# Chemical Composition and Flavor Characteristics of Cider
## 3 Results and Discussion
### Volatile Compounds of Cider fermented with Different Non-Saccharomyces Yeasts
#### 3.2.1 SPME-GC-MS Analysis of Cider Fermented with Different Non-Saccharomyces Yeasts
The total concentration of volatile flavor compounds in cider samples is shown in Figure 2A. The total content of volatile compounds in cider was significantly higher than that in FAJ, and the total content of volatile compounds in co-inoculation fermented cider (except the Sc+Km group) was higher than that in Sc. To clearly observe the differences in volatile compounds between the cider samples, the data obtained by GC-MS were used to prepare a heatmap, as seen in Figure 2B; the color (from blue to red) indicates that the relative content changed from low to high. The volatile flavor components of several cider samples were significantly different in species and contents, which may be caused by different species of non-_Saccharomyces_ yeast [5]. There were more volatile compounds in the yeast-inoculated samples than in FAJ, indicating that microorganisms produced many metabolites during fermentation. Furthermore, ciders produced by co-culture fermentation had more species of volatile compounds than Sc; this result was in accordance with a previous study [25]. As shown in Figure 2C, the total concentrations of esters, alcohols, and acids in different cider samples were different. The total concentration of esters in FAJ without yeast inoculation was significantly lower than that of other cider samples. Except Sc+Km having the highest total ester concentration, there was no significant difference between the total ester concentration in the co-inoculation fermented groups and the Sc. Figure 1: The concentration of organic acid in apple juice and cider samples fermented with different non-_Saccharomyces_ yeast. Results represent the mean \(\pm\) SD for three independent experiments. Different letters represent significant differences at the 95% confidence level (Duncan’s test). Sc, _S. cerevisiae_ monoculture. Sc+Zb, _S. cerevisiae_ and _Z. bailii_ co-inoculation. Sc+Rm, _S. cerevisiae_ and _R. mucilaginousa_ co-inoculation. Sc+Km, _S. cerevisiae_ and _K. marxiamus_ co-inoculation. Sc+Dh, _S. cerevisiae_ and _D. hansenii_ co-inoculation. FAJ, apple juice. Based on relevant reports on the aroma threshold [26; 27], the aroma values of the odor activity values (OVA) were calculated and the characteristic aroma components were determined. OVA and aroma description reference the predecessors' research [23; 26; 28]. OVA is the ratio of the amount of a compound to the aroma threshold of that compound. An OVA greater than 1 is the aroma component that is of great importance in the flavor of apple juice or cider, which is called the characteristic aroma component of apple juice or cider. The contribution to the aroma of cider will increase with increasing OVA. As shown in Supplementary Table S2, there were certain differences in OVA between apple juice and the five fermentation groups. FAJ has a great difference compared to other fermentation groups. There were 16 aroma substances with an aroma value greater than 1. 1-Butanol, 3-methyl-, 3-hexen-1-ol, (Z)-, hexanal, furfural, butanoic acid, 2-methyl-, ethyl ester, butanoic acid, 3-methyl-, ethyl ester, decanoic acid, ethyl ester, hexanoic acid, octanoic acid, and dodecanoic acid had high aroma values and significantly affected the overall aroma of the six groups of samples. The 3-hexen-1-ol, (Z)-, hexanal, furfural and butanoic acid, and 3-methyl-, ethyl ester aroma component imparted grassy, spicy, and almond flavors to the FAJ group only. After fermentation, the flavorful substance was not detected (3-hexen-1-ol, (Z)-, hexanal, furfural and butanoic acid, and 3-methyl-, ethyl ester). Hexanoic acid, ethyl ester and acetic acid, 2-phenyl ethyl ester were detected in other fermentation groups, except for FAJ; they give the cider its green apple flavor and sweet honey flavor. The aroma value of the co-inoculation fermented groups was higher than that of the Sc group.