Effects of fermented soya bean on digestion, absorption and diarrhoea

For many centuries Asian people have consumed soya beans in various forms of traditional fermented soya bean foods. Major desirable aspects of fermented soya bean foods are their attractive flavour and texture, certain nutritional properties, and possible health promoting effects. This study describes effects of fermented soya beans on gastrointestinal physiology and addresses digestion, absorption and diarrhoea.

Using an in vitro digestion model it appeared that fermentation increased solubility and absorbability to a large extent as a result of protein and carbohydrate degradation. The level of water-soluble dry matter increased during fermentation with Rhizopus sp. (tempe) from an initial 7 up to 27%, and during fermentation with Bacillus sp. from an initial 22 up to 65%. In vitro digestibility was only slightly higher for the fermented soya beans. Soya beans are more or less pre-digested by the action of the micro-organisms and can therefore serve as a source of easily available nutrients.

Tempe extracts did not inhibit the growth of Escherichia coli. Enterotoxigenic E. coli (ETEC) induced haemagglutination of red blood cells was strongly inhibited by tempe extracts, and in vitro adhesion of ETEC to brush border membranes isolated from the small intestine of piglets was inhibited up to 95% by several tempe extracts.

Perfusion of small intestinal segments of anaesthetised piglets showed an inverse relationship between osmolality and net fluid absorption (linear correlation). ETEC infection of small intestinal segments prior to perfusion resulted in an osmolality independent reduction of net fluid absorption of approximately 400μl/cm ². Both cooked soya bean and tempe were able to minimise this reduction in net fluid absorption induced by ETEC. However, sodium losses as a result of ETEC infection were lower and dry matter and total solute absorption were higher for tempe when compared to cooked soya bean. A fraction containing high-molecular-weight components (> 5kDa) was shown to be mainly responsible for the observed protective effect of tempe. Several possible mechanisms of action are outlined and discussed.

Soya beans fermented with Rhizopus microsporus showed better protection against ETEC-induced diarrhoea compared to cooked and especially toasted (commercial) soya beans in weaned piglets in vivo . Furthermore, fermentation of cooked soya beans, especially with Bacillus subtilis , resulted in increased feed efficiency probably as a result of increased digestibility. These characteristics imply the potential of using fermented soya beans in individuals suffering from diarrhoea and malnutrition.