Pistachio Skin Phenolics Are Destroyed by Bleaching Resulting in Reduced Antioxidative Capacities

Pistachio shells split naturally prior to maturity leading to their unique crack-shell form. Within 24 h of harvest, hull-trapped moisture may cause shell staining. The illegal process of bleaching has been used to restore a desirable white color to pistachio shells. It is not known whether bleaching adversely affects phytochemical levels in pistachios. Therefore, we identified for the first time multiple pistachio skin phenolics as quercetin (14.9 μg/g), luteolin (10.0 μg/g), eriodictyol (10.2 μg/g), rutin (1.6 μg/g), naringenin (1.2 μg/g), apigenin (0.2 μg/g), and the anthocyanins, cyanidin-3-galactoside (696 μg/g) and cyanidin-3-glucoside (209 μg/g). We investigated the effects of bleaching (0.1−50% hydrogen peroxide) on phenolic levels and antioxidative capacities in raw and roasted nuts. Because of their flavylium cation structures, anthocyanins were the most sensitive to bleaching. Bleaching decreased total anthocyanin levels [μg/g of skins (% hydrogen peroxide)]: 905 and 549 (0%); 653 and 145 (0.1%); 111 and 18.4 (5%); 6.1 and 3.2 (25%); 0 and 0 (50%) for raw and roasted nuts, respectively. Bleaching also reduced antioxidative capacity [μM/g of Trolox (% hydrogen peroxide)]: 945 and 725 (0%); 940 and 472 (0.1%); 930 and 455 (5%); 433 and 370 (25%); 189 and 173 (50%), for raw and roasted nuts, respectively. Raw nuts preserved phenolic levels and antioxidant capacity better than roasted nuts, suggesting contributing effects of other substances and/or matrix effects that are destroyed by the roasting process. The destruction of bioactive phenolics in pistachio skins may negatively impact the potential health benefits arising from pistachio consumption. Keywords: Pistacia vera; pistachios; skin phenolics; antioxidant; bleaching; roasting