First, we investigated the time course of changes in the composition and content of isoflavone along with the fermentation time (0–48 h) of a yellow-soybean tempeh as shown in Fig. 1. The content of the three types of isoflavone-aglycon increased with fermentation time, and became approximately twofold after 24 h fermentation compared
with that before fermentation (0 h). Among the nine types of isoflavone-glucoside, the content of malonylglucoside increased transiently from 12 to 24 h of fermentation. However, the contents of monoglucoside and acetylglucoside decreased with fermentation time. As a result, the total content of isoflavone (both aglycon and glucoside) decreased as the fermentation process progressed. Therefore, a sufficient increase in the isoflavone content, especially that of its aglycon form, could not be expected during tempeh fermentation