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Pol. J. Food Nutr. Sci. 2007;57(Special issue 4C):601–605
The purpose of this study was to compare the effects of the different temperatures and time of apple convective drying on the phenolic compounds content and their antioxidant activity. The apple were washed, peeling and cut into cubes of 1 cm before processing, and was dried at three different temperatures of 50, 60, 70oC during 14 h of convection drying method. Phenolic profiles determined by HPLC and antioxidant activity (ABTS, DPPH, FRAP) of apples were analyzed after 2, 7, 10 and 14 hr of drying. The main phenolic compounds identified and quantified in dried apples was chlorogenic acid, p-coumaric acid, phloretin-2’-O-glucoside, phloretin-2'-O-xyloglucoside, (+)-catechin, (-)-epicatechin, procyanidins B1, B2, C1. The total phenolics content in fresh apple was 224.82 mg/100 g of dw. Procyanidins were the most predominant phenolic group and contributed to 73.68% of total phenolics content. The total content of procyanidins ranged from 165.64 mg/100 g dw in fresh apple to 67.51, 44.05, and 32.26 mg/100 g dw in dry apple after it was exposed to 14 h of hot-air drying process at 50oC, 60oC, and 70oC, respectively. p-Coumaric and chlorogenic acids during drying process were successively degraded. Dihydrochalcones content decreased with the rise of temperature and the time of processing. After 14 hours of drying at 60oC and 70oC, 100% loss of compounds belonging to dihydrochalcones occurred in apples. The antioxidant activity of tested dried apple samples was very strongly correlated with the content of phenolic compounds. The obtain results ranged from 5.51 µmol T/100 g dw for ABTS to 41.10 µmol T/100 g dw for DPPH and 28.56 µmol T/100 g dw for FRAP for fresh apples. Drying processes might destroy some of the phenol compounds what strongly correlated with antioxidant activity losses. The higher losses of antioxidant capacity, especially for DPPH and ABTS, was observed after 7 hours of drying process. Our results indicate that the drying process together with the rise of temperature and processing time could diminish the phenolic compounds content and antioxidant activity of apples. According to these results, it would seem clear that the most appropriate drying temperature in order to preserve polyphenols content and the antioxidant capacity of apple would be around 50oC or around 70oC provided that the drying time is short.