Key volatile compounds in red koji-shochu, a Monascus-fermented product, and their formation steps during fermentation

•Red koji-shochu has characteristic odors cheese, milky, oily, and fruity.•Key compounds in red koji-shochu were identified by comparison with white koji-shochu.•Ketones and short-chain acids in red koji-shochu derive from red koji.•Red koji includes higher protease and lipase activity than white koji.•High concentration of higher alcohols is due to the corresponding acetate esters in red koji-shochu.

Red koji, which refers the solid culture grown koji mold on the steamed rice, is one of the ingredients of Asian fermented foods including the Japanese spirit shochu. This study was aimed at elucidating the characteristic flavor and key volatile compounds of red koji-shochu as well as the mechanism of their formation. Sensory evaluation showed that red koji-shochu has the distinctive flavors cheese, sour, milky, and oily. Fifteen key volatile compounds of red koji-shochu were identified by gas chromatography-mass spectrometry and high-performance liquid chromatography, and by comparison between red koji-shochu and white koji-shochu, as another typical shochu. The mash analysis revealed that ketone compounds and short-chain acids derive from red koji. Furthermore, although other key compounds were produced by yeast, it is highly likely that their concentrations were affected directly or indirectly by the high activities of protease and lipase in red koji.