1.2.3 Soyabean

Although not suited for small-scale extraction of the oil, the soyabean has, since 1945, become the most important source of both vegetable oil and protein and is therefore briefly characterized.

Figure 4: Soyabean.

Source: S. Rehm, G. Espig, 1984, p. 94

The soyabean or soyabean, Glycine max, is a member of the Papilonaceae, which includes some forty species of frequently twinning shrubs, distributed generally in the Asia and Australasia region. It is considered as having its origin in northeastern China, although the genus has two major centres. One is in eastern Africa, the second in the Australasian region with a secondary centre in China. From China soyabean spread to the neighbouring countries Korea, Japan and South-East Asia and finally around the world. As a cultivated crop it remained basically confined to Asia until the beginning of the century, when the USA developed soyabean into a major commercial crop.

The wet subtropics provide the best climate for the soyabean with average annual temperatures of around 25° C and optimal rainfall of 500 to 750 mm per year. The plant is extremely photoperiodic, with most varieties only flowering with day-light less than 14 hours a day. Day- light periods shorter than 12 hours lead to dwarf growth and reduced yields. All varieties are adapted to specific conditions. Cultivation of certain varieties is limited to particular geographic latitudes.

As shown in Table 1, the world production of soyabean has more than doubled in the last decade and a half and currently stands at over 100 million tons per year. The USA is the world's largest exporter, and together with China and Brazil, accounts for over 90 % of the world production (Africa: 0.2 %). Highest domestic consumption is in Asia, where it has been a basic food for centuries. Soyabean is mainly cultivated for its seeds commercially used for human consumption, stock food, and the extraction of oil. It is presently the world's most important grain legume in terms of total production and international trade.

The fruit is normally a short hairy pod, which can vary from 2 to 10 cm in length and 2 to 4 cm in width according to variety. The number of pods per plant can vary from a few dozen to several hundred, depending mainly on climatic conditions. They usually contain three, occasionally more, small, hard or ovoid seeds, usually between 5 and 10 mm in diameter. Seed weight varies considerably in the range of 5 to 40 g per one hundred seeds. The oil content of the seeds, as the major characteristic for the purpose of this booklet, varies between I 5 and 22 %, which is, at any event not sufficient for efficient extraction with small-scale technology.

Harvesting can be done by all methods from entirely manual to fully mechanized, depending on cost factors in a concrete context. In regions where labour is plentiful, plants are pulled by hand, thrown into heaps and threshed with sticks. In general, wind-rowing is not to be recommended, except in those circumstances where field conditions preclude natural drying, for instance in some West African areas.

Yields obtained range from 0.5 tons per ha in West and East Africa, 1.0 tons/ha in Central and southern Africa, 1.5 to 2 tons/ha in East Europe and most of Asia to an average in the USA of 2.5 tons/ha. These relatively low average yields should be judged against yields of nearly 6 tons/ha achieved by commercial growers in the USA, which also indicate the huge potential for increased production in tropical countries.

As mentioned earlier, the cultivation of soyabeans would never be economic without the double potential for vegetable oil and the protein-containing meal which accounts for 40% of the production value. Due to the low oil content, the modern process of solvent extraction is usually applied which, in turn, is only relevant for large- scale industrial operations.

Soya oil normally contains 10 % linolenic, 55 % oleic and 30 % linoleic acid with up to 50 % variation in a specific component. Without going into details, one might say that these components make the oil without further processing rather poor and unstable in flavour for direct human consumption. As an industrial rawmaterial, it is mainly used for the production of margarine.

Soyabeans contain a toxic factor which blocks the activity of the digestive enzyme trypsin. Before feeding whole seeds to pigs or poultry, this trypsin inhibitor should be destroyed by heating. Since soyabeans are normally heat treated during processing, oil cake is generally inhibitorfree.