## Materials and Methods
### The Extraction of Flavonoids  
Mature berry skins were manually peeled off in frozen status. Then, the skin was ground into powder, which was freeze-dried at \(-40\)\({}^{\circ}\)C. The extraction of flavan-3-ols was consistent with the method described by [PERSON] et al. [36]. The procedure of extracting free flavan-3-ol monomers was as follows: 0.100 g dried skin powder was mixed with 1.0 mL acetone/H\({}_{2}\)O (7:3, \(v/v\)) and 0.005 g ascorbic acid. The mixture was shaken for 15 min and then centrifuged for 15 min at 8000 rpm. The extraction of residue was conducted twice. All the supernatants were pooled, and 400 \(\upmu\)L of the pooled supernatants was dried rapidly with a stream of dry nitrogen at 30 \({}^{\circ}\)C. The dried samples were dissolved in 200 \(\upmu\)L acidified methanol with 1% (\(v/v\)) HCl and then neutralized with 200 \(\upmu\)L aqueous sodium acetate (200 mM). The extraction of proanthocyanins is as follows: dried skin powder (0.100 g) was mixed with a solution (1.0 mL) of 0.3 M HCl in methanol with 50 g/L phloroglucinol and 5 g/L ascorbic acid. The mixture was incubated for 20 min at 50 \({}^{\circ}\)C and then combined with 1.0 mL aqueous sodium acetate (200 mM). The mixture was centrifuged for 15 min at 8,000 rpm. The residue was extracted twice. All the supernatants were collected and stored at -40 \({}^{\circ}\)C until analysis. Flavonols and anthocyanins were extracted in accordance with the procedure reported by [PERSON] et al. [37]. Dried skin powder (0.100 g) was macerated and sonicated in 50% (\(v/v\)) methanol in water (1.0 mL) for 20 min. The extraction was then conducted with centrifugation for 10 min at 12,000 rpm. The supernatant was collected and the residue was extracted twice. All the supernatants were pooled and stored at \(-40\)\({}^{\circ}\)C.  
High Performance Liquid Chromatography-Mass Spectrometry (HPLC-MS) Analysis of Flavonoids in Berry Skins  
In accordance with the method described by [PERSON] et al. [38], flavan-3-ols in matured grape skin was analyzed using high performance liquid chromatography/triple-quadrupole tandem mass spectrometry (HPLC-QqQ-MS/MS). An Agilent 1200 series HPLC coupled with a Poroshell 120 EC-C18 column (150 \(\times\) 2.1 mm, 2.7 \(\upmu\)m) and diode array detector (DAD) and an Agilent 6410 QqQ instrument equipped with an electrospray ionization source were used. The analysis of flavonoids in mature grape skin was performed, consistent with the method described by [PERSON] et al. [39]. Agilent 1200 series HPLC-MSD trap VL linked simultaneously to a Zorbax EclipseXDB-C18 column (250 \(\times\) 4.6 mm, 5 \(\upmu\)m) and variable wavelength detector was used. Anthocyanins in mature grape skin were analyzed using Agilent 1100 series HPLC-MSD trap VL (Agilent, Santa Clara, CA, USA) coupled with a reversed phase Zorbax SB-C18 column (250 \(\times\) 4 mm, 5 \(\upmu\)m) and diode array detector (DAD) in accordance with the method reported by [PERSON] et al. [40]. The content of flavan-3-ols was determined using (+)-catechin (C), (\(-\))-epicatechin (EC), (\(-\))-epigallocatechin (EGC), and (\(-\))-epicatechin-3-O-galactic (ECG), which were used as external standards. Flavonols and anthocyanins were quantified using quercetin-3-O-glucose and malvidin-3-O-glucose as external standards, respectively. All the concentrations of flavonoids were expressed as mg/kg berry fresh weight in this study.