## 3 Results and Discussion
### Profiling of Melanoidin Pyrolysis Compounds in the Three Kinds of Daqu  
Melanoidin is the final product of the Maillard reaction, and the pyrolysis products of melanoidin were analyzed by pyrolysis at high temperature. According to Table S2, a total of 50 pyrolysis products were detected from three kinds of _Daqu_. Overall, 50, 39, and 39 kinds of pyrolysis products were detected from BQ, WQ, and YQ, respectively, and the number of pyrolysis products in BQ was 11 more than that in WQ and YQ, respectively (Figure 3a), which indicated that the structure of melanoidin in BQ was more complicated. According to PCA analysis (Figure 3b), there were significant differences in the composition of the pyrolysis products between the three kinds of _Daqu_, and there were also differences in the composition of the melanoidin pyrolysis products between the parallel samples of each kind of _Daqu_, indicating that the Maillard reaction in saucetflavor _Daqu_ is complex. Melanoidin pyrolysis products in saucetflavor _Daqu_ were mainly composed of pyrrolopyrazines, furans, pyrazines, pyrroles, indoles, phenols, pyrans, and other heterocyclic substances. Among them, pyrrolopyrazines, furans, phenols, and indoles had the highest proportion in melanoidin pyrolysis components, but the proportion in the three types of _Daqu_ is quite different. The melanoidin is formed by polymerizing sugar degradation products and then the cross-linking of carbohydrates, proteins, polyphenols or other macromolecular substances to produce high molecular mass polymers [22; 23]. Several reports have elucidated the partial structure of melanoidin were heterocyclic substances such as furan and pyrrole [24; 25]. Figure 2: Analysis of differences in volatile components between three types of _Daqu_ (**a**), R\({}^{2}\)X (cum) = 0.978, R\({}^{2}\)Y (cum) = 0.996, Q\({}^{2}\) (cum) = 0.980; the proportion of volatile substances (**b**); the heatmap of heterocyclic substances in BQ, WQ, and YQ (**c**). In BO, 4-amino-1-Pentanol (14.34%), 5,10-Diethoxy-2,3,7,8-tetrahydro-1H,6H-dipyrrolo [1,2-a:1',2'-d]pyrazine (8.01%), 3,5-dimethoxy-Phenol (4.95%), hexahydro-Pyrrole[1,2-a] pyrazine-1,4-dione (4.66%), Butanal (4.15%), 9H-Pyrido[3,4-b]indole (4.06%), etc., were the main substances with the high proportion in BO. Pyropyrazines, indoles, and phenols were the main components of BO melanoidin. According to the results of the comparison of peak areas, 5,10-Diethoxy-2,3,7,8-tetrahydro-1H,6H-dipyrrolo[1,2-a:1',2'-d]pyrazine, Indole, Pyrrole and Phenol in BO were significantly higher than those in YQ and WQ (Figure 3c). The results showed that the Maillard reaction produced a large number of pyrazines in BO, and even after a large number of pyrazines participated in the formation of melanoidin, high concentrations of pyrazines could still be detected in BO. Melanoidin components in YQ were mainly pyrrolopyrazines, furans, and phenols, including 5,10-Diethoxy-2,3,7,8-tetrahydro-1H,6H-dipyrrolo[1,2-a:1',2'-d]pyrazine (11.81%), 4-amino-1-Pentanol (9.63%), \(p\)-Cresol (9.48%), hexahydro-Pyrrole[1,2-a]pyrazine-1,4-dione (7.22%), 3-methyl-1,4-diazabicyclo[4.3.0]nonan-2,5-dione (5.18%), Indole (4.22%), hexahydro-3-(phenylmethyl)-Pyrolo[1,2-a]pyrazine-1,4-dione (3.78%), Furfural (3.74%). The difference between YQ and BO in the proportion of pyrolysis compounds was smaller than that of WQ (Figure 3a). The detection peak area of hexahydro-Pyrrole [1,2-a] pyrazine-1,4-dione was higher than that of WQ and BQ, suggesting that pyrazines in YQ were mainly involved in the formation of melanoidin, so the content of pyrazines detected in YQ was lower than that in WQ and BQ. There was a big difference between WQ and BQ in the proportion of pyrolysis products.