 2 months of hand picking of the opened fruits (seedcotton). The seeds are a by product of the ginning process (removal of the lint) and need further cleaning (removal of the fuzz) and drying to a moisture content of 8% before they can be stored under cool and dry conditions. Annually grown castor is ready for harvesting within 5-6 months after sowing. Whole panicles are cut at 2 week intervals and left to dry on a floor for about a week before seeds are collected, cleaned and stored.

In large scale production systems outside Asia, the major oilseed crops are harvested with grain combine harvesters with some specific adaptations according to the type of crop. This is followed by artificial seed drying and storage in stores or silos with controlled aeration. Uniform plant type and seed ripening are important characteristics of cultivars suitable for mechanized harvesting (Weiss, 2000).

Perennial oil crops

Oil Palm

In contrast to dry oilseeds which can be stored for a considerable period of time before oil extraction, freshly harvested fruit bunches of the oil palm have to be taken to the oil mill as quickly as possible for steam sterilization in order to deactivate the enzyme lipase and kill all micro organisms in the wet fruit mesocarp. This will prevent a rapid rise in free fatty acid content and general degradation of the palm oil. On large oil-palm plantations the oil mill is always strategically positioned and connected to the plantation blocks by a network of roads (or rail tracks) to ensure efficient transport of the fresh fruit bunches. Oil-palm schemes including smallholders generally include a nucleus plantation with an oil mill serving both the plantation as well as these so called outgrowers. The plantation's transport system also collects the crop from the outgrowers at a number of collection points, to ensure timely arrival of all harvested fruit bunches at the mill. Oil-palm harvesting is still a manual operation, with the aid of a chisel or Malayan knife on a long pole, since all mechanical devices developed so far have proven too costly.

After cracking the kernels to remove the shell and drying them to 6% moisture content, they can be stored for a considerable period of time before oil extraction, just like other oilseeds.

Coconut

Coconut is largely a smallholder's crop. Fruit harvesting and all post-harvest handling is performed manually. The nuts are left to dry in a shaded place. Husked and opened nuts are dried in a kiln or hot air dryer, the copra (endosperm) is removed and further dried to 6% moisture content. Well-dried copra can be stored for some time before oil extraction. Humid conditions during storage increase the chances of aflatoxin contamination and general degradation of oil quality.

Olive

Manual harvesting is still predominant in olive orchards. Mechanical harvesting aids include trunk and branch shakers. A further step towards full mechanization is the recent development of self propelled overhead harvesting machines in combination with hedge rowed olive shrubs. Olive and oil-palm fruits are similar in that the high moisture content of the mesocarp is favourable to enzymatic action. Olive fruits should also be processed soon (within 3-4 days) after harvesting to minimize degradation of the oil (Young et al., 1986).

Oil extraction

Modern extraction of oilseeds includes some basic operations similar to traditional methods still applied in some rural areas, such as seed crushing and cooking of the ground seed mass before oil extraction under pressure (Weiss, 2000). However, many innovations in processing machinery (e.g. continuous oil expulsion by screw presses) and extraction technology (solvent extraction in particular) have resulted in much higher extraction efficiency and larger mill output, as well as in cleaner crude oils and almost oil free meals.

Oil extraction is performed by 4 different methods (Young et al., 1986):

• in a single mechanical operation with high pressure expellers,
• by a combination of pre pressing with an expeller followed by solvent extraction of seeds,
• by solvent extraction of seeds,
• by wet expulsion of fruits with a high water content in addition to the oil in the mesocarp, such as those of the oil palm and olive.

The various stages involved in the four methods of processing seeds or fruits into crude oil and meal are depicted in Figure 2.

Oil content is the main factor determining the method of extraction to be applied to seeds (Carr, 1989). Mechanical pressing alone or more often in combination with solvent extraction is generally used for seeds with a high oil content (>40%), including rapeseed, sunflower, groundnut, sesame, safflower and also copra and palm kernels. Direct solvent extraction is more efficient for seeds with a low oil content (< 20%) such as soya bean, but can also be used for high oil containing but permeable raw materials like copra and palm kernels.

Oilseeds

Pretreatment. Seedlots may first require additional cleaning and drying on arrival at the oil mill. Hulling or decorticating involves seed cracking and separation of hulls from kernels. It is applied to most seeds, except the very small ones like safflower and rapeseed, in order to increase oil-extraction efficiency and also reduce crude fibre content in the residual meals. This is followed by grinding to reduce particle size and flaking by rolling, then heating the seed mass in stack cookers (at 12% moisture content and 85-95°C) and subsequent redrying (to 2-3% moisture) before the seed mass is extracted for oil. Cooking serves many purposes: disruption of oil cells, reduction of oil viscosity, coagulation of the proteins and fixation of phospholipids (facilitates separation of oil from the cake and reduces refining costs), general reduction of microbial load, inactivation of enzymes and detoxification. A new development in oilseed processing equipment is the extruder, a mechanical device similar to a screw press (but without choke) that replaces traditional grinding, flaking and cooking operations and produces a seed mass ready for oil extraction in a very short time (Weiss, 2000).

