Effect of soaking, dehulling, cooking and fermentation with Rhizopus oligosporus on the oligosaccharides, trypsin inhibitor, phytic acid and tannins of soybean (Glycine max Merr.), cowpea (Vigna unguiculata L. Walp) and groundbean (Macrotyloma geocarpa Harms)

Soybean, cowpea and groundbean, three locally grown legumes in West Africa, were processed into tempe, an Indonesian-type fermented food. Changes in oligosaccharides, trypsin inhibitor, phytic acid and tannins were monitored during the pretreatments (soaking and soaking-dehulling-washing-cooking) and fermentation with Rhizopus oligosporus. About 50% of raffinose and more than 55-60% of sucrose and stachyose were lost during the pretreatments of the beans. Stachyose decreased during fermentation with a reduction of 83.9%, 91.5% and 85.5% respectively for soybean, cowpea and groundbean while raffinose remained fairly constant. Galactose, the predominant sugar, glucose, fructose, maltose and melibiose increased during the first 30 and 36 h of fermentation of cowpea and groundbean, but decreased thereafter. Soaking the beans for 12-14 h had no effect on the level of trypsin inhibitor of the beans while it increased the phytic acid content to 1.7% in soybean and to 0.8% and 0.7% in groundbean and cowpea. Cooking soaked and dehulled soybean, cowpea and groundbean for 30, 7 and 15 min respectively resulted in 82.2%, 86.6% and 76.2% in trypsin inhibitor. However, a slight increase in trypsin inhibitor was observed during soybean fermentation. Phytic acid decreased during fermentation by 30.7%, 32.6% and 29.1% respectively in soybean, cowpea and groundbean at the harvesting time. Tannins mainly located in seed coat were removed as a result of pretreatments mainly dehulling. These changes are beneficial especially in infant feeding based on cereal and legume-based foods. (C) 2002 Elsevier Science Ltd. All rights reserved.