Fluctuations in antioxidant capacity and polyphenolic content of small fruits in storage at -20°C for one year

Fruits are excellent sources of antioxidant compounds as well as vitamins, minerals and dietary fiber, therefore, the US Department of Agriculture’s healthy eating guidelines include a recommendation for consumption of at least 2.5 cups of fruits per day. Freezing at -20 °C is one of the most popular and convenient ways of storing small fruits long-term. The typical recommended storage time it is 8-12 months. In the present study we monitored fluctuations in antioxidant capacity (Ferric Reducing/Antioxidant Power assay (FRAP), 2, 2-diphenyl-1-picrylhydrazyl (DPPH) assay, and 2, 2´-azinobis (3-ethylbenzthiazoline-6-sulphonic acid) (ABTS) assay), total phenols (TP, Folin–Ciocalteu assay), total flavonoids (TF, colorimetric assay with Al2Cl3) and anthocyanins (TA, pH differential method) of 10 small fruits during one year storage at -20 °C. Sweet cherries, sour cherries, strawberries, red currants, raspberries, blackberries, hawthorn, cornelian cherries, and red and white grapes were chosen for the study. The first set of fruits (sweet cherries, sour cherries, strawberries, red currants and raspberries) was frozen in June 2009, and the second set of fruits (blackberries, hawthorn, cornelian cherries, red and white grapes) was frozen in September 2009. Phytochemical content was determined at equal time intervals. Initial TP contents in fruits ranged between 60.12-891.15 mg GAE/100g FW, TF content between 17.90-536.76 mg CE/100 g FW and TA content between 0-121.82 mg CGE/100g FW. The dynamic evolution of antioxidant capacity reflected the transient changes in phytochemical composition of analyzed fruits in storage. Fluctuations in phytochemical content observed during storage depended greatly on the type of fruit in question. Most of the analyzed fruits showed significant changes in phytochemical content and antioxidant capacity during and after one year of storage at -20°C.