Abstract: To select high-efficiency strains suitable for semi-dry black douchi fermentation, this study compared and analyzed the koji-making effects of six Aspergillus oryzae strains and one Lichtheimia corymbifera strain with large differences in enzyme-producing capacity. The results showed that A. oryzae HN3.042, DM1 and DM2 had a strong ability to metabolize amylase, and the fermented douchi had a lower L* value. A. oryzae 24M-1, 30M-1, and PNM003 had a strong ability to produce protease, aminopeptidase, and carboxypeptidase, resulting in greater utilization of protein substrates and higher contents of total acid and amino nitrogen in the fermented douchi. In terms of volatile flavor substances, the douchi fermented with stains HN3.042, DM1, and DM2 had a relatively higher content of 1-octen-3-ol, isovaleric acid, and 4-methylpentanoic acid. The douchi fermented with strains 24M-1, 30M-1, and PNM003 had a relatively higher content of trans-2-nonenal, 3-octanone, and isophorone, resulting in richer and better flavor quality. L. corymbifera QM3 had lower enzymatic activity when compared with A. oryzae, resulting in poorer physicochemical indicators, lower contents of free amino acids and volatile flavor substances and poorer quality in douchi. The study suggests that the quality of douchi is related to the enzyme-producing capacity of the koji mold. The predominance of amylase in the koji enzyme system is conducive to the formation of color and acidic flavor substances in douchi, while with the predominance of proteolytic enzymes is conducive to the formation of amino nitrogen and good flavor quality in douchi.

Key words: enzyme system; Aspergillus oryzae; Mucor; douchi; flavor