## 3 Results and Discussion  
### Qualitative and Quantitative Analysis of Volatile Compounds  
A total of 115 volatile compounds were detected in the strong-flavor daqu, including 31 esters, 13 alcohols, 7 acids, 13 aldehydes, 13 nitrogenous compounds, and so on. Ester compounds are mainly formed by microorganisms in daqu using alcohols and carboxylic acids. Almost all of the 31 ester compounds detected in this study were found in the flavor analysis of strong-flavor baijiu [13; 14; 15]. Figure 1 shows the variation in volatile compounds at dissimilar stages of the production process of strong-aroma daqu. As illustrated in Figure 1a, when elongating the production duration, the maximum change in volatile compound type was observed after 4 days of daqu production, which belongs to the material rapid growth stage. This is probably owing to the fact that multitudinous new substances are produced by microbial fermentation [16; 17], such as through the Maillard reaction, and the reaction substrate is sufficient at this stage, thus achieving speedy growth. In the last four periods, the species of compounds changed were unremarkable, all within 10 species. At this time,  
the variety of components was comparatively stable, but the concentrations of the compounds still changed. As depicted in Figure 1b, the species of acid compounds reached a maximum value in the first four periods after constant growth and then exhibited a decreasing trend before reaching relative stability. A similar trend existed for aldehydes, esters, and alcohols, which presented a comparatively balanced and harmonious level in the final period, suggesting that the daaqu gradually matured. It is noteworthy that nitrogenous and sulphur-containing compounds are two extremely important categories in the study of baijiu flavor, with nitrogenous compounds usually showing a roasted aroma. [PERSON] et al. [18] detected a variety of pyrazine compounds such as 2,6-dimethylpyrazine and 2-ethylpyrazine in strong-flavor baijiu such as Gujinggong and Jiananchun. These pyrazines are often produced by the Maillard reaction. In addition to the above aroma, there also are certain health care effects [19]. Sulphur-containing compounds, on the other hand, have a considerably low olfactory threshold and usually have tremendously high odor activity values [20], thus making a considerably high contribution to the overall flavor development of baijiu. In this study, the nitrogenous compounds only appeared at stage C, and 13 species in total were detected. The sulfur compounds were only produced in the form of methyl mercaptan in the final stage, which indicates that in the latter stages, although the total types and concentrations of esters and alcohols changed very little, the overall flavor still changed significantly. The concentration changes in volatile compounds in the different production processes of strong-flavor daqu are exhibited in Table 1. We found that some compounds could be detected in all the daqu production processes, such as ethyl acetate, ethyl caproate, ethyl pentadecanoate, ethyl octadecenoate, etc. Ester compounds give baijiu an important fruit aroma but also have certain health effects, such as ethyl acetate, which has a protective effect on acute liver injury. Similar to dibutyl phthalate, nonanoic acid, octanoic acid, 4-hydroxy-2-butanone, and 2,5-di-tert-butylphenol could only be detected in A; 12 species, such as ethyl valerate, hexyl formate, and ethyl heptanoate, were only present in stage B; 10 compounds, including isopentyl formate, 2,8-dimethylundecanoic acid, and methyl ester, were only detected in phase C. The mere compounds in D were methallyl alcohol, crotonyl alcohol, 2-methylhexadecanoic acid, 11,14-eicosadienoic acid, methacrolein, 2-vinyl-2-butenal, (E,E)-2,4-hexadienal, 2-Ethyl-5-methyl-pyridine, ethyl 5-amino-1,2,3-thiadiazole-4-carboxylate, 3,5-dimethylpyrazole, methyl anthranilate, 3,8  
Figure 1: Variation in volatile compounds in dissimilar production process stages of strong-flavor daqu.