Immersion freezing of strawberries to improve postharvest quality

GALETTO, CD; VERDINI, RA; ZORRILLA, SE; RUBIOLO, AC

Viña del Mar

Congreso; 10th International Congress on Engineering and Food; 2008

International Association of Engineering and Food

Strawberry is a non-climacteric fruit highly appreciated for its excellent organoleptic properties, but also an extremely perishable fruit due to its soft texture, high softening rate and high sensitivity to fungal attack. Strawberries should be harvested ready for consumption, which means there is a very short period of fruit at its best quality. Research all over the world has proved that freezing is one of the most effective treatments to ensure long-term preservation. However, freezing produces changes in the histological structure of strawberries depending not only on speed of freezing and size of ice crystals but also on the structure of the individual types of tissues. Immersion freezing (IF) allows high rate of freezing with low cost. The aim of this work was to evaluate the effects of IF in CaCl2 solutions combined with pretreatment with pectin methylesterase (PME) on the quality parameters of strawberries. Fresh strawberries (Fragaria ananassa, cv. Camarosa) were used for the studies. The fresh fruits were randomly distributed among the experiments. Fresh non-pretreated strawberries were used as a reference (FRESH-CONTROL). One group of strawberries was frozen in a 30% (w/v) CaCl2 solution at -20°C (FROZEN-CA). In the enzyme test, strawberries were first dipped in PME solution at 38 ºC for 30 min and then they were frozen in the CaCl2 solution at -20°C (PME-FROZEN-CA). Another group of strawberries traditionally frozen were used as control (FROZEN-CONTROL). After 55 days of frozen storage, the strawberries were thawed for analysis. Weight loss, texture, total and cell-bound calcium content, pectin content, and degree of esterification of pectins were analyzed. Immersion freezing significantly decreased weight loss compared to traditional freezing from 9.6 to 4.7%. However, all freezing treatments similarly reduced firmness in comparison with FRESH-CONTROL from 16.4 to 4.3 N. Immersion freezing significantly increased total calcium content, but the relationship cell-bound/total calcium was similar for all frozen strawberries. In addition, the relationship cell-bound/total calcium was lower for all frozen strawberries than for FRESH-CONTROL, the relationship being 0.36 and 0.71, respectively. Both pectin content and degree of esterification of pectins were similar for all frozen strawberries and FRESH-CONTROL. These results increase the knowledge in the area of new technologies for fruit preservation by freezing.