Characterization of selected phenolic compounds in Georgia-grown muscadine grapes
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Muscadine grapes (Vitis rotundifolia Michx.) are indigenous to the southeastern United States but they grow wild from Delaware to the Gulf of Mexico. The increasing interest in grapes and wine is due to their antioxidant properties and claimed health effects. This interest has expanded to the muscadines as potential nutraceutical products. Major polyphenolics in muscadine grapes were determined by HPLC. Total phenolic content of grapes and leaves was determined by the Folin-Ciocalteu reagent method. Total antioxidant capacity was assessed as TEAC (trolox equivalent antioxidant capacity), and total anthocyanin content was determined spectrophotometrically by a pH differential method. The stability of unpasteurized muscadine grape juices, as affected by temperature and addition of ascorbic acid, was examined by following the kinetics of antioxidant capacity, total phenolic content, and total and individual anthocyanins during an 8 week storage study. Ellagic acid, myricetin, quercetin, kaempferol, resveratrol, the diglucosides of delphinidin, petunidin, malvidin, cyanindin, and peonidin, as well as petunidin monoglucoside were found in grape skins. Gallic acid, catechin, epicatechin, and delphinidin-3,5-diglucoside were found in seeds. Phenolics in muscadine leaves were myricetin, quercetin, gallic acid, ellagic acid and kaempferol with average concentrations of 213, 15, 28, 10 and 3% higher than in fruits, respectively. Phenolics in seeds contributed 75% of the total phenolic content in grapes and accounted for 95% of the total antioxidant capacity of the fruits. Leaves had on average, 667% more antioxidant capacity than the fruits. Anthocyanins accounted for about 11.5% of the total phenolics in grapes. Delphinin-3,5-diglucoside, petunidin-3,5-diglucoside and malvidin-3,5- diglucoside were the most abundant anthocyanins in the fruits. The addition of ascorbic acid negatively affected the stability of anthocyanins in juices and was accentuated by high storage temperature. Added ascorbic acid increased the antioxidant capacity of muscadine juices but interfered with the Folin-Ciocalteu method of determination of total phenolics. The by-products of muscadine processing and production like seeds, pomace, and leaves have the potential to be used as nutraceuticals due to their high content of polyphenolics.