Functions of Coffee Fermented by <i>Rhizopus oligosporus</i>

Rhizopus oligosporus (R. oligosporus) is a fungus belonging to the Mucoraceae and is usually used to ferment soybeans in East Asia. R. oligosporus-fermented soybeans are called tempeh and are mainly eaten in East Asia. The recent researches have demonstrated that R. oligosporus can also ferment the beans other than soybeans. In this study, raw coffee beans were fermented with R. oligosporus, and then the effects of coffee prepared from the fermented beans were investigated. The amounts of fats and saccharides were lower in the R. oligosporus-fermented coffee beans than in the unfermented coffee beans, but the amount of dietary fiber was increased in the fermented coffee beans. The R. oligosporus-fermented coffee exhibited decreased total polyphenol levels, and its antioxidant activity was also reduced. However, the concentration of chlorogenic acid, which is one of polyphenols, in the R. oligosporus-fermented coffee was higher than that in the unfermented coffee. In comparison to the unfermented coffee, the R. oligosporus-fermented coffee inhibited α-amylase and α-glucosidase activity more strongly, inhibited cancer growth more weakly or to a similar level, and inhibited H₂O₂-induced reactive oxygen species production more weakly. The inhibition of α-amylase and α-glycosidase activity is considered to contribute to the prevention of obesity and diabetes. Fermenting raw coffee beans with R. oligosporus may contribute to improving certain effects of coffee, especially the preventative effects against obesity and diabetes.