# Chemical Composition and Flavor Characteristics of Cider
## 3 Results and Discussion
### Volatile Compounds of Cider fermented with Different Non-Saccharomyces Yeasts
#### 3.2.2 GC-IMS Analysis of Cider Fermented with Different Non-_Saccharomyces_ Yeasts  
As shown in Supplementary Table S3, 29 volatile compounds were identified in cider samples by GC-IMS, including 14 esters, 8 alcohols, 3 aldehydes, 3 ketones, and 1 acid, which proved esters and alcohols were the main contributors to the cider aroma. If the concentration of volatile compounds is relatively high, monomers (M) and dimers (D) can be produced [29].  
Cider samples from citon samples from the topographic map are shown in Figure 3A,B, in which the migration time of the ions is plotted in the abscissa and the retention time of gas chromatography is plotted in the co-ordinate. The background of the topographic plots is blue, and the reactive ion peak abscissa (RIP) appears at 1.0. The points on either side of the RIP represent volatile compounds; the redder the dot, the higher the concentration. As shown in Figure 3A, there were large differences between FAJ and other yeast-inoculatediders, which may indicate that yeasts are important for cider fermentation. However, the volatile compounds in the ciders were similar, except for FAJ. Sc topographic map was used as a reference, and topographic maps of other samples were subtracted to obtain the difference comparison model. If the concentrations of the compounds are similar, the background is white. The closer the color is to red, the higher the concentration is compared to the reference concentration and, for blue, vice versa. The results are shown in Figure 3B. FAJ is different from the five ciders, but the ciders were similar among them. The Sc+Zb group had the lowest concentration of aroma substance among the co-inoculation fermented groups. The aroma substance concentration of the Sc+Dh group was significantly different from that of the other co-inoculation fermented groups.  
Comparison of the fingerprints of the obtained peaks allowed the identification of differences in specific volatile compounds. The results are shown in Figure 3C. The concentration of the substances in the yellow frame had apparent differences between FAJ and other ciders; they were much higher in other ciders than those in FAJ and they were also different from each other. Regarding ethyl lactate and 1-butanol, the contents of these two compounds in Sc+Rm were slightly higher than the other four groups of cider. 1-Butanol has a typical nasal aroma, while ethyl lactate has a faint fruity, fatty aroma. For acetic acid, 2-phenyl ethyl ester and octanoic acid, ethyl ester, the contents of those two compounds in Sc were the lowest of the five groups of ciders (except for the FAJ). The contents of 2-methyl-1-propanol, 3-methyl,1-butanol, acetone, and ethanol had little difference in the five cider groups; the content of the Sc group was the lowest. There were some volatile compounds in FAJ that had higher contents than those of other cider samples, such as hexanal, cis-3-hexenyl acetate, furfural, and 3-pentanone. The black frame content was lower in Sc and FAJ cider than in other samples. The substances in the black box include ethyl butyrate, which can provide the fruit aroma of cider [23], indicating that the fruit aroma of the co-culture fermentation cider is more intense. The concentration of red frame substance was higher in FAJ but lower in other ciders. Except for 1-hexanol, the concentration of red frame substance differed slightly in the fermentation group.  
Figure 3D is the cluster analysis plot of the cider samples. According to the plot, the FAJ group clustered into one category, the Sc group and the Sc+Zb group clustered into one category, and the rest of the co-inoculation fermented groups (Sc+Dh, Sc+Km, and Sc+Rm) clustered into one category.