Bacillus plays a unique role in Baijiu brewing, but its functions have not been fully explored. In this study, Aspergillus niger, Saccharomyces cerevisiae and Thermoactinospora rubra were used as chassis microorganisms in a fermentation system. Bacillus subtilis and Bacillus licheniformis, which are common in Maotai-flavoured Baijiu, were studied in the fermentation. Their metabolic differences and effects on the formation of Baijiu flavour in the process of stacking fermentation were analysed. The combined fermentation system of B. licheniformis showed the highest glucose metabolism, good flavour production, the greatest number of flavour compounds and the highest contents of important compounds, including ethanol (80.51%) and tetramethylpyrazine (0.35%). The maximum concentrations of glucan, maltotriose, maltose monohydrate and glucose were 349.64, 70.98, 64.34 and 51.31 mg g −1 , respectively. These results suggested that the metabolism of sugar and its derivatives was closely related to the overall flavour of the fermentation system.

Yeast extract (YE) is a kind of umami additives used in fermented condiments, it also has special yeasty flavour (YF), which is not widely accepted by some consumers. In this paper, the technology of mixed microbial transformation was applied to eliminate or cover up YF during mixed fermentation. FG10, which is produced by Hubei Angel Yeast Co. Ltd, is a kind of YE that has the distinctive YF and has been applied to the market. It had good sensory evaluation after 6 h with Saccharomycopsis fibuligera and Lactococcus lactis. At 6 h, compared with single fermentation, the number of bacteria of L. lactis was enhanced by 34.28%, and the number of bacteria of S. fibuligera was reduced by 47.93%. The yeasty flavoured substances propionic acid and butyric acid were completely removed. The result of GC-MS showed that acetal, phenyl acetaldehyde, 2,4-dimethylbenzaldehyde, benzaldehyde, isoamyl alcohol, 5methyl-2-furan methanol, phenylethanol were generated or increased. Furthermore, the amino acids of YE were significantly different after fermentation by different microorganisms (P < 0.05), The flavour compounds and amino acids in YE before and after fermentation had closely correlation (r = 0.94), the consumption of amino acids in mixed fermentation promoted the variety of corresponding flavour compounds, which boosted the formation of free YF YE. In addition, taste substances lactic acid generated 6.27 AE 0.115 g L À1 and nucleotides improved 15.05%, which were beneficial to natural YF free seasoning with YE as raw material.

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