Abstract: To select high-efficiency strains for semi-dry black douchi fermentation，this study compared and analyzed the effects of koji-making and fermentation quality differences of 6 strains of Aspergillus oryzae and one strain of Lichtheimia corymbifera with large differences in enzyme production capability. The results showed that HN3.042 and Aspergillus oryzae DM1 and DM2 have a strong ability to metabolize amylase, which is conducive to the formation of black color in douchi. Strains 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, HN3.042, DM1, and DM2 had a relatively higher content of acid substances such as 1-octen-3-ol, isovaleric acid, and 4-methylpentanoic acid in the fermented soybeans. Strains 24M-1, 30M-1, and PNM003 had a relatively higher content of trans-2-nonenal, 3-octanone, and isophorone, resulting in a richer and better flavor quality of the fermented douchis and being more suitable for soybean processing. The strain QM3 had lower enzymatic activity compared to Aspergillus strains, resulting in lower physicochemical indicators, free amino acid contents, and volatile flavor substances in the fermented soybeans and poorer quality. The study suggests that the quality of fermented soybean paste is related to the enzyme production capacity of the koji strain used in fermentation. A koji enzyme system with amylase as the dominant enzyme is conducive to the formation of color and acidic flavor substances in fermented douchi, while a koji enzyme system with proteolytic enzymes as the dominant enzymes is conducive to the formation of amino acid nitrogen and good flavor quality in fermented douchi.

Key words: enzyme system, Aspergillus oryzae, Mucor, douchi, flavour