Changes of volatile flavor substances in different layers of fermented grains in the mechanized production of strong-flavor Baijiu

In this study, using fermented grain in the mechanized brewing process of strong-flavor (Nongxiangxing) Baijiu as the research object, based on headspace solid-phase microextraction combined with gas chromatography-mass spectrometry (HS-SPME-GC-MS), the volatile flavor substances in fermented grains at the upper, the middle and the lower layers were analyzed, and the ester substances with the detection rate >65% were analyzed by principal component analysis (PCA). The results showed that the main volatile flavor substances in different layers of fermented grains in the fermentation process were acids, esters and alcohols, and their content changes were similar to some extent. The relative contents of acids and esters fluctuated greatly. Among them, the relative content of acids showed a downward trend as a whole, while esters showed an upward trend, and alcohols maintained a dynamic balance. At the end of the fermentation, the contents of volatile flavor substances in fermented grains at the lower layer was the highest (4.27 mg/g), followed by the middle layer (2.71 mg/g) and the upper layer (1.38 mg/g). In the whole fermentation process, there were 22 kinds of ester compounds with the detection rate >65%. Principal component analysis (PCA) of esters showed that the whole fermentation process could be divided into two stages of 0-45 d and 45-123 d. In the early stage of fermentation (0-45 d), the fermented grains in the middle and the lower layers were positively correlated with ethyl butyrate, ethyl acetate and L(-)-ethyl lactate. At the late stage of fermentation (45-123 d), the fermented grains at the upper layer were positively correlated with ethyl caproate, and fermented grains at the middle and the lower layers were positively correlated with 22 ester substances.