Grape Pomace Polyphenol Quantification

The total polyphenol contents of grape pomace extracts and purified extracts were determined using the Folin–Ciocalteu colorimetric method [58 ,59 ]. Briefly, 20 µL of sample was mixed with 1.58 mL water and 100 µL of Folin–Ciocalteu reagent. After 8 min, 300 µL of 20% sodium carbonate solution was added. The mixture was vortexed for 10 s and then incubated at 40 °C for 30 min. The absorbance was measured at 765 nm using ultraviolet-visible spectrophotometer (Shimadzu, Japan). Total polyphenolic contents of samples were expressed as gallic acid equivalents (GAE) in mg L⁻¹ of bulk solution. It is important to note that individual polyphenols can show different gallic acid equivalency [60 (link),61 ]. Although two different mixtures of polyphenols result in the same GAE, the distribution of the individual polyphenols in a given mixture can significantly differ from the other mixture. Therefore, the composition of the grape pomace extract and recovered polyphenols were further characterized via HPLC.