Baijiu is a traditional solid-state distilled liquor prepared from grain fermentation of China, with a more than 2,000 years of history. As the national liquor of China, Baijiu occupies an important position in China’s food industry. In 2018, the total sales revenue of Baijiu industry reached 536.383 billion yuan and total annual profit was 147.645 billion yuan. Because of the complex raw materials, special technology and co-fermentation of multiple strains, the composition of Baijiu is complex. Of note, although the trace components only account for 2% of Baijiu, they contribute to the flavor and functional quality of Baijiu. As reported, these trace components mainly include alcohols, aldehydes, ketones, esters, acids, carbonyl compounds, nitrogen compounds, sulfur compounds, short peptides etc. Among them, volatiles play an important role on aroma profile and functional quality of Baijiu, while non-volatiles are the precursor of volatiles and the contents of them will influence the contents of volatiles in Baijiu, thereby affecting the flavor and functional quality of Baijiu as well. This perspective mainly introduces the technological process of Baijiu production, summarizes recent progress in the research on the trace components in Baijiu, especially for the influence of trace components on the flavor and functional quality of Baijiu.

Baijiu is the national liquor of China, which has lasted in China for more than 2000 years. Abundant raw materials, multi-strain co-fermentation, and complex processes make the secrets of baijiu flavor and taste still not fully explored. Acid substances not only have a great influence on the flavor and taste of baijiu, but also have certain functions. Therefore, this paper provides a systematic review for the reported acid substances, especially for their contribution to the flavor and functional quality of baijiu. Based on previous studies, this paper puts forward a conjecture, a suggestion, and a point of view, namely: the conjecture of “whether acid substances can be used as ‘key factor’ for baijiu quality “; the suggestion of “the focus of research on acid substances in baijiu should be transferred to evaluating their contribution to the taste of baijiu”; and the view of “acid substances are ‘regulators’ in the fermentation process of baijiu”. It is worth thinking about whether acid substances can be used as the key factors of baijiu to be studied and confirmed by practice in the future. It is hoped that the systematic review of acid substances in baijiu in this paper can contribute to further in-depth and systematic research on baijiu by researchers in the future.

Daqu has gained wide attention because it is an essential source of microorganisms and flavor in baijiu production. In this study, HS-SPME combined with GC-MS/O was used to analyze the volatile flavor components of Strong aroma baijiu Daqu. DI-GC-O was used to choose the best extraction fiber to extract the representative overall aroma profile of Daqu. A total of 139 compounds were identified in the six different maturity stages of Daqu, and these compounds are of different types and concentrations. HS-SPME combined with GC-MS/O was used to analyze the aroma active substances in the finished Daqu, and a total of 43 aroma compounds were identified. The OAVs of 21 aromatic compounds were calculated based on the quantitative analysis results of MHS-SPME. Eighteen compounds with OAVs ≥ 1 made significant contributions to the overall aroma of Daqu, including guaiacol, 4-ethyl-2-methoxy phenol, 2-ethyl-3,5-dimethylpyrazine, etc.