Chemical and nutritional properties of soybean carbohydrates as related to nonruminants: A review

Soybean [Glycine max (L.) Merril] carbohydrates make up approximately 35% of soybean (SB) seed and 40% of soybean meal (SBM) dry matter (DM). Approximately half of these carbohydrates are nonstructural in nature, including low molecular weight sugars, oligosaccharides, and small amounts of starch, while the other half are structural polysaccharides, including a large amount of pectic polysaccharides. The small amounts of free galactose, glucose, fructose, and sucrose make up the low molecular weight sugars. Galacto-oligosaccharides (raffinose, stachyose, and verbascose) comprise approximately 5% of the SB DM, while starch represents less than 1%. The structural carbohydrates can be divided into cotyledon meal and hull polysaccharides. The primary cotyledon meal polysaccharides are arabinogalactan and an acidic polysaccharide that is similar to pectin, whereas the hull contains pectin, hemicelluloses, and cellulose. A portion of SBM carbohydrates are digested and another portion fermented in the animal's gastrointestinal tract. The low molecular weight sugars and starch are essentially 100% digestible by nonruminants, while the oligosaccharides are not digested by gastrointestinal enzymes, but are fermented by bacterial populations in the ileum and colon of pigs. This fermentation may result in both positive and negative effects. The nutritional significance of the structural SB carbohydrates in nonruminant animal diets remains virtually unknown. Many factors affect the nutritional value of SB carbohydrates, including cultivar and genotype, processing, and addition of exogenous enzymes. A better understanding of the nutritive contribution of SB carbohydrates is necessary as regards their impact on animal nutrition broadly defined.