Soy-derived products contain a lot of α-galacto-oligosaccharides (αGOS), which cannot be digested in human's digestive tract. The most common αGOSs present in foods are melibiose, raffinose and stachyose. However, αGOS can be utilized and fermented by intestinal microbes, which causes flatus and gastrointestinal disorders. Lactobacillus acidophilus with high active α-galactosidase was inoculated into soymilk to make soymilk yogurt. Analyses of αGOSs in soymilk and soymilk showed that soymilk contained 0.7% raffinose, 2.5% stachyose and no melibiose; while soymilk yogurt contained 0.7% raffinose, 2.0% stachyose and no melibiose. Analyses of αGOSs in soymilk yogurt made by extensive longer inoculation with L. acidophilus showed no influence on reducing the amount of any αGOSs. Enzymatic characterization showed the optimal pH of L. acidophilus α-galactosidase was pH 5.5. The α-galactosidase was found to possess good activity in the pH range from 5.0 to 7.0, which was enough for degrading α-GOSs in the fermentation process. However, the activity was almost lost dramatically when the pH was lower than 5.0, which may account for that α-GOSs could not be removed completely in soymilk yogurt, since there was a lower pH than 5.0 in later stage of fermentation process
