# Chemical Composition and Flavor Characteristics of Cider
## 4 Discussion  
Wine yeast strains were added through co-inoculation fermentation to several cider samples to study their influence, having the following findings. Modulatory effects on physicochemical properties vary depending on the microorganisms utilized. The pH and glycerol of apple cider after inoculation were higher than those of apple juice. Sc has the highest acidity. In addition to Sc+Rm, the ethanol concentration of the co-inoculation fermented groups is low. Acetic acid is the major organic acid in cider. The acetic acid content of the Sc and FAJ groups was significantly lower than that of the co-inoculation fermented groups. Malic acid content was reduced in the co-inoculation fermented groups compared to that in the Sc group. In the aroma spectrum of cider samples, esters, alcohols, and acids are the most abundant and diverse volatile compounds in the samples. The contents of the corresponding substances of the main volatile esters, alcohols, and acids in cider are absent or very low in FAJ, such as decanoic acid, ethyl ester, phenylethyl alcohol, and octanoic acid. Phenylethyl alcohol, which can produce a rose scent, was relatively abundant in cider samples and varied greatly among the groups. This indicates that the aroma of cider is mainly produced in the fermentation process of yeast and has little relationship with the volatile aroma components of the raw material itself. This is consistent with the results of previous studies [36]. According to [PERSON], apples contain non-volatile glycoside precursors of substances such as phenylethyl alcohol. When apples are broken, the glycosides of phenylethyl alcohol can be hydrolyzed by plant enzymes or by enzymes produced by yeast during fermentation. These release the aroma of phenylethyl alcohol. Alternatively, phenylethyl alcohol may also be formed by yeast during fermentation, capable of synthesizing phenylethyl alcohol from substrates present in apples. The contents of ethyl lactate and 1-butanol in the Sc+Rm were the highest of all of the cider samples. Total acids and esters were highest in Sc+Dh, and total acid concentrations were significantly higher in Sc+Dh and Sc+Rm than in Sc. This indicates that the yeast species have an important effect on cider fermentation. There were 16 aroma substances with an aroma value greater than 1.0. The contents of hexanoic acid, ethyl ester and octanoic acid, ethyl ester in the Sc were the lowest among the five groups of ciders. The OVA of octanoic acid, ethyl ester with apple fragrance and decanoic acid, ethyl ester with cream fragrance in the Sc+Dh group was higher than in the other fermentation groups. Octanoic acid and acetic acid have a vegetable cheesy flavor and lauric acid has a mild fatty, coconut, bay oil flavor. The content of octanoic acid, acetic acid, and dodecanoic acid (OVA \(>\) 1) in the Sc+Dh group was higher than in the other cider samples. We found that the aroma of the cider fermented by the \(D\). _hansenii_ strain is characterized by a mild creamy aroma. The aroma values of the co-inoculation fermented groups were mostly higher than those of the Sc group. The difference between aroma types is that the cider fermented by \(Z\). _hailii_ has a prominent sweet aroma, the apple wine fermented by \(R\). _mucilaginosa_ has a strong hops aroma, the apple wine fermented by \(K\). _marxianus_ has a strong mellow and sweet aroma, and the apple wine fermented by \(D\). _hansenii_ has a rich milk aroma. These differences may be due to the different types of yeast in the fermentation of different levels of raw materials for apples in the process of adsorption, oxidation, hydrolysis, and polymerization [32]. The contents of most volatile flavor compounds were positively correlated with the co-inoculation fermented groups. Each yeast, according to its characteristics, leads to differences in metabolic substances and aroma components. Studies have found that the fermentation of apple juice can improve the undesirable flavor components, and co-culture fermentation can enhance the aroma of apple wine.