The production of HMF has been reported to be enhanced by acid
catalysis (37). Therefore, HMF production in HP-koji was predicted to
be accelerated by a large amount of citric acid produced by the whitekoji
mold (29). To investigate the effect of citric acid on the production
of HMF, we compared the HMF concentration of HP-koji prepared
from white koji (HP-Wk) with that of HP-koji prepared from yellow
koji (HP-Yk), which does not produce much citric acid. As shown in
Table 2, the antioxidant activity and phenolic content of HP-Yk were
almost the same as those of HP-Wk. Conversely, the HMF concentration
of HP-Wk was three times that of HP-Yk. Recent research has
shown that HMF exhibits some physiological effects, such as a
protective effect on human hepatocytes, antioxidant activity, and
anti-tyrosinase activity (36,38–40). Therefore, HP-Wk, which contains
high level of HMF, is a good candidate as a new functional food.