## Results and Discussion
### Total Phenols, Total Flavonoids, Total Anthocyanins, and Color Attributes
#### 2.3.1 Esters  
Esters are mainly formed during the wine fermentation process under multiple enzymatic reactions [46]. Therefore, the production rate of esters in wine is essentially determined by the population and activity of yeasts [19]. Regarding their molecular and synthetic feature, esters in wine can be further divided into acetate esters, ethyl esters, and other esters [34]. It is known that esters are the major volatile compounds that contribute the fruity and floral notes to the overall aroma of wine [47]. Acetate esters are synthesized from the metabolism of lipids and amino acids linked with acetyl-CoA under a series of enzymatic reactions and different yeast strains and esterase levels significantly affect the distribution of acetate esters in wine [19; 48; 49]. In this study, acetate esters were the major ester group in the bog bilberry syrup wine (Table 3). The wine supplemented with 150 mg/L DAP showed the highest acetate esters level compared to other wines. Additionally, ethyl acetate, isoamyl acetate, and phenethyl acetate were the predominant individual acetate esters in these wine samples. These acetate esters represented more than 99% of the total acetate ester content (Table 3). Regarding their flavor notes, ethyl acetate provides wine with the pineapple, fruity, solvent, and balsamic aroma, whereas isoamyl acetate exhibits the banana, fruity, and sweet note to wine. Phenethyl acetate has been reported to contribute to wine with the flowery flavor [35]. The concentration of ethyl acetate and isoamyl acetate in the wine samples were higher than their thresholds, indicating that these volatiles could significantly contribute their flavor notes to the overall aroma of the wine. The wine supplemented with 150 mg/L DAP showed much higher ethyl acetate concentration compared to other wine samples. The similar enhancement was also found on the content of isoamyl acetate in the wine with 150 mg/L DAP. Additionally, the increase in the DAP amount resulted in a significant enhancement on the accumulation of phenethyl acetate in the wine after the fermentation. However, its OAV value in these wines was below 1, indicating that the flavor note of phenethyl acetate might not significantly contribute to the overall aroma of the bog bilberry syrup wine. Our results were consistent with the previous studies where acetate esters showed a significant increase in the content in wine supplemented with nitrogen resource [10]. We speculated that the addition of high DAP amount (150 mg/L) in the bog bilberry syrup before the fermentation could stimulate the growth of the yeasts, which could activate the expression of alcohol acetyltransferases in the yeasts. These eventually led to the formation of more acetate esters in the wine [46; 50]. The major ethyl esters in the wine samples included ethyl hexanoate, ethyl lactate, ethyl octanoate, ethyl 2-hydroxy-4-methylpentanoate, ethyl decanoate, diethyl succinate, ethyl 9-decenoate, ethyl dodecanoate, and ethyl myristate. Ethyl esters are biosynthesized from ethanol and fatty acids and these esters also contribute unique flavor notes to the overall aroma of wine [34; 51]. Although numerous ethyl esters were yielded in these wine samples after the fermentation, only some of them showed their OAV values above 1 (Table 3). The addition of DAP in the bog bilberry syrup wine also showed the effect on enhancing the accumulation of these esters in the wine (Table 3). For example, ethyl butanoate is known to have the strawberry, apple, and banana notes [35]. The ethyl butanoate level in the wine supplemented with 60 mg/L DAP started to be higher than its threshold, and its highest level was observed in the wine with 150 mg/L DAP. Similarly, ethyl hexanoate showed higher OVA value in the wine with 90 mg/L DAP, which could provide the wine with a better aroma with the fruity, green apple, banana, brandy, and wine-like aroma notes [35]. Additionally, the OAV value of ethyl octanoate was higher than 1 in the wine supplemented with 150 mg/L DAP (Table 3).