Extraction. The earlier batch type hydraulic presses have been largely replaced by single-shafted or twin shafted screw presses, which allow continuous operation and expel the oil more efficiently. Presses used for single stage mechanical high-pressure expulsion and for pre expelling in combination with solvent extraction are similar. They consist of a horizontal wormshaft or auger assembly, which revolves within a tapering steel barrel with slots for drainage of expelled oil and choke gears on each end to regulate pressure and discharge of the cake. In the case of prepressing, the speed and choke are adjusted to allow a faster throughput at much lower pressure. The objective here is not maximum (>90%) oil extraction as in the first method, but to obtain a seed cake of good permeability and reduced oil content (15-20%) for the next stage of solvent extraction (Young et al, 1986). The operation of most solvent extractor equipment is based on percolation of hexane (petroleum derivative; boiling point 65-70°C) through the granulated cake at 40-60°C, producing a miscella of solvent with oil and an extracted cake with only 1% oil content. Direct solvent extraction operates on the same principle, but with extra attention to seed pretreatment (rigorous cleaning and larger particle size). Solvent extraction plants require high standards of operation to minimize the risk of fire and explosion (inflammable mixtures of dust and hexane vapours).

Cleaning. The oil expelled from screw presses requires removal of suspended solids or "foots" by settling and filtration. The miscella (hexane + oil) from the solvent extractor is first filtered to remove foots. The oil is then recovered by evaporating the solvent and finally cleaned of all traces of hexane by steam stripping. The cake is passed through a desolventizer/toaster to remove all hexane and excess moisture.

Cake processing. The cake discharged from the press or solvent extractor is ground into a meal by hammer or revolving disc mills. The meal is often further processed into pellets to avoid excessive dust formation during handling and transport.

Final products. In some instances the quality of crude oil may be good enough for direct use as salad or cooking oil, but generally it will require further refining first. Meals produced by high pressure expulsion have a higher oil content (3-6%) than those produced by solvent extraction (0.5-1%).

Oil palm

The objectives of sterilization of fresh fruit bunches, usually with steam in pressurized horizontal boilers (final temperature reaching 130°C), are as much loosening and softening of the fruits as arresting free fatty acid formation (Hartley, 1988). Fruits are then stripped from the bunches in rotating drums and subsequently digested in steam jacketed vessels (95°C) to break up the mesocarp before oil extraction. Most modern palm-oil mills use twin shafted screw presses. The liquor draining from the presses a mixture of oil (65%), water (25%) and solids (10%) is passed over screens (to remove larger impurities) into settling (clarification) tanks (heated by steam coil) to separate the oil from water and solids (sludge). The oil is further clarified in centrifugal separators, vacuum dried and sent to storage tanks for crude palm oil. Various methods of bio degradation have been developed to convert the de oiled sludge (palm oil mill effluent) into environment friendly products.

The cake extruded from the press consists of fibre with some residual oil and nuts, which are separated by pneumatic, mechanical or hydraulic means. The fibre mass can be used as fuel for the mill boilers. The nuts are dried and size graded before feeding into centrifugal nutcrackers. Separation of the kernels from the broken shells, formerly in a clay bath in which the kernels float and shell bits sink to the bottom of the tank, now takes place in a specially designed hydrocyclone. The kernels are redried before storage. Oil extraction of kernels is similar to that of oilseeds.

Olive

Olive fruits are crushed and mashed into a paste before extraction by repeated mechanical pressing. In contrast to oil palm and most oilseeds, oil extraction takes place without heating (Di Giovacchino, 1997). The oil is separated from the watery mix ("margine") by clarification in a similar way to palm oil, but again without heating. Oil from the first pressing is usually of such good quality that further refining is unnecessary (virgin olive oil). Residual oil in the cake may be further recovered by solvent extraction and used for nonfood purposes. The remaining cake is usually not used as stockfeed but can serve as organic fertilizer.

Small scale oil extraction in rural areas

One of several traditional methods of extraction involves boiling of pounded (macerated) oilseeds, fresh coconut endosperm, or oil-palm fruits in water and skimming off the floating oil. The "ghani" mills of South Asia and parts of South East Asia (e.g. Burma(Myanmar)) express oilseeds by friction in a mortar and pestle device driven by animal or electric power. The extraction efficiency of these methods is low (60-70%) and the remaining cake may still contain 10% oil. The development of hand operated screw and hydraulic presses, but especially the recent introduction of inexpensive small scale expellers, similar in design to the horizontal screw presses of large oil mills and powered by small diesel engines or by electricity, have greatly increased the extraction efficiency of village oil mills, as well as the quality of edible oils. Solvent extraction is seldom used in small scale operations on account of costs of equipment and also the safety risks involved (Weiss, 2000).

