STUDIES ON THE MYCELIAL GROWTH OF ASPERGILLUS-ORYZAE ON RICE GRAIN

Rice koji, a solid-state culture of Aspergillus oryzae on steamed rice grains, is used as an enzyme source and raw material for sake ferment ation, and the quality of koji is regarded as the most important facto r determining the quality of sake. However, the control of solid-state culture is very difficult compared with submerged culture. In Japanes e breweries, the control of koji making is traditionally done by obser ving features such as mycelial distribution and odor. In order to deve lop an automatic control system for koji making, we first studied the mycelial growth mode in rice grains from the respiratory activity, usi ng a gas analyzer. The relationships between th properties of rice gra ins and the growth mode was studied, and it was recognized that the gr owth mode was affected not only by nutritional conditions and moisture , but also by physical properties like hardness and the shape of rice grains. We also studied the features and odor of koji in order to appl y the traditional method of controlling koji making to a novel automat ic control system by means of an expert system, or fuzzy control syste m. Mycelial distribution in rice koji was measured to elucidate the st eric growth mode in the solid-state culture. The mycelial density show ed an exponential distribution against the distance from th surface of rice koji. On the basis of this phenomenon, mycelial distribution was expressed by a mathematical model and shown quantitatively. The mycel ial distribution and enzyme composition in rice koji made in sate fact ories were examined and the relationships between the mycelial distrib ution and the quality of koji were studied. The results suggested that when the degree of mycelial penetration is low, the activities of aci d protease and acid carboxypeptidase are high. The volatile compounds produced by growing A. oryzae on steamed rice during koji making were collected with a Tenax column and analyzed by GC and GC-MS. Eight kind s of alcohols, three kinds of aldehydes, five kinds of ketones, and an ester were identified. The production of volatile compounds was affec ted by th oxygen concentration, and, aldehydes and ketones increased s ignificantly with a deficiency of oxygen. The production of most of th e volatile compounds was maximum at the middle stage of growth, and de creased thereafter. The tendency of 1-octen-3-ol was different from ot her volatile compounds; it increased with th cultivation time and did not decrease.