## 4 Discussion
It was indicated that ethyl acetate might be mainly produced by YQ in high-temperature _Daqu_. In addition, contents of hexadecanoic acid ethyl ester, n-propyl 9,12-octadecadienoate, and phenethyl acetate were high in the YQ sample. Hexadecanoic acid ethyl ester has a creamy and fruity aroma. Ethyl linoleic is the main ingredient of the empty cup flavor of sauce-flavored baijiu, and it has a good effect on hypertension, coronary heart disease, and other vascular diseases [45]. [PERSON] et al. [42] reported this compound for the first time in baijiu. N-propyl 9,12-octadecadienoate was rarely identified in baijiu, so it is vital to investigate how it affects the flavor of baijiu. Phenethyl acetate contributes to the rosy aroma; it is an important aromatic compound in sauce-flavored baijiu [46]. L-ascorbyl dipalmitate was discovered in high-temperature _Daqu_ for the first time and was not reported in other _Daqu_ samples previously. Additional research on its contribution to baijiu flavor is necessary. Among the alcohols detected in the three _Daqu_ samples, the contents of phenethyl alcohol and 2,3-butanediol were the highest, and phenethyl alcohol has rosy, fruity, and sweet bread aromas, and 2,3-butanediol has a sweet aroma [47]. Thus, it is speculated that these two substances were crucial aromatic compounds in high-temperature _Daqu_. Benzyl alcohol has basimine oil and hyacinth aromas [46]. In addition, acetic acid (vinegar) and butanoic acid were the main aromatic compounds in sauce-flavored baijiu [43]; acetic acid and alcohol could be further synthesized into esters via an esterification reaction [48]. Acetoin is a precursor of tetramethylpyrazine, which has a strong creamy and fatty aroma, and they provide important flavors for baijiu [49]. Pyrazines have roasted peanut, hazelnut, cocoa, and nutty aromas, and they were detected in sauce-flavored baijiu in higher proportions than other flavored types of baijiu. [27]. Furthermore, tetramethylpyrazine has medical effects, such as treating cardiovascular diseases and improving learning disabilities [50; 51]. In this study, the contents of pyrazines in three kinds of _Daqu_ were the highest, and tetramethylpyrazine was the most abundant, comprising 31-46% of the total pyrazines. Additionally, the most abundant types of pyrazines were found in WQ (Figure 2). Nevertheless, the pyrazine content was significantly lower compared to the southern high-temperature _Daqu_[27; 48]. The flavor compounds were formed via numerous metabolic reactions in _Daqu_, and they were largely affected by complex microbial communities. Furthermore, there are enormous differences in microbial community diversity among the three samples, which may be related to the _Daqu_ production process, temperature, environment, and other factors. The microbial diversity analysis results showed that compared with the WQ sample, BQ and YQ showed higher microbial species richness and diversity (Table 2). The results of the microbial community composition analysis showed that the two most common bacterial phyla were Firmicutes and Actinobacteria, and the most common fungus phylum was Ascomycta (Figure 3a,c), which is consonant with previous reports [17]. They are not only the dominant microorganisms in high-temperature _Daqu_ but also the dominant microorganisms in other baijiu _Daqu_[52]. _Bacillus_, _Kroppenstedtia_, _Virgibacillus_, _Oceanobacillus_, and _Scopulibacillus_ were predominant bacterial genera, and _Thermoascus_, _Thermomyces_, _Byssochlamys_, and _Aspergillus_ were dominant fungal genera in the three _Daqu_ samples (Figure 3b,d). These results were different from previous studies in some southern regions, owing to the differences in climate conditions between the north and the south; the south is richer in the composition of brewing microorganisms [15; 17; 48]. It was demonstrated that the microorganism composition in high-temperature _Daqu_ was distinctive between different producing regions and generated different microbial community structures.