Refining

A large proportion of freshly extracted oils from oilseeds is consumed directly without further refining by the local populations of Asia as is crude palm oil in West Africa. However, crude vegetable oils contain impurities dirt, meal fines, water, phosphatides (phospholipids), free fatty acids, partial glycerides, waxes, oxidation products, pigments (carotenoids and chlorophyll) and traces of metals of which some cause progressive degradation of quality over time and also interfere with manufacturing processes (Stauffer, 1996; Young et al., 1986). For those reasons, industrially processed vegetable oils are subjected to further refining to obtain so called RBD (refined, bleached, deodorized) quality oils. The main operational steps of the refining process, the methods applied and impurities removed successively are shown in Figure 3. Degumming is the separation of phosphatides from the oil by adding hot water or steam and sometimes also phosphoric acid, followed by centrifugation. It improves the results of the next step in the refining process, i.e. neutralization or deacidification, and is particularly important for solvent extracted oils, e.g. of soya bean and rapeseed, which contain up to 2-3% phosphatides. Treatment of the oil with a dilute caustic soda solution will convert the free fatty acids into soap, which is removed by repeated centrifugal action and washing with hot water. The refined oil is then vacuum dried before the next step of bleaching to remove pigments, oxidation products and other remaining impurities by adsorption to activated bentonite clay and filtration.

The oils of sunflower, sesame, safflower and maize contain relatively large amounts of waxes, which cause clouding at low temperatures. Before bleaching, these oils are therefore cooled to 5°C, mixed with water, and allowed to stand for several hours before the aqueous wax suspension is removed by centrifugation (Stauffer, 1996).

Finally, the oil is deodorized by injecting live steam through hot (200-275°C) oil to remove undesirable odours and flavours. The result after cooling and final filtration is refined vegetable oil of RBD grade, containing about 0.02% free fatty acids, with a zero peroxide value and a very pale yellow colour.

In the case of palm and lauric (coconut and palm kernel) oils, which generally contain more (2-5%) free fatty acids but are low in phosphatides, physical refining with steam can be used to distill off all free fatty acids instead of neutralizing these chemically by alkali refining. This method reduces the number of steps in the refining process and the loss of oil in the by products (Carr, 1989; Young et al., 1986).

Modification

Refined oils are often processed further to modify their chemical and physical properties and so increase their usefulness in different food products. Such processes include hydrogenation, interesterification and fractionation. Hydrogenation or hardening increases oxidative stability and converts liquid oils to semi solid plastic fats for use in margarines, shortenings and other fats. The hydrogenation of oils is usually applied during the refining process before the final stage of deodorization (Carr, 1986). Interesterification also changes melting and crystallization characteristics by rearranging the positions of fatty acids in the triglycerides, but without producing trans fatty acids as can happen during hydrogenation. It finds application in producing a so called "non-trans" dietary margarine from soya bean and other oils high in polyunsaturated fatty acids. It also improves plasticity and creaming properties of shortenings (Stauffer, 1996).

Fractionation or winterization is a process by which different fractions are separated by slow cooling of heated (70-75°C) RBD grade oils down to 6-10°C (Young et al., 1986). This induces crystallization of triglycerides with high melting points, which can then be removed by filtration or centrifugation from the liquid fraction. For example, fractionation of palm oil yields palm stearin and olein. The solid fractions can be used as component of margarines, shortening and frying fats and the olein fraction as salad oils with low cloud point, i.e. no precipitation at refrigerator temperatures.

Genetic resources

Plant breeding tends to narrow the genetic variation of a crop and often a few excellent cultivars dominate in technology-based agriculture to the exclusion of all else (Simmonds, 1981). Continued progress in crop improvement, which also takes into account future changes in environmental, agronomic and socio-economic demands, requires easy access to adequate sources of genetic variability. The collection, conservation and characterization of germplasm comprising wild and domesticated plant types, landraces and old cultivars of hundreds of crop species has developed over the past 50 years into a highly specialized activity in so-called genebanks established by national and international agricultural research organizations (FAO, 1996). The management strategies of such ex situ plant genetic resources are evolving from mere collection and maintenance of individual accessions to optimization of genetic diversity and to active exploitation of the genetic potential for crop improvement. These latter aspects are enhanced by the application of biometrical and molecular genetics (van Hintum, 2000). The International Plant Genetic Resources Institute (IPGRI) has the mandate of promoting agricultural biodiversity in general and of coordinating global genebank activities with emphasis on plant genetic resources in developing countries (IPGRI, 1999).

