To develop a water absorption model for sake brewing rice, the validity of a model that incorporates white cores and cracks in a space ruled by the diffusion law was investigated numerically and compared with the actual phenomenon. The results showed that both white cores and cracks have the effect of increasing the water absorption rate, which was reproduced in the simulation. The changes over time in the water distribution inside the rice were compared with the actual water absorption images, allowing the model to be evaluated visually. Comparison of the change in moisture content between the actual samples and the simulation showed that the incorporation of cracks brought the model closer to the change in moisture content of the actual samples. These results indicate that the water absorption model proposed in this study can simulate the water absorption of rice for sake brewing with higher accuracy than a model using the diffusion law alone.

A koji extract medium is commonly utilized for yeast extended cultivation in sake brewing. However, its production from rice koji is a time-consuming process. Therefore, we developed an alternative medium using a food additive yeast extract and glucose（yeast extract medium or YEM）. We compared the growth of twelve sake yeast strains in a commercial koji extract medium（CKE）, diluted koji extract medium（DKE）, and yeast extract medium（YEM）. The results showed that yeast growth in YEM was equal to or better than that in CKE. Subsequently, pilot-scale sake brewing experiments were conducted using yeasts cultured in CKE and YEM. It was found that the progress of moromi mash and the quality of the sake produced were similar between the two. Furthermore, results of practical-scale sake brewing experiments carried out in four sake breweries in Gunma Prefecture demonstrated that yeast cultures in YEM posed no issues in practical applications.

The influence of mizu-koji and/or the dehydration of steamed rice on the sake moromi process was investigated taking starch retrogradation into account. When the enzymes were extracted at 5 ℃ from the koji, the relative activities of α-amylase, glucose production, and acidic protease extracted for 2 hours were 81, 80, and 74 ％ based on 5 hours’ extraction, respectively. Particularly, the longer the extraction time was, the more the acidic protease activity was increased during the 5 hours. When the steamed rice was dehydrated under forced-air at 10 ℃, the moisture content was decreased time-dependently from 39.8 ％ to 14.1 ％ after 440 minutes. Small sake brewing tests were carried out with mizu-koji for 5 hours at 6 ℃ and/or the dehydration of steamed rice under forced-air for 5 hours and the changes in Báume degrees, contents of maltose, and glucose were monitored. As a result, the mizu-koji treatment without the dehydration of steamed rice increased constantly in both Báume degree and maltose content, while the dehydration of steamed rice increased notably in glucose content during the initial stage of moromi. This indicates that the mizu-koji treatment should increase the elution of acidic protease and suppress the inactivation of α-amylase. On the other hand, the dehydration should increase the absorption of more water-containing enzymes into the rice and glucoamylase, thought to be the main component for glucose production, which could produce glucose not only on the surface but also inside the rice grain. Furthermore, the generated glucose inside the grain might be tend not to diffuse due to high viscosity in the rice grain, which might easily inhibit the α-amylase activity and consequently lead to a decrease in the content of maltose.