Effect of freezing on the chemical composition of strawberries

Abstract

One of the most popular ways of berries preservation is freezing. Freezing prolongs the shelf life of berries, inhibits the growth of microorganisms and helps maintain their nutritional value. Most vitamins and other biologically active compounds remain in frozen products. The object of the investigation was fresh and frozen strawberries (Fragaria × ananassa (Duchesne ex Weston) Duchesne ex Rozier) ʽRumbaʼ, ʽAsiaʼ, ʽSonsationʼ and ʽMalwinaʼ cultivars grown on a farm in Joniškis district, Lithuania. The research aim was to evaluate changes in the chemical composition of strawberries during the freezing process. 3 kg of fresh strawberries of each cultivar were placed in polyethylene bags for refrigeration (3 bags of 1.0 kg each) and frozen at –37 °C. In the fresh and frozen strawberries were determined amount of dry matters, soluble solids, ascorbic acid by standard methods. The total phenolic content was determined spectrophotometrically using the Folin-Ciocalteu reagent. Total anthocyanins were established by measurement of absorbance at 538 nm with two-ray UVS 2800 spectrophotometer. The antiradical activity was determined using the DPPH method. The decrease in dry matter and soluble solids of frozen strawberries was insignificant compared to fresh ones. Only in ʽRumbaʼ berries, soluble solids content decreased by 8%. In different cultivars of frozen strawberries, vitamin C content was lower from 6 to 37%. The freezing process significantly reduced the total amount of phenolic compounds in the strawberries of all investigated cultivars but did not have a significant effect on anthocyanin’s content. The total anthocyanin’s content decreased for only 0.02–0.05 mg 100g-1. However, the antioxidant activity of strawberries after freezing decreased significantly. The freezing process reduced the nutritional value of the studied cultivars of strawberries although some of the changes were not significant.