Comparison of methods for compositional characterization of grape (Vitis vinifera L.) and apple (Malus domestica) skins

A fundamental prerequisite for upgrading of fruit skins in press residues from juice, cider, and wine processes to food ingredients or supplements is the provision of methods for evaluation of the phenolics and cell wall carbohydrates in the materials. This study compared the monosaccharide yields obtained for different wine grape skin (Cabernet Sauvignon, Merlot, and Shiraz) and apple skin (Red Delicious (RD) and Golden Delicious (GD)) samples by trifluoroacetic (TFA) acid vs. hydrochloric (HCl) acid hydrolysis. Recovery values of monosaccharide standards after the acid treatments were also compared. TFA hydrolysis (2 M, 121 °C, and 2 h) resulted in higher monosaccharide yields from the fruit skin samples than HCl hydrolysis under the same conditions—after recovery value adjustments. Analyses of the phenols after extensive, sequential aqueous methanol (60%, w/w) extraction of the fruit skins confirmed that grape skins are a good source of anthocyanin pigments, notably the 3-glucosides of malvidin and cyanidin, and demonstrated that apple skins are a potential source of catechins.