## 4 Discussion
plantarum_ TUST-232, the R-B mixed juice showed great changes in the composition and relative content of some main volatile compounds, such as hexanal, (ZZ)-2-heptenal, 2-octenal, benzaldehyde, 1-octen-3-one, hexanoic acid, and decanoic acid. These compounds have been identified as important volatile compounds in the foods fermented by _Lactobacillus_[47]. Among the main volatile compounds, due to the fermentation of _L. plantarum_ TUST-232, the relative contents of acids and aldehydes were 31.61% and 42.27%, respectively, and they played a key role in the characteristic flavor of the R-B mixed juice. Previous studies showed that the relationship between the volatile compounds and fermentation substrates was complex [48]. The fermentation of _L. plantarum_ TUST-232 increased the content of some organic acids, which not only improved the acidity of the beverage, but also changed the relative contents of some volatile compounds. Malolactic fermentation (MLF) is a process via which _L. plantarum_ converts malic acid into lactic acid and carbon dioxide through the secretion of the malolactic enzyme (MLE). After 14 h of fermentation, the content of malic acid in the mixed juice decreased significantly and the content of lactic acid increased significantly, indicating that the MLE secreted by _L. plantarum_ TUST-232 was highly active in performing MLF during the fermentation [49]. At the same time, MLF made it possible for _L. plantarum_ to grow well without adding nutrients before fermentation. In addition, the MLF can improve the flavor and taste of beverages, produce many volatile secondary compounds, and improve the stability [50]. Lactic acid participated in ester metabolism during the fermentation, which led the formation of some esters. After fermentation, the relative content of olefins decreased by 17.13%, which was related to the participation of malic acid in the metabolic pathway of olefins [48]. The relative content of acetic acid increased significantly after fermentation, indicating that _L. plantarum_ TUST-232 activated the acetate kinase route of the phosphogluconate pathway [51]. _L. plantarum_ could consume lactic acid and release the same amount of acetic acid under aerobic conditions [47]. After fermentation, the R-B mixed juice showed great changes in the relative contents of several main aldehydes, such as the disappearance of furfural and heptanal, which could help to reduce the \"caramel-like\" flavor and the strong fruity odor of the beverage, respectively [52]. Meanwhile, a significant increase was observed in the relative contents of 2-octenal and 2-undecenal; the former has the aromas of fat, meat, cucumber and chicken, while the latter has a strong aroma of unsaturated aldehyde fragrance, both of which may improve the flavor of the beverage. The relative contents of ketone, ester, and alcohol also varied in their effects on the flavor of the beverage. After fermentation, some ketones could synergistically enhance the fruity aroma of the fermented beverage [45]. The beverage contained some fatty acids, such as methyl ketones (2-nonanone, 2-octanone), which could have been formed by \(\beta\)-oxidation of fatty acids and decarboxylation of the produced \(\beta\)-keto acids [47]. Before fermentation, 6,10-dimethyl-5,9-undecaden-2-one was not detectable, but it was detected to reach a certain amount after fermentation, endowing the beverage with a sweet rose aroma. The formation of esters was mainly related to the consumption of sugars by _L. plantarum_[53]. Due to a lower odor threshold of esters than alcohols, the esters in the beverage after fermentation may produce a great impact on the aroma of the beverage, and the relatively low contents of esters such as methyl laurate, decyl propionate, and benzyl salicate may have positive effects on the beverage flavor [46]. Meanwhile, the formation of esters was also related to the abundance of alcohols in the beverage. During fermentation, _L.