Changes in Selected Physical Property and Enzyme Activity of Rice and Barley Koji during Fermentation and Storage

Koji are solid-state fermentation products made by inoculating steamed grains with the spores of fungi, particularly Aspergillus spp. This research was undertaken to identify the fermentation and storage conditions optimal for the production and maintenance of selected hydrolytic enzymes, such as alpha-amlyase and protease, in koji. Steamed rice and barley were inoculated with 2x10(11) Aspergillus oryzae spores per kilogram of grains and fermented for 118 h in a growth chamber at 28 to 32 degrees C with controlled relative humidities. Samples were drawn periodically during fermentation and storage at -20, 4, or 32 degrees C, and alpha-amylase and protease activity, mold counts, a(w), moisture contents, and pH of collected samples were determined. It was observed that the a(w), moisture contents, and pH of the koji were influenced by the duration of fermentation and temperature of storage. The alpha-amylase activity of both koji increased as the populations of A. oryzae increased during the exponential growth phase. The enzyme activity of barley koji was significantly higher than that of rice koji, reaching a peak activity of 211.87 or 116.57 U at 46 and 58 h, respectively, into the fermentation process. The enzyme activity in both products started to decrease once the mold culture entered the stationary growth phase. The protease activities of both koji were low and remained relatively stable during fermentation and storage. These results suggest that rice and barley koji can be used as sources of alpha-amylase and desired enzyme activity can be achieved by controlling the fermentation and storage conditions.