Barley Koji: Exploring the Effects of Steeping and Fermentation on Malt Quality Parameters

Koji has long been used in Japan to supply enzymes, flavors, and nutrients for traditional fermented food and beverages. Its application to brewing offers a unique, versatile, and innovative alternative to specialty malts. This study assessed the brewing suitability of kilned barley koji produced with a barley-koji strain of Aspergillus oryzae—here termed barley koji malts (BKM)—by evaluating the effects of steeping times (6, 7, and 8 h) and solid-state fermentation (SSF) durations (40, 44, and 48 h) on malt quality and metabolomic profiles. BKM exhibited desirable malting quality parameters, including extract levels comparable to standard base malts, rapid production times, and high color formation under a base malt kilning profile. Shorter steeping times yielded improved enzymatic and technological performance, while variation in SSF duration produced distinct color development. This study demonstrates the potential of BKM as a versatile and innovative alternative for specialty malts in craft brewing applications.