## 4 Discussion  
High-temperature _Daqu_ is a crucial saccharification and fermentation starter for baijiu, and its quality determines the yield and quality of sauce-flavored baijiu [38]. One of the main factors contributing to the distinctive aroma of northern Chinese sauce-flavored baijiu is the high-temperature _Daqu_, which plays a key role in the manufacturing process. Therefore, the present study comprehensively analyzed the physicochemical indexes, volatile flavor components, and microbial community compositions of three kinds of high-temperature _Daqu_, which offers a more thorough and scientific assessment of _Daqu_ for production applications, and also provides a scientific foundation for the processing standardization of northern Chinese sauce-flavored baijiu. The physicochemical indexes of _Daqu_ determined its quality and whether it was suitable for fermentation. During the _Daqu_ production process, the moisture content should not be more than 13.0% [17]. As displayed in Table 1, the moisture content of three _Daqu_ samples met these standards. BQ showed the highest moisture, and WQ showed the lowest moisture because of the free- and well-ventilated storage conditions, and WQ was located in the outer layer, resulting in more moisture loss, while BQ was located in the inner layer, with less moisture loss [39]. However, this result was not completely consistent with the southern Chinese high-temperature _Daqu_[35]. The divergence of moisture in _Daqu_ was mainly due to the differences in climate and humidity of the fermentation environments in the north and south of China [4]. The research showed that the acidity of high-temperature _Daqu_ should be approximately 1.0 [40], and the acidities of the three _Daqu_ types were in accordance with the acidity indexes. The saccharifying and liquefying powers refer to the ability of microorganisms with saccharifying functions in _Daqu_ to convert starch into sugar [41]. The microorganism metabolizing the saccharifying function of hydrolase in _Daqu_ is mainly _Aspergillus_[17]. Furthermore, the results of the microbial community composition analysis showed that _Bacillus_ and _Aspergillus_ were the most prevalent in the BQ sample (Figure 3), so it was indicated that BQ showed the highest saccharifying power and the lowest starch, while WQ had the highest liquefying power and starch. The results of  
Figure 6: Correlation heatmap between the key flavor compounds and dominant genera. The asterisks show significant correlations (different flavor compounds in Table S1; _***_, 0.01 \(<\)_p \(<\)_0.05; _***_), 0.001 \(<\)_p \(<\)_0.01; _***_, \(p<\)_0.001). previous studies showed a different phenomenon where RQ and YQ showed the highest saccharifying power in the different colored high-temperature _Daqu_ of Sichuan, and WQ showed the highest saccharifying power in the different colored high-temperature _Daqu_ of Guizhou [35; 39; 41]. The esterification power of _Daqu_ represents the capacity of microbe-produced esterification enzymes to produce esters. [42]. The experimental results showed that the highest esterifying power of BQ was 288.36 U, and it was indicated that BQ has a strong ability to synthesize esters. The volatile components in the three _Daqu_ samples were detected using HS-SPME-GC-MS. A total of 96 volatiles were detected in the three samples; BQ contained 55, WQ contained 59, and YQ contained 61. The primary volatile flavor compounds in the _Daqu_ samples were esters, alcohols, acids, phenols, ketones, and pyrazines (Table S1). Esters, often with fruity and flowery aromas, were thought to be the main contributors to the aromatic character of _Daqu_ samples [43]. One of the four main ester components in baijiu is ethyl acetate, which is also the main ester in sauce-flavored baijiu and has a fruity flavor [4; 44]. As displayed in Table 2, the content of ethyl acetate was the highest among the more than ten ester compounds identified in the three _Daqu_ samples. YQ had the highest amount of ethyl acetate, but BQ had a higher concentration.