Critical Importance of α-Glucosidase Contained in Rice Kernel for Alcohol Fermentation of Rice Polish

Abstract

In our previous study, we proposed a new alcoholic beverage called nuka-sake, which is made from uncooked rice polish without any enzyme source such as koji. In nuka-sake brewing, the uncooked rice polish serves not only as the fermentation material but also as the enzyme source. In the present study, the results of both laboratory-scale nuka-sake brewing runs with various grades of rice polish and analysis of amylolitic enzyme distribution in rice polish suggested that α-glucosidase (EC 3.2.1.20) is a key enzyme of parallel fermentation in nuka-sake brewing. Miglitol, a specific inhibitor of α-glucosidase, strongly inhibited glucose production from rice polish. To obtain further evidence regarding the contribution of α-glucosidase, this enzyme was purified from rice grain kernels ( Yamadanishiki cultivar) and supplied for both rice polish saccharification and nuka-sake brewing. The purified α-glucosidase promoted both glucose production from rice polish and alcohol fermentation in nuka-sake brewing. Based on these results, it was considered that the α-glucosidase contained in rice polish plays an important role in glucose production. This role may be a rate-limiting factor for parallel fermentation in nuka-sake brewing. Moreover, oligosaccarides accumulated during the saccharification of uncooked rice polish, implying the contribution of not only α-glucosidase but also α-amylase (EC 3.2.1.1). Through this result, it can be speculated that the starch contained in rice polish will be decomposed to glucose as a result of the action of α-glucosidase and α-amylase contained in rice polish.