## 3 Results and Discussion
### Classification Comparison of Volatile Compounds in Rose Beverage
Conversely, approximately half of the ester compounds were significantly upregulated in the SFR sample, indicating that yeast fermentation strongly promoted the conversion of acids and alcohols into esters. This observation is consistent with the decreasing trend in alcohol content observed in the same sample. Among the esters, isoamyl acetate (3-methylbutyl acetate) and ethyl octanoate were the most significantly upregulated in SFR samples, imparting a stronger fruity aroma to the rose beverages. Furthermore, we observed that the content of 2-phenethyl acetate was higher in the co-fermented rose beverage (LSFR) than in the monoculture samples. 2-phenylethyll acetate is an aromatic ester found in fermented rose beverages, possessing floral, fruity, and honey-like aromas. It can be formed by the esterification of 2-phenylethyll alcohol with acetate or through the transesterification with acetate [64]. Our earlier analysis showed that the phenylethyll alcohol content was lowest in the LSFR sample among the fermented beverages, providing a plausible explanation for the origin of 2-phenethyl acetate. This result suggests that co-culturing L. plantarum with S. cerevisiae could promote the conversion of 2-Phenethyl acetate from phenylethyll alcohol. Terpenes (monoterpenes and sesquiterpenes) are considered major aromatic compounds in roses. A total of 51 terpen were detected across the rose samples (Table S2). Notably, four novel volatile compounds were identified in the fermented rose beverages, potentially due to the synthesis of specific terpen by the fermenting strains. Among the detected terpenes, 35 compounds were upregulated in the LFR group, while only 9 compounds were upregulated in the SFR group. This finding differs from a study by [PERSON] et al. [65], which reported reduced terpene content in orange juice after LAB fermentation. This discrepancy might be attributed to strain-specific characteristics. The _L. plantarum_ strain used in this study has strong \(\beta\)-glucosidase production and can release terpenoids through hydrolysis to enrich the aroma of rose beverages [66]. Aldehydes and ketones contribute to the complexity of food flavor, modulate aroma release, and play a decisive role due to their low odor thresholds [67]. Following fermentation, the content of most aldehydes, including benzaldehyde, furfural, and nonanal, decreased, consistent with previous findings [21]. As flavor components in food, aldehydes were unstable and could easily be converted into acids and alcohols under fermentation, which might be the major reason for their decreased levels [68]. 2,4-Dimethyl benzaldehyde exhibited the highest content in aldehydes, which was increased by 4.6- and 3.8-fold in LFR and LSFR after fermentation. A previous study has shown that 2,4-dimethyl benzaldehyde was likely the primary contributor to the sour odor produced by LAB [69]. Hence, the addition of yeast can effectively improve the sour flavor brought by LAB fermentation. Ketone is a predominant volatile compound in many fermented foods, especially in yogurt [70]. In this study, we identified 19 ketone compounds in four rose beverages, of which 1 and 3 ketones were significantly upregulated in SFR and LFR samples, respectively. In addition, we also find that the relative contents of three flavor compounds, including isophorone (3,5,5-trimethylcyclohex-2-en-1-one) [71], 2-heptanone (heptan-2-one) [21], and 2-nonanone (nonan-2-one) [72], were much higher in the co-fermentation sample than those in the monocultured samples. This result further confirmed that the synergistic effect of _L. plantarum_ and _S. cerevisiae_ is conducive to the accumulation of flavor substances. Acids enhance food flavor by staying below the odor threshold and balancing unpleasant odors [73]. In the rose beverages, seven aromatic active acid compounds were detected: geranic acid ((E)-3,7-dimethyl-2,6-octadienoic acid), decanoic acid, octanoic acid, citronellic acid (3,7-dimethyl-6-octenoic acid), nonanoic acid, hexanoic acid, and 2-methylbutyric acid.