# Chemical Composition and Flavor Characteristics of Cider
## 3 Results and Discussion
### Volatile Compounds of Cider fermented with Different Non-Saccharomyces Yeasts
#### 3.2.4 Combination of SPME-GC-MS and GC-IMS Results for Multiple Analysis of Volatile Compounds  
PCA was used to show the distribution of the cider samples. In Figure 4A, the principal components (PCs) PC1 and PC2 represent 95.3% and 1.8% of the total variance, respectively, so the two PCs elucidated 97.1% of the total variables in this model. This result showed that they were able to explain the total variance analysis. As shown in Figure 4A, three groups were observed, which indicated that there were significant differences in flavor between the FAJ, Sc, and co-inoculation fermented groups. As shown in Figure 4B, volatile compounds with VIP > 1 were isoamyl acetate, ethyl hexanoate, 3-methyl-1-butanol, acetic acid, ethyl 3-methyl butanoate, ethanol, ethyl 2-methyl butanoate, octanoic acid, ethyl ester, 1-butanol, 3-methyl-, 1-hexanol, ethyl butanoate, 2-methyl-1-propanol, acetone, and butan-1-ol. This indicates that they have an important effect on the flavor of cider. This is consistent with the result of previous research [2]. RDA was used to analyze the correlation between the physicochemical properties of cider and volatile compounds. As shown in Figure 4C, FAJ was positively correlated with malic acid, tartaric acid, citric acid, and glucose. The Sc group was positively correlated with succinic acid, total acid, glycerol, and alcohol content. Among them, Sc showed a strong correlation with total acidity and succinic acid. The highest total acidity and succinate were found in Sc (Table 1, Figure 1). The succinic acid taste is strong, bitter, and salty, which can make wine taste strong and increase the mellow feeling. The positive correlation of Sc with glycerol and alcohol was weak. The positive correlation of Sc with glycerol and alcohol was weak. Some studies have found that, during the fermentation process of cider, a part of the alcohol can be converted into acids. While yeast uses sugar to produce alcohol, it also produces other by-products, such as glycerol, higher alcohols, and organic  
Figure 4: The PCA analysis **(A)**, VIP plot **(B)**, and RDA analysis (**C**) of volatile flavor compounds identified by GCMS and GC/IMS in apple juice and cider samples fermented with different non-_Saccharomyces_ yeast. PCA and variable importance for predictive components (VIP) plots were analyzed based on PLS modeling. The upward and downward triangles represent the volatile flavor compounds identified by GCMS and GC/IMS, Sc, _S. cerevisiae_ monoculture. Sc+Zb, _S. cerevisiae_ and _Z. bailii_ co-inoculation. Sc+Rm, _S. cerevisiae_ and _R. mucilaginous_ co-inoculation. Sc+Km, _S. cerevisiae_ and _K. maraxianus_ co-inoculation. Sc+Dh, _S. cerevisiae_ and _D. hansenii_ co-inoculation. FAJ, apple juice. acids [31]. These make cider sweet with acid, and acid is also astringent. Meanwhile, Sc+Zb and Sc+Rm of the co-inoculation fermented groups were also affected. In the process of co-culture fermentation, the growth of non-_Saccharomyces_ yeast is inhibited by _S. cerevisiae_ and this further influenced the non-_Saccharomyces_ yeast in the cider fermentation process in terms of growth and metabolic activity [17]. The co-inoculation fermented groups were positively correlated with acetic acid content. Acetic acid is the main volatile acid in cider; it can enrich wine's taste and aroma. A diagram showed the positive correlation between acetic acid and pH. Fruit wine contains higher alcohols and aldehydes, both of which are oxidized so that a large amount of acetic acid is accumulated in fruit wine. Acetic acid will further esterify with alcohols, which will significantly increase the concentration of fruity esters, such as acetic acid, isoamyl ester and acetic acid, 2-phenyl ethyl ester, and endow fruit wine with special fragrance [32]. In the wine fermentation process, organic acid and alcohol esterification reaction, besides giving wine an ester aroma fragrance compound, can also lead to a higher alcohol content drop [33]. This is consistent with the results of the present study.