Research on the origin of the “grain aroma” in Nongxiangxing baijiu based on the flavor profile of steamed brewing grains

The unique flavor profile of Nongxiangxing baijiu is intricately linked to its interlocking brewing processes. However, limited research exists on how the raw grains used in brewing affect baijiu's flavor through the simultaneous distillation of raw and fermented materials. This study used Simultaneous Distillation Extraction coupled with Gas Chromatography-Mass Spectrometry to analyze the flavor composition of five brewing materials and one brewing adjunct: sorghum, rice, maize, millet, wheat, and rice husks-after steaming and the flavor quality of the crude baijiu produced through different brewing techniques. The results revealed that the steaming of raw grains contributes seven new flavor compounds to the crude baijiu, including propionic anhydride, p-vinylguaiacol, (E)-2-decenal, nonanal, α-terpinene, hexanal, and 2-methylnaphthalene. These substances are important sources forming the characteristic "grain aroma". Additionally, the content of nine flavor compounds (n-pentanoic acid, ethyl hexanoate, ethyl octanoate, (E)-2-nonenal, benzaldehyde, ethyl palmitate, propanol, isoamyl alcohol, and n-hexanol) significantly increased. Further analysis using Ultra-High Performance Liquid Chromatography-Quadrupole-Exactive Mass Spectrometry and Gas Chromatography-Quadrupole Mass Spectrometry revealed that the oxidative decomposition of medium- and long-chain fatty acids and the Maillard reaction of free amino acids during grain cooking are important pathways for generating 11 key flavor substances including nonanal, hexanal, (E)-2-decenal, and benzaldehyde. The contents of these reaction products are significantly increased in the crude baijiu produced by the simultaneous distillation of the raw and fermented materials process, collectively shaping the rich connotation of "grain aroma". In conclusion, this study reveals the dual mechanism of "grain aroma" formation: the direct transfer of flavor substances from cooked grains and the transformation of grain endogenous substances during the technological process.