The effects of calcium chloride and sucrose prefreezing treatments on the structure of strawberry tissues

The structural changes in strawberry tissues during prefreezing treatments, freezing and thawing were studied by means of textural and drip loss measurements as well as by bright-field and confocal laser scanning microscopy (CLSM), and by Fourier transform infrared (FT-IR) microscopy. Calcium chloride or sucrose were used as dipping pretreatment agents before freezing. In the calcium chloride test series a full factorial screening design was used. The three-level factors included were the CaCl₂concentration of the dipping solution, the dipping time and the temperature of the solution. Two kinds of sucrose treatments were used. Strawberries were either dipped in water–sucrose solutions or were sprinkled with crystallized sucrose. The CaCl₂or sucrose pretreatments did not significantly affect the drip loss of thawed strawberries compared to water dips or untreated reference samples. The pretreated strawberries were firmer than the untreated control samples but some of the CaCl₂- and sucrose-treated strawberries were less firm than the strawberries dipped in water. According to microscopical studies, both CaCl₂and crystallized sucrose pretreatments affected the microstructure of strawberry tissues. These pretreatments especially affected pectin, protein, lignin and structural carbohydrates in the vascular tissue and cortex compared to the untreated reference samples. CaCl₂-treatment had an even stronger effect than sucrose on the above compounds except lignin. The pretreatments did not seem to affect the epidermis, hypodermis or pith.