An overview of genebanks with large germplasm collections of major vegetable oil crops is presented in Table 12. Germplasm of tree crops such as oil palm and coconut cannot be stored for long periods as "seeds" and genebanks consist of large field collections. This also applies to the olive, which is mostly vegetatively propagated. Germplasm of the annual oil crops is effectively conserved as stored seed.

Oil-palm genebanks of NIFOR in Nigeria and MPOB in Malaysia consist of large field collections with 1000 and 1700 accessions respectively, which were collected from the centre of high genetic diversity in south-east Nigeria and other sites in Africa in the period 1956-1994 (Rajanaidu et al., 2000). Smaller collections are also maintained in other oil-palm research centres in the tropics. The Coconut Genetic Resources Network (COGENT) and IPGRI coordinate the conservation of more than 700 accessions present in field collections in the Philippines, Indonesia, India and several other countries (Bourdeix, 1999). Progress has been made with cryopreservation of oil-palm and coconut embryos and pollen (Assy-Bah & Engelmann, 1992; Rohani et al, 2000). This may provide a safe and less expensive alternative for long-term storage of genetic resources to field genebanks, which require considerable resources of land, staff and upkeep and remain vulnerable to losses by natural disasters and diseases.

The International Olive Oil Council (IOOC) supports the collection and conservation of wild olive germplasm and landraces in field genebanks in Spain (OWC, Cordoba) and other countries. Genetic resources of tung-oil trees and wood-oil trees appear to be limited to the maintenance of small work collections by NARS in China, the United States and Malawi.

For soya bean, groundnut and maize large collections of more than 13 000 accessions each are available in the genebanks of the relevant international agricultural research centres: AVRDC (Taiwan), ICRISAT (India), CIMMYT (Mexico) and IITA (Nigeria). In the United States there are major germplasm collections for soya bean (INTSOY), sunflower (NPIS), cottonseed (NSSL), safflower (USDA) and linseed (USDA). China maintains large collections of soya bean (CAAS) and castor (ICGR & IOCR) and Russia of sunflower (VIR & VNIIMK), linseed (VIR) and castor (VIR). The NBPGR in India has the principal germplasm repository for sesame. Rapeseed germplasm is distributed over several genebanks of national agricultural research systems (NARS) in Europe, North America, China and India.

Collection of germplasm for all oil crops mentioned in Table 2 except rice is of minor importance or non-existent except for jojoba (United States) and niger seed (Ethiopia, India).

Breeding

Breeding and genetics have contributed considerably to the establishment and improvement of oil crops (Knowles, 1989a). Traditionally, most oil-crop breeding has been in the domain of public agencies, but in America and Europe many private companies are now also involved in oil-crop breeding and seed production. Private sector activities in Asia include rapeseed, sunflower and cottonseed in India and oil palm in Malaysia, Indonesia and Papua New Guinea.

The general objective of plant breeding is the development of crop cultivars with the potential of providing maximum economic benefits to the growers. This usually requires simultaneous selection for plant type and vigour, ecological adaptation, yield, quality and other characters. Disease and pest resistances may assume the highest priority in breeding, especially when these have become a threat to the profitability or even survival of the crop. Selection progress depends on the breeding plan applied, which in turn is to a large extent influenced by the species’ life cycle (annual or perennial), and mating system (self-pollinating or cross-pollinating) (Simmonds, 1981). The life cycle and mating system of major oil crops is indicated in Table 13.

Perennial oil crops

Practically all woody perennials are cross-pollinators (Simmonds, 1981), including oil palm, coconut and olive. Breeding plans in the oil palm are comparable to those applied in maize and sunflower, including methods of reciprocal recurrent selection within genetically divergent subpopulations and F₁ hybrid seeds as final result. The floral biology of the oil palm (monoecious, with female and male flowers on separate inflorescences) and large multiplication factor (>5000) enable efficient and economically viable mass production of seeds, even though it depends entirely on hand pollination. The discovery around 1940, that pure stands of palms yielding the preferred thin-shelled Tenera fruits can only be obtained by crossing Dura with Pisifera palms, has been another strong impetus to the exclusive use of F₁ hybrid seed in all oil-palm plantings. On the other hand, in coconut about 95% of all plantings are still open-pollinated progenies after mass selection, partly due to longer breeding cycles (>10 years) and because this monoecious palm carries only a few (20-60) female flowers (and large numbers of male flowers) on each inflorescence. Consequently, the multiplication factor is low (50-100) and emasculation is required as an additional operation of seed production. The considerable hybrid vigour exhibited by hybrids between certain Tall and Dwarf populations has encouraged the initiation of advanced breeding programmes based on reciprocal recurrent selection also in the coconut. These have already lead to F₁ hybrids yielding 30-100% more than the best open-pollinated selections and so justifying the additional efforts and costs of F₁ hybrid seed production. The olive represents a classic example of fruit-tree breeding, in which cultivars are clones from phenotypically or genotypically selected trees. Seed germination is usually poor and olive seedlings have a long (4-9 years) juvenile phase.

