Regulating yeast flavor metabolism by controlling saccharification reaction rate in simultaneous saccharification and fermentation of Chinese Maotai-flavor liquor

Maotai-flavor liquor is produced by simultaneous saccharification and fermentation (SSF), in which filamentous fungi produce hydrolases to degrade the starch into fermentable sugar. Saccharomyces cerevisiae simultaneously transforms the sugars to ethanol and flavor compounds. The saccharification rate plays an important role in regulating the liquor yield and flavor profile. This work investigated the effect of saccharification rate on fermentation by regulating the inoculation ratio (1:0.1, 1:0.5, 1:1, 1:5, 1:10) of S. cerevisiae and Aspergillus oryzae, the main saccharification agent. We found no significant difference in reducing sugar content among the mixed cultures with different ratios. This indicated a balance of the saccharification rate and the sugar consumption rate, in which the former was controlled by the interaction between A. oryzae and S. cerevisiae, and the latter controlled the metabolism of the two species. The ethanol yield was the highest in ratios of 1:0.5, 1:1, and 1:5, while the total production of flavor compounds was the highest for the ratio of 1:0.5, which was mainly attributed to the vigorous metabolism of S. cerevisiae. The inoculum ratio of 1:10 produced the second highest content of flavor compounds in which a large number of alcohols and esters were derived from the vigorous metabolism of A. oryzae. This indicated that the saccharification rate significantly influenced the flavor metabolism. This study improves understanding of the interaction and cooperation between A. oryzae and S. cerevisiae in co-culture fermentation for Chinese liquor making.