## 2 Results and Discussion
### Phenolic Compounds
#### 2.3.1 Non-Anthocyanin Phenolic Compounds
Quercetin and its glycosides appeared to be the predominant flavonoids (Table 4). The result was consistent with a previous report [45]. The freeze concentration treatment did not significantly affect the total content of flavonoids in the wines after fermentation (Table 4). However, some individual flavonoids were observed to be different between the treated and the control wines. For example, quercetin-3-O-rutinoside and quercetin-3-O-rhamnetin were only detected in the treated wine. The treated wine contained higher content of kaempfer-3-O-glucose, but lower content of quercetin, taxifolin, and isorhamnetin. After one-year aging, the content of flavonoids in the treated wine was similar to that in the control except that higher contents of kaempfer and isorhamnetin were observed (Table 4). Glycosic conjugated flavonoids showed a content decrease after one-year aging, whereas an increase in their aglycones was observed. The decrease of flavonoid glycosides was mainly due to condensation, oxidation, and copigmentation reactions [46], as well as hydrolysis of the glycosides to produce corresponding aglycones [47]. Particularly, the copigmentation of flavonoids with anthocyanins has been reported to result in color enhancement of wine after aging, since the copigmented anthocyanins have a more stable structure [44; 45]. Flavonols also provide bitterness and astringency to wine [48]. However, the freeze concentration treatment did not favor the flavonoid contribution to wine quality due to its negligible influence on flavonoid contents. These wines after the fermentation process contained eight phenolic acids, including three hydroxycinnamic acids and five hydroxybenzoic acids (Table 4). These phenolic acids existed in the wines at a low content. Hydroxycinnamic acids can serve as strong antioxidants and can be oxidized to yield brown pigments [48]. Besides, they also play important roles in stabilizing anthocyanins and improving the color of wine [48]. After the fermentation, gallic acid appeared to be the predominant hydroxybenzoic acid in these wines, and its content was higher in the control (Table 4). Similarly, caffe acid was also observed in a higher content in the control. After one-year aging, hydroxycinnamic acids showed a content increase, whereas a decrease in the content of hydroxybenzoic acids was observed (Table 4). As a result, caffe acid appeared to be the predominant phenolic acid in these aged wines. The increase of hydroxycinnamic acids could be caused by the hydrolysis of esters during aging [46]. Besides, these molecules can also participate in polymerization and/or oxidation [9; 49]. Hydroxybenzoic acids, like gallic acid, also experience oxidation, esterification, and/or acetylation during aging, resulting in content decrease [46; 50]. Stilbenes have been confirmed to show health-promoting benefits in wine [42]. In the present study, a total of five stilbenes was detected in the treated and control wines (Table 4). There was no consensus on stilbene evolution during aging, since both increases and decreases of them have been reported in different studies [42; 51]. The stilbenes in our study showed significant decrease during aging, and no significant content difference was found between the treated and control wines. Therefore, freeze concentration treatment did not affect the content of stilbenes in wine.