Breeding objectives include:

• For oil palm: maximizing oil yields (per palm and per ha) and more recently quality (change in fatty acid composition) of palm oil (Gascon et al, 1989; Hartley, 1988). Disease resistance (Fusarium wilt in Africa, sudden death in South America, crown disease and Ganoderma in South-East Asia) is sometimes locally given high priority.
• For coconut: diseases, including those threatening the survival of the whole crop in certain regions (e.g. the various wilts and yellowing diseases), are a very important factor, but unfortunately breeding has not been able to provide answers in most cases, mainly because of lack of genetic resources with adequate host resistance (Bourdeix, 1999; Satyabalan, 1989).
• For olive: cold tolerance, early bearing, yield, regular production and some efforts to disease resistance (Brousse, 1989; Lavee, 1990).

Annual self-pollinating oil crops

Soya bean and groundnut are strictly self-pollinating species with very little natural outcrossing and therefore requiring painstaking pollination methods to achieve cross-breeding. In other self-pollinating oil crops like cotton, sesame, safflower and linseed, natural cross-pollination may be higher, 1-10%, and in rapeseed (B. napus and B. juncea) even up to 30%. Selfing in these species will therefore require isolation of the flowers by bagging during anthesis. The breeding process includes line and pedigree selection after crossing and backcrossing, all leading to homozygous and uniform cultivars. These are true to type and can be multiplied in seed blocks with simple precautions such as guard-rows and minimum distances (specific for each crop) to avoid illegitimate outcrossing. F1 hybrid cultivars with considerable hybrid vigour for yield have been successfully developed during the past two decades for cotton and B. napus rapeseed. In cotton such F1 hybrids find application in South Asia and China, where seed production is economically feasible because of low labour costs for emasculation and hand pollination (Hau et al., 1997).

The following are examples of crop specific breeding objectives, in addition to the general aims of higher yields and oil content:

• For soya bean: less sensitivity to photoperiod and temperature, early maturing, disease resistance (rust, downy mildew, anthracnose, bacterial pustules and blight, viruses), improved oil quality (fatty acid composition) and elimination of beany flavour (Fehr, 1989; Weiss, 2000).
• For groundnut: earliness and drought tolerance, reduced seed dormancy, disease resistance, e.g. rusts, leaf spots, bacterial wilt and aflatoxin-producing Aspergillus flavus (Clavel & Gautreau, 1997; Coffelt, 1989).
• For cotton: earliness, disease resistance (bacterial blight and Fusarium wilt) and low gossipol content by glandless plants (Hau et al., 1997; Kohel, 1989).
• For sesame: compact plants, earliness, disease resistances (little progress so far) and non-shattering (Ashri, 1989; Weiss, 2000).
• For safflower: spineless plants, early maturity, thin seed hulls, resistance to foliar diseases in particular (Knowles, 1989b; Weiss, 2000).

Annual cross-pollinating oil crops

Open-pollinated, composite (mixture of improved selections) or synthetic (mixture of inbred lines) cultivars of sunflower, maize and castor are still widely used in countries with predominantly smallholder and subsistence farming systems. Breeding procedures, which involve recurrent mass or family selection, are relatively simple and seed can be produced in well-isolated seed blocks at fairly low costs. Elsewhere, F₁ hybrid cultivars completely determine the production of these three crops because of the much higher economic returns obtained as a result of hybrid vigour and plant uniformity. Breeding procedures are complex, requiring a number of measures including the development of genetically diverse subpopulations (e.g. by reciprocal recurrent selection). These form the source of inbred lines which, after confirmation of good combining ability for all desired agronomic and physiological characteristics, are selected as parents of F₁ hybrids. Large-scale seed production is based on monoecious flowering (requiring mechanical or manual removal of male flowers on the female parent line), as is the case for maize and castor, or on cytoplasmic male sterility (CMS) of the female parent, as in sunflower and also recently developed castor hybrid cultivars. The situation is somewhat different in rapeseed. Considerable hybrid vigour for yield is present in B. rapa and also in the self-pollinating B. napus, but only in recent years have F₁ hybrid cultivars started to replace the open-pollinated varieties, particularly in North America and Australia. Seed production of F₁ rapeseed hybrids is based on self-incompatibility and CMS (Banga, 1998).

