Effect of continuous system ultraviolet light application on total phenolic content and antioxidant activity of commercial apple juices

Ultraviolet light treatment is one of the innovative non-thermal food techniques used in the food industry to ensure microbial safety. In this study, the effect of continuous system UV-C light treatment on the total flavonoid content (TFC), total phenolic content (TPC), antioxidant activity (by DPPH and ABTS methods), color values, browning index and concentration of some phenolic compounds of commercial apple juices were determined. Moreover, the kinetic modeling of the changes in these properties of apple juices was also carried out. A total dose of 240 J/cm2 was used in the continuous system UV-C light treatment, and a 5,68 log reduction was achieved on E.coli O157:H7 at this dose. While the initial TFC and TPC values were 2.53 mg CE/100mL and 11.00mg GAE/100mL, respectively, after 60 minutes of UV-C light treatment, these values were 1.80 mg CE/100 mL and 9.17 mg GAE/100 mL, respectively. Decreases in TFC and TPC were of the zero- and second-order kinetic models, respectively. On the other hand, UV-C light treatment decreased the antioxidant activity of apple juices determined by DPPH and ABTS methods by 50 and 30%, respectively, while increasing their browning index by 2.3 times (p˂0.05). While the decrease in antioxidant activity of apple juices by DPPH method was in accordance with the zero-order kinetic model, the decrease in antioxidant activity determined by ABTS method was in accordance with the second-order kinetic model. Application of UV-C light treatment for 60 minutes had a statistically significant effect on the color values of apple juices such as a*, b*, a*/b*, color index, chroma angle and hue angle (p˂0.05) while it did not influence the L* color (~26.5), pH (3.62) and Brix (11.7%) values of apple juices (p˂0.05). UV-C light treatment significantly decreased the gallic, protocatechuic, chlorogenic, gentisic, vanillic and caffeic acid contents of apple juices (p˂0.05), but the effect of this treatment on p-hydroxybenzoic acid content was statistically insignificant (p˃0.05). While the degradation kinetics of caffeic acid was of the second-order kinetic model, the degradation kinetics of other components was of the zero-order kinetic model.