Changes in free phenolic content and HMF concentration
during the stepwise-heating process It was expected that free
phenolics, which have antioxidant activity, would be released from
the cell wall of rice by the enzyme reaction in HP-koji. HMF is a wellknown
Maillard reaction product, and it has already been reported to
show antioxidant activity (36). Therefore, the free phenolic content
and HMF concentration of HP-koji were expected to have increased
through enzyme and Maillard reactions, respectively. To investigate
the antioxidant components of HP-koji, we measured its free phenolic
content and HMF concentration. Heat processing caused a timedependent
increase in the free phenolic content and HMF concentration
(Fig. 4A). In particular, the HMF concentration of HP-koji
increased significantly after heat processing. The increases in the
free phenolic content and HMF concentration were accompanied by
increases in the ORAC and SOSA, respectively (Fig. 4). Therefore, a part
of the antioxidant activity of HP-koji is attributed to the generation of
free phenolics and Maillard reaction products such as HMF. However,
despite the increase in the free phenolic content, increases in phenolic
acids such as ferulic acid and p-coumaric acid, which are the key
phenolics in rice, were not observed (data not shown). In the future, it
would be informative to identify the specific antioxidant phenolics
generated by the heat processing of koji.