Crop-specific breeding objectives include:

• For sunflower: reduced plant height, oil quality (high oleic acid content), and resistance to lodging, to diseases (e.g. downy mildew and rust), to broomrape and birds (Fick, 1989; Vear, 1992).
• For rapeseed: triple-zero (no erucic acid, low in glucosinolates and linolenic acid) cultivars, day-neutral B. napus cultivars suitable for the subtropical winters or tropical highlands, herbicide tolerance, resistance to diseases (e.g. black leg, leaf spot and black rot), non-shattering and resistance to lodging (Downey & Röbbelen, 1989; Renard et al., 1992).
• For castor: short plants, early maturing, indehiscent and thin-walled capsules (non-shattering) and disease resistance (Atsmon, 1989; Weiss, 2000).

Molecular breeding

Plant biotechnology has evolved, particularly during the past decade, into an applied science providing powerful additional tools for plant breeding with the potential of increasing selection efficiency and creating new approaches to hitherto unattainable objectives. Molecular breeding has basically two main applications of plant biotechnology: molecular markers and transgenic plants. Molecular marker technology is applied, also in many major oil crops, for germplasm characterization and management, detecting genetically divergent breeding subpopulations (e.g. to predict hybrid vigour), accelerating gene introgression from related species and for MAS (molecular marker-assisted selection). MAS enables early selection of important major genes (e.g. disease resistance or oil quality) with molecular marker(s) closely linked to the gene controlling the trait. In the case of polygenic traits (e.g. components of yield) a more complex QTL (quantitative trait loci) analysis is required for the identification of significantly linked markers. A prerequisite to such a QTL analysis is the availability of a saturated genetic linkage map (Mohan et al, 1997).

Generally, successful genetic transformation is still limited to characters controlled by major genes for which gene isolation and transfer is relatively easy. It also requires the possibility of routine application of transformation technologies and regeneration of plants from in-vitro explants or cell cultures. These are well established in crops like soya bean, rapeseed, cotton, maize and oil palm but not yet in sunflower and coconut. Tolerance to glyphosate or glufosinate herbicides and insect resistance based on the Bt gene (derived from Bacillus thuringiensis) are the main characters so far successfully expressed and commercialized. The global area of transgenic crops in 2000 (James, 2001) was 44.2 million ha (compared to 1.7 million ha in 1996), of which more than 80% is grown in the United States and Canada and the rest in Europe, South America, China, South Africa and Australia, while so far none are grown in South-East Asia. Transgenic soya bean has the lead with 25.8 million ha, followed by transgenic maize (10.3 million ha), transgenic cotton (5.3 million ha) and rapeseed/canola (2.8 million ha). Other interesting projects concerning genetic transformation in oil crops, such as changing the fatty acid composition of the oil by manipulating oil-synthesis pathways, have just started to be commercialized for rapeseed in the United States (Downey & Taylor, 1996), but are still in the research phase for the oil palm in Malaysia (Corley & Stratford, 1998).

The prevailing negative public perception of transgenic crop plants in several industrialized countries (Europe in particular) as well as developing countries can be a temporary obstacle to the introduction and unrestricted cultivation of transgenic cultivars of important oil crops in South East Asia. One could add to this also the lack of adequate legislation for proprietary rights and biosafety in some countries.

A seed supply system involves a complex chain of activities and processes from "gene to marketable seed". Plant breeding and release of new cultivars is followed by the multiplication of seed over several generations (from breeders to foundation and certified seed), seed quality control, processing and conditioning, storage and transport, demand assessment, marketing and distribution (Jaffee & Srivastava, 1992). The extent of recurrent demands for fresh seed of improved cultivars of annual oilseed crops depends on factors such as ease of on-farm reproduction and cost of seed in relation to crop revenue.

Annual oil crops

Soya bean and groundnut cultivars are pure lines easily reproduced on-farm a number of times before genetic (mixing with other cultivars) and physiological (seed-borne diseases and pests) deterioration of seed quality induces the grower to purchase fresh seed of certified quality. Rapid loss of seed viability under hot and humid conditions can also be a factor stimulating more frequent demands for fresh seeds of soya bean and groundnut cultivars. Seeds of other self-pollinating crops such as rapeseed (B. napus and B. juncea) and sesame can be stored for longer periods of time, but higher natural outcrossing may accelerate genetic decline in farm-saved seed and so necessitate more frequent purchases of fresh seed. In open-pollinated, composite or synthetic cultivars of cross-pollinating crops such as sunflower and B. rapa rapeseed the genetic decline in farm-saved seed is even faster because of the mating system and segregation. A seed replacement rate of 20% (i.e. fresh seed of certified quality purchased once in every 5 years) is considered adequate for most self-pollinated oilseed crops and 30-50% for cross-pollinators to maintain cultivar identity and optimum crop production. Most F1 hybrid cultivars require the purchase of fresh seed for each new crop to avoid a dramatic genetic decline in production already in the first generation of farm-saved seed.

