Exploring the impact mechanisms on different mechanized airing approaches during second round heap fermentation of sauce-flavor Baijiu: From physicochemical parameters, microbial diversity to volatile flavor compounds.

The airing process of sauce-flavor Baijiu is a critical operation that serves the functions of cooling, homogenizing, and facilitating microbial proliferation and metabolism. Comprehensive analysis of physicochemical parameters, bacterial and fungal community of fermented grains, and volatile flavor compounds of soy-sauce (Jiangxiang) and mellow-sweet (Chuntian) typical base liquors among traditional (CT) and two different mechanized (JXA and JXB) airing operations were investigated. The results indicated that the dynamic variation patterns of moisture content, total titratable acidity, starch content, lactic acid, acetic acid, pH, and dominated microbial composition among CT, JXA, and JXB were similar, while minor bacterial genera with relative abundance including unclassified Micrococcineae, unclassified Rhizobiales, etc, and dominated fungi such as Torulaspora, Hyphopichia, Candida, Pichia, and Penicillium were profoundly influenced by mechanized airing operations, especially by JXB. A total of 100 and 101 volatile flavor compounds were qualitatively and quantitatively detected from soy-sauce (Jiangxiang) and mellow-sweet (Chuntian) typical base liquors. Mechanized airing operations were more consistent with CT for mellow-sweet (Chuntian) typical base liquors, but 2,3-dimethyl-5-ethylpyrazine, 2,3-dimethylpyrazine, tetramethylpyrazine and ethyl benzoate etc. were more abundant in soy-sauce (Jiangxiang) typical base liquors by mechanized airing operations, which were mainly associated with Leuconostoc, Acetoanaerobium, Limnohabitans and Bradyhizobium etc. This study provides a theoretical evidence for understanding the relationships among physicochemical parameters, microbial communities and volatile flavor compounds during second round heap fermentation of sauce-flavor Baijiu, laying a foundation for further elucidating the mechanized brewing mechanisms.