Seasonal Variation of Qualitative and Quantitative Composition of Phenolic Compounds and Antioxidant Activity in Apple (Malus domestica Borkh.) Fruits

The aim of this study was to explore the peculiarities of the qualitative and quantitative composition of phenolic compounds and antioxidant activity variation in apple (Malus domestica Borkh.) fruit samples during their growth season. The highest total concentration of phenolic compounds (20.97±0.74 mg g-1) was detected during the initial period of the phenological fruit growth stage (31 days after full bloom - DAFB), and the lowest (1.61±0.05 mg g-1) – during the fruit maturation period (143 DAFB). Chlorogenic acid was the predominant compound in apple samples throughout the vegetation period – it comprised 24-32.12% of all identified and quantitatively evaluated phenolic compounds in apples. The highest concentration of chlorogenic acid (20.97±0.74 mg g-1) was detected at the beginning of the fruit development period.

In total, 6 quercetin glycosides were detected in apple samples: hyperoside, isoquercitrin, rutin, reynoutrin, avicularin and quercitrin. The highest total concentration of quercetin glycosides (2.75±0.08 mg g-1) was detected at the beginning of the fruit development period (31 DAFB), and the lowest (0.23±0.01 mg g-1) – during the fruit maturation period.

The antioxidant activity in apple sample extracts was analysed by applying 2,2-diphenyl-1-picrylhydrazyl (DPPH) and ferric reducing antioxidant power (FRAP) spectrophotometric assays. The strongest antiradical and reducing activity (TEDPPH reached 81.02 mmol g-1, and TEFRAP - 715.63 mmol g-1) was detected at the beginning of the fruit development period. The statistical correlation analysis showed a very strong positive correlation between the total amount of the identified phenolic compounds and the reducing activity of the apple extracts (Spearman’s correlation coefficient – R=0.927, p<0.01). There was also a strong positive correlation between the total amount of phenolic compounds and the antiradical activity of the apple extracts (R=0.770, p<0.01). Among individual compounds that have been identified and quantitatively evaluated via high-performance liquid chromatography (HPLC), the strongest correlation with antiradical and reducing activity was observed for quercetin glycosides - isoquercitrin (respectively, R=0.851 and 0.845, p<0.01) and hyperoside (R=0.770 and 0.891 p<0.01).