The market value of seed of annual oilseed crops also depends very much on seed rates and multiplication factors (Table 14). The seeds of improved soya bean and groundnut cultivars have therefore a lower market value (about 1.5 times the value of the oilseed crop) compared to open-pollinated seed of sunflower or rapeseed (4-6 times the crop). The seed price of F1 hybrid cultivars is often 12-18 times that of the oilseed crop (Louwaars & van Marrewijk, 1996). In Europe and America the seed supply systems are almost completely privatized. Plant variety protection (plant breeders' rights) and the strong demand by the growers for high quality seed enables seed companies to recover a reasonable profit margin also on low-value seeds of self-pollinated crops. Private and public sectors collaborate closely in creating a legal and regulatory environment favourable to the development of a dynamic private seed industry (van der Vossen, 1996). Seed supply systems in South and South-East Asia vary considerably in the degree of development and private sector involvement, but they all have in common that state seed corporations are the main suppliers of improved seed of self-pollinated crops such as rice, wheat, pulses and oilseeds. These are strategically important food crops produced almost entirely by small and resource-poor farmers from on-farm saved or informally produced seed. The improved seed supplied by the public sector plays an essential role in periodically refreshing farmers' seed stocks of popular cultivars and also in facilitating the introduction of new improved cultivars. However, seed production targets to meet optimum seed replacement rates (10-20%) are seldom achieved, except for rice in Indonesia and the Philippines. Public seed system projects have been inclined to give priority to cereals and the quantities of improved seed of pulses and oilseeds are often not more than 1-5% of total annual seed requirement. The seed market for self-pollinated crops remains unattractive to the private seed industry because of low profit margins and insecurity with regard to plant variety protection (van der Vossen, 1997).

Perennial oil crops

The annual demand for oil-palm seed of the "D x P" hybrid type is 120-140 million "seeds" (Anonymous, 1996). More than 85% of this quantity is used for new planting and replanting of old oil-palm fields in South-East Asia, the rest in West Africa and Central and South America. These seeds are supplied by more than 25 specialized seed production centres of large private oil-palm companies or public research institutes in Malaysia (40-50 million), Indonesia (35-60 million), Papua New Guinea (10-12 million), Costa Rica (15-25 million), Nigeria (3 million), Ivory Coast (2 million) and 1-2 million seeds each in Benin, Ghana, Democratic Republic Congo, Cameroon and Colombia. An additional service often provided by these centres includes pregermination of the seed by heat treatment before distribution. The price of oil-palm seeds is not related to crop revenue but rather based on production costs plus a reasonable profit margin. There is considerable variation in seed quality, but some suppliers have a reputation for consistently producing seed of very high quality (e.g. producing almost 100% Tenera palms). Hybrid seed production of coconut is comparable to that of oil palm, except that the low multiplication factor and large seed (fruit) size are serious impediments to mass production and distribution. Coconut "seed" production is mainly a public sector responsibility, e.g. in India, Indonesia, the Philippines and Ivory Coast. An estimated 15% of new coconut plantings over the past decade have been raised from hybrid seeds (Bourdeix, 1999).

Demand and supply

World demand for oils and fats is expected to grow steadily from 117 million t in 2000 (see Table 4) to at least 175 million t (84% of vegetable origin) in 2020 (Mielke & Mielke, 1999). Major determining factors are the ever-increasing world population, by an estimated 1.5. billion to 7.5 billion, and a rise in per-capita consumption of oils and fats as a result of individual income growth in several countries. In developing countries edible oils and fats are essentially used for food, and average consumption per person will still be low to moderate (14-21 kg/year). However, the dietary intake of oils and fats in most industrialized countries has reached saturation levels (42-52 kg/year/person) and observed growth in consumption largely concerns nonfood applications.

The major vegetable oils are commodities with considerable possibilities for mutual substitution, and the future share of each in the total supply will be determined by available land resources, oil yield per ha (highest for the oil palm) and production costs per t of oil (lowest for palm and kernel oils). Growth in production of annual oilseed crops may slow down in some countries as a result of scarcity of arable land and water resources, competition with cereal production, or abolition of agricultural subsidies (e.g. rapeseed and sunflower in the European Union). Availability of soya bean oil depends primarily on the demand for the economically more valuable soya bean meal for livestock production, which is expected to develop more slowly than the demand for oils and fats. On the other hand, palm-oil production will continue to expand considerably, particularly in South-East Asia, and reach an estimated 40 million t per year in 2020. By that time palm-oil supplies will be capable of providing 27% of the total annual demand for vegetable oils (148 million t) against an estimated 25% for soya bean oil, 15% for rapeseed oil, 11% for sunflower oil, 4% each for groundnut and cottonseed oils, 3% for maize oil and 2% for olive oil. Lauric oil production will increase to about 10 million t (7%) over the same period, with equal contributions from copra and palm kernels. The remaining 2% of the supply includes sesame, linseed, safflower and castor oils.

