## 3 Materials and Methods
### Total Phenols, Total Flavonoids, Total Anthocyanins, and Color Attributes  
The content of total phenols was determined using Folin-Ciocalteu method with minor modifications [62]. Briefly, the wine sample (0.1 mL) was mixed with 0.1 mL of Folin-Ciocalteu reagent and 2.8 mL of distilled water. The resultant mixture was incubated in the darkness for 8 min. Afterwards, the mixture was mixed with 2 mL of 7.5% (\(v/v\)) sodium carbonate solution, and then kept in the darkness for 2 h. The absorbance of the mixture was recorded at 765 nm on a spectrophotometer (Unico Instrument Co. Ltd., Shanghai, China). Gallic acid was used as the external standard, and the content of the total phenols in wine sample was expressed as mg gallic acid equivalents per liter of wine (mg GAE/L). Each measurement was carried out in triplicate.  
Total flavonoid content determination followed an aluminum chloride assay with minor modifications [63]. Briefly, 5 mL of 30% (\(v/v\)) wine sample was mixed with 0.3 mL of 5% (\(w/v\)) NaCO\({}_{3}\) solution. The resulting solution was kept for 6 min. The mixture was mixed with 0.3 mL of 10% (\(w/v\)) Al(NO\({}_{3}\))\({}_{3}\) solution and then kept for another 6 min. The resultant mixture was then mixed with 4 mL of 1 mol/L NaOH solution and 0.4 mL of distilled water and incubated at room temperature for 15 min. The absorbance of the mixture was recorded at 510 nm on the same spectrophotometer. Distilled water was used as the blank. Rutin was used as the external standard, and the total flavonoid content of the wine sample was expressed as mg rutin equivalents per liter of wine (mg:rutin/L). Each determination was performed in triplicate.  
Total anthocyanin content in bog bilberry syrup wine was assayed using the pH differential method [64]. In brief, the bog bilberry syrup wine (1 mL) adjusted the pH to 1.0 and 4.5 using acetate buffer solution. Then, the absorbance of the resultant sample was calculated using  
\[\text{A (absorbance)}=(\text{A}^{\text{PH1.0}}\text{5}_{\text{20 nm}}-\text{A}^{\text{PH1.0}}\text{7}_{\text{00 nm}})-(\text{A}^{\text{PH4.5}}\text{5}_{\text{20 nm}}-\text{A}^{\text{PH4.5}}\text{7}_{\text{00 nm}}). \tag{1}\]  
The total anthocyanin content was calculated using the equation below and expressed as mg cyanidin-3-\(O\)-glucose equivalents per liter of wine (mg Cyanidin-3-\(O\)-glucose/L):  
\[\text{Total anthocyanin content (mg/L)}=(\text{A}\times\text{MW}\times\text{ DF}\times 1000)/(\varepsilon\times 1) \tag{2}\]  
where MW is the molecular weight of cyanidin-3-\(O\)-glucose (449.2 g/mol), DF represents dilution factor, \(\varepsilon\) is the molar extinction coefficient (26,900 L \(\times\) mol\({}^{-1}\times\) cm\({}^{-1}\)), and \"1\" indicates the light length path (cm). Each measurement was conducted in triplicate.  
CIELab space method was used to determine the color attributes of the bog bilberry syrup wine samples [4]. The wine sample was filtered through a 0.45 \(\upmu\)m filter membrane and the transmittance at 440, 530 and 600 nm were determined using a UV-VIS spectrophotometer (Unico Instrument Co. Ltd., Shanghai, China). Distilled water was used as the blank.