Effects of roasting and cooking on the lipid composition of raw groundnut ( Arachis hypogaea ) seeds : dietary implications

Introduction The cultivated species of the groundnut plant, Arachis hypogaea, is an annual herb belonging to the Papilionaceae division of the family Leguminoseae. It is a spreading, sunloving plant, widely grown in tropical, sub-tropical, Mediterranean and warm temperate climates. Groundnut is also known as earth-nut, peanut-monkey-nut, Manilla-nut, Chinese nut, goober pea, pindar, pistache de terre, larachide. In various Nigerian vernaculars: epa (Yoruba); gya’da (Hausa); okpakpa (Ibo) 1 . The groundnut is a legume with seed rich in oil (48-49 %) and protein (about 26 %). Groundnuts are the main export crop in Nigeria’s northern states and the bulk of the crop is produced north of latitude 11 o N. The crop does best in light sandy soil, which allows good drainage and ease of harvesting; 500 mm of rain can be sufficient if well distributed. Production in excess of village market sales has varied from less than 100 000 tons in an unfavourable year to 1 000 000 tons of shelled groundnuts in a favourable year. Nearly all this commercial crop is processed in local oil mills and the oil mills and the oil and cake sold at home and abroad 2 . The groundnut is widely consumed in Nigeria and in various other parts of West Africa. Its nutty, agreeable flavour makes it a particularly suitable vegetable food when roasted. It is often cooked and eaten mixed with maize. Its high protein content makes it especially valuable as a tissue builder. The groundnut is one of the most important sources of edible vegetable oils and it is cultivated principally for this purpose. The oil comprises about 50 % of the total kernel. The content of the oil has been stated to depend on the richness of the soil and the conditions of cultivation rather than on the variety grown 1 , the percentage being greater in nuts grown in warm than in temperate climates. It is non-drying, edible oil, liquid at ordinary temperature, golden yellow in colour. About 82 % of the oil consists of the low-melting point glycerides of oleic and linoleic acids, while the remaining 18 % is made up of palmitic, stearic, arachidic and linoceric acids. The oil has an iodine number of 86.8. According to Hilditch and Williams 3 , Nigerian and West African groundnut oils have the following mean glyceride components, fatty acids per cent of weight, Nigerian (West African): oleic, 61.8 (59.1); linoleic, 19.7 (20.6); palmitic, 8.6 (9.2); stearic, 3.7 (5.5); arachidic, behenic, lignoceric, 6.3 (5.6). The oil is thus of a high nutritive value readily digestible and, except for its lower vitamin content, has often been ranked as nutritionally the equal of butter fat. In Nigeria the oil is next only to palm oil as culinary and edible oil being regarded in the South as a relish reserved for cooking on more important occasions instead of palm oil which is cheaper. Groundnut oil is, however, more commonly used than palm oil in Northern Nigeria. It is also used as an illuminant oil and like palm kernel oils, as a basis for pomade. Abroad, groundnut oil is used principally as edible oil especially as salad oil in place of olive oil. Considerable quantities of it are hydrogenated to produce a hardened fat which is employed in the manufacture of margarine. It is used as a lubricant and for oiling wool. Lower quality oils are employed in soap making. Groundnut oil is one of the official oils in the British Pharmacopoeia. It has been successfully used as a fuel in diesel engines 1 .