The oil and protein content together account for about 60% of dry soybeans by weight; protein at 40% and oil at 20%. The remainder consists of 35% carbohydrate and about 5% ash. Soybean cultivars comprise approximately 8% seed coat or hull, 90% cotyledons and 2% hypocotyl axis or germ.
The majority of soy protein is a relatively heat-stable storage protein. This heat stability enables Soya food products requiring high temperature cooking, such as tofu, soymilk and textured vegetable protein (Soya flour) to be made.
The principal soluble carbohydrates, saccharides, of mature Soya beans are the disaccharide sucrose (range 2.5–8.2%), the trisaccharide raffinose (0.1–1.0%) composed of one sucrose molecule connected to one molecule of galactose, and the tetrasaccharide stachyose (1.4 to 4.1%) composed of one sucrose connected to two molecules of galactose. While the oligosaccharides raffinose and stachyose protect the viability of the Soya bean seed from desiccation (see above section on physical characteristics) they are not digestible sugars and therefore contribute to flatulence and abdominal discomfort in humans and other monogastric animals; compare to the disaccharide trehalose. Undigested oligosaccharides are broken down in the intestine by native microbes producing gases such as
carbon dioxide, hydrogen, nitrogen, methane, etc.
Since soluble Soy carbohydrates are found mainly in the whey and are broken down during fermentation, Soy concentrate, Soy protein isolates, tofu, Soy sauce, and sprouted Soya beans are without flatus activity. On the other hand, there may be some beneficial effects to ingesting oligosaccharides such as raffinose and stachyose, namely, encouraging indigenous bifidobacteria in the colon against putrefactive bacteria.
The insoluble carbohydrates in Soya beans consist of the complex polysaccharides cellulose, hemicellulose, and pectin. The majority of Soya bean carbohydrates can be classed as belonging to dietary fiber.
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