Dietary Content and Gastrointestinal Function of Soybean Oligosaccharides in Monogastric Animals

Soybean is a major ingredient in non-ruminant animal diets throughout the world. There is an extensive body of information suggesting that soybean is an excellent source of high quality protein, whereas less attention has been paid to soybean oligosaccharides. Soybean oligosaccharides, also referred to as ┙-galacto-oligosaccharides, oligosaccharides of the raffinose family or simply ┙-galactosides, are water-soluble, low-molecular weight carbohydrates raffinose, stachyose and verbascose. In maturing seeds, oligosaccharides are formed by successive addition of galactosyl moieties to a sucrose primer. Alpha-galactosides are characterized by the presence of ┙(1→6) linkages between galactose moieties which are bonded via ┙(1→3) to terminal sucrose. Unlike other oligosaccharides, soybean ┙galactosides can be extracted directly from the raw material and do not require enzymatic manufacturing processes. Soybean oligosaccharides comprise approximately 4% of the soybean dry matter (DM) and during processing in the preparation of soybean meal (SBM) they are not removed or destroyed. Therefore, in SBM, ┙-galactosides represent approximately 5-6% but could be as high as 8% DM. Other processed soybean products, however, may contain significantly less oligosaccharides than SBM. The oligosaccharide content of soy protein concentrates (SPC) is as low as 3% DM while soy protein isolates (SPI) contain only trace amounts of oligosaccharides. Soybean oligosaccharides appear to be indigestible in the upper intestinal tract of monogastric animals due to the absence of ┙-galactosidase enzyme. However, they are easily fermented by the lower gut microflora, resulting in the production of various gases and short-chain fatty acids. Studies have shown considerable microbial fermentation of ┙galactosides in the small intestine with some authors referring to soybean oligosaccharides as bifidogenic factors which stimulate the growth of beneficial bacteria and others claiming that increased consumption of oligosaccharides may lead to negative effects in the large intestine of mammals, such as flatulence, diarrhea, and excessive dietary protein decay.