Estabilidade e potencial antioxidante de antocianinas do resíduo agroindustrial da uva Isabel cultivada no Vale do São Francisco

Abstract

Large amounts of wine by-products are produced as a result of processing grapes in Brazil. Considering the presence of bioactive pigments in this material, the aim of this work was to investigate the antioxidant potential and stability of anthocyanins from the grape juice pomace. The grape residue (Vitis labrusca, cv. Isabella) was ceded by an industry producing of pure grape juice, located in the region São Francisco Valley, in the city of Petrolina-PE. The anthocyanin content was determined using pH-differential method. The isolated anthocyanin were identified and quantified by chromatography analysis using a RP-HPLC-DAD system. The antioxidant activity of the purified anthocyanins were evaluated for their ability to scavenge free radicals DPPH• (1,1-diphenyl-2-picrylhydrazyl) and ABTS•+ [2,2'-azino-bis (3-ethylbenzthiazoline-6sulfonic acid)], using gallic acid as standard. Purified anthocyanins were diluted in ethanol solution 95%: HCl 1.5N; 85:15 v/v (pH = 1.0) to evaluate the light stability, and distributed into test tubes, exposed and not exposed to light for a period of 840 hours (35 days) at 21°C ± 1°C, monitored by spectrophotometer readings. The effect of temperature on the stability of anthocyanins was evaluated using test tubes containing purified anthocyanins diluted into water, acidified with HCl (pH = 1.0), and subjected to temperatures of 50°C, 70°C and 90°C in thermostatic bath. Absorbance readings were made in the control sample (0h), and after 1, 3, 5 and 7 hours of heating. The absorbance data were used to calculate the rate constant (k) and half-life (t1/2) of anthocyanins through first-order kinetics equations. The average content of anthocyanins was 31.66 ± 0.63 mg. 100g-1 residue (expressed as malvidin-3-glucoside). A total of six majority different anthocyanins were identified: cyanidin-3-O-glucoside, delphinidin-3-O-glucoside, malvidin-3-O-glucoside, malvidin-3,5-O-diglucoside, pelargonidin-3-O-glucoside and peonidin-3-O-glucoside, with different concentrations. Malvidin 3-glucoside was the most abundant (61.66%) of the total observed, while malvidin-3,5-O-diglucoside (14.88%) was the second. The anthocyanin fraction showed high antioxidant potential against the radical DPPH• (EC50: 0.26 g. g DPPH•-1; TEC50: 1.37 minutes; AE: 2.79). After 5 minutes of reaction, at concentrations of 5-40 μg. mL-1, showed variations in the percentage scavenging between 48 and 94%, respectively, though there is a high correlation between anthocyanin content and antioxidant capacity (R = 0.93). Against the radical ABTS•+ anthocyanins exhibited significant action in capturing the radical with value of 199.35 uM Trolox. g-1. The half-life (t1/2) of the anthocyanin pigments, in the absence of light, was 3.320h (138 days) and in the presence of light was 505h (21 days), with the follows rate constants (k): 2.09 x 10-4 h-1 e 1.37 x 10-3 h-1, respectively, showing that presence of light contributed significantly to the instability of anthocyanins. In the study of heat stability, the rate constants (k) were: 1.84 x 10-2 h-1 (50°C); 7.82 x 10-2 h-1 (70°C); and 1.99 x 10-1 h-1 (90°C). The half-life of the samples occurred after 3.5h at 90°C and would occur after 8.9h and 37.7h for the samples at 70°C and 50°C, respectively, indicating the higher instability of anthocyanins to high temperatures. Anthocyanins from cv. Isabella grape juice residue are a promising source of natural dye which has antioxidant action with potential application in food products with an acidic pH, packaged in opaque packaging, and not subjected to severe heat treatment during processing.