The estimated share of palm oil in the international trade of oils and fats in 2020 (65 million t) will be 45%, of soya bean oil 22%, and 6% each for brassica oilseeds and sunflower oils. Exports of palm, palm-kernel and coconut oils from Malaysia and Indonesia combined may then account for 60% of world trade. About 13 of the 22 main producing countries are net importers of oils and fats, with China, India and the European Union being the largest (Mielke & Mielke, 1999).

With such a dominant share in the world trade, it is not surprising that large stocks of palm oil following years of record production can have a depressing effect on prices, as was the case in the years 2000-2001 (<US$ 230 per t of crude palm oil). The production of perennial crops like the oil palm cannot be adjusted within a short span of time, but annual oilseeds (e.g. rapeseed and sunflower) usually respond quickly with a decrease in sown area because production becomes uneconomic at such low market prices. Old stocks disappear and the balance between demand and supply is re-established in subsequent years with concurrent recovery in price levels (>US$ 400 per t). In other words, prices for vegetable oils will continue to fluctuate with recurrent dips due to overproduction, but each time followed by a fairly quick recovery and a generally upward trend over the longer term (Mielke & Mielke, 1999, 2001).

The demand for oil meals will increase to some 290 million t per year by 2020 and soya bean will continue to provide 55% of total demand (Mielke & Mielke, 1999). Rapeseed meal (11% of total demand) will show the largest growth in China and India, while Russia, Ukraine and Argentina will mainly produce sunflower meal (7% of total demand). Groundnut meal will show declining growth, palm-kernel meal a sharp increase in line with palm oil production and copra meal production will not show much growth. The largest net exporters of oil meals are the United States, Argentina and Brazil; the largest importers the European Union and China.

Research

In the plantation crop oil palm, maximum economic returns remain the highest priority. The oil palm outstrips all other oil crops as the highest and most efficient producer of vegetable oil. In South-East Asia oil yields of 7 t/ha have been obtained recently under optimum conditions of climate, soil and crop management. Nevertheless, combined efforts in plant breeding, crop physiological and agronomic research are expected to result in annual oil yields of more than 10 t/ha in the medium-long term. It is necessary to bear in mind, however, that these yields can only be sustained by sound agronomic practices including soil conservation methods and systematic fertilizer applications. Nevertheless, such production levels should provide opportunities for meeting increasing demands for palm oil from existing areas under oil-palm cultivation after they have been replanted with higher yielding cultivars. Further large-scale conversion of natural forests into plantations, as is taking place in some South-East Asian countries, would then no longer seem necessary.

Coconut does not appear to have such a bright future as an oil crop. In the world market it already faces increasing competition from palm-kernel oil and in the longer term possibly also from rapeseed that is genetically modified to produce lauric oil. On the other hand, as a smallholder crop in the coastal areas of the tropics, coconut will continue to be a very important supplier of multifunctional food and other products to the local communities. Sometimes, it is practically the only tree crop that can be grown in the prevailing ecosystem. A quickly growing world market for healthy and environment-friendly products should also offer new opportunities for the coconut export trade. This will require more research to develop viable coconut-based farming systems and novel processing technologies for local industries to manufacture diversified coconut products. Growing networks of internationally-supported coconut research on the application of biotechnology offer prospects of eventually solving some of the major constraints in coconut production, devastating yellowing and wilt diseases in particular.

Soya bean and groundnut in South-East Asia are considered more as food crops rich in proteins and fats that are grown with other crops simultaneously or in rotation. Research is, therefore, focused on higher yields and crop security within the context of existing or new cropping systems. Early maturity, resistance to biotic and abiotic stress factors and greater capacity for symbiotic nitrogen fixation will continue to be important criteria in selection and research programmes in progress in Indonesia, Thailand and other South-East Asian countries.

Progress in changing the fatty acid composition of seed oils by genetic modification is considerable. The transgenic rapeseed producing lauric oil has shown that metabolic engineering of plant storage lipids is possible without affecting the physiology and agricultural performance of the crop (Murphy, 1994). Before long a range of genetically modified cultivars of major (annual) oil crops will be available that produce custom-made oils for nutritional, industrial and pharmaceutical purposes (Vageesbabu & Chopra, 1999; White & Benning, 2001). However, production costs will ultimately define their market potential. For instance, coconut and palm-kernel oils are likely to remain cheaper alternatives to lauric oil from rapeseed, unless the latter were to have technological advantages or be protected by trade barriers. On the other hand, the application of genetic modification techniques may make it possible to produce certain specific oils for industrial and pharmaceutical applications, that are now only present in wild or underutilized plants, at required quantities and at lower costs.