The conversion of soybeans to tempe is achieved through fermentation by Rhizopus. This fermentation process leads to hydrolysis of both proteins and lipids. The present work investigated certain biochemical changes which accompany the conversion of soybeans to tempe. The contents of non-protein nitrogen and free $ alpha$-amino nitrogen increased from 2.34 to 15.14%, and 2.03 to 5.22%, respectively after 48 h fermentation. SDS electrophoresis showed that a substantial quantity of the proteins in raw soybeans were hydrolysed by the Rhizopus to low molecular species (molecular weight $<$13,000 Daltons). Trypsin inhibitor activity found in tempe was lower than that of soybean and soaked soybean (an intermediate step in tempe preparation). The protein digestibilities of tempe and soaked soybean were higher than that of soybean. Reversed phase HPLC showed that the peptide separation profile of tempe was different from that of soybean and soaked soybean. The ESI/MS of the RP-HPLC fractions gave molecular weight of soybean peptides ranging from 1962 Da to 22,699 Da and tempe peptides ranging from 569 Da to 16,688 Da. The fatty acid compositions of tempe, soybean and soaked soybean were similar; relatively high levels of linoleic acid followed by oleic, linolenic and stearic acids were found. The acid values increased from 1.49 to 11.42 during the fermentation of soybeans. The total soluble carbohydrate contents of soybean, tempe and soaked soybean as well as the types and quantities of individual sugars were similar. The fermentation of soybean by Rhizopus had only a minor effect on the proximate composition of soybean; however, the soybean and fungal enzymes contributed primarily to changes in protein composition.

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