Effects of osmotic dehydration treatments with sugar solution and calcium on the quality of freeze-thawed blueberry

Calcium salts are commonly used as specific texture-improving additives for preserved fruits and vegetables, pickles, and candied fruits. Osmotic dehydration modified freezing parameters of agro-products, through its high osmotic effect and solute-solvent exchange, effectively preserves the quality of blueberries in terms of color and active substances while facilitating the penetration of additives into fruit and vegetable cells. This study aims to investigate the impact of combining calcium salts as cryoprotectants with high-concentration sugar solution immersion during osmotic dehydration at room temperature on the textural quality of thawed blueberries. Comparison of the effects of three calcium salt cryoprotectants represented by calcium propionate, calcium lactate, and calcium chloride and treatments with concentrations of 0.5, 1.5, 3.0, and 5.0 g/mL calcium propionate on the quality of freeze-thawed blueberries. The effects on freeze-thaw quality were assessed by examining the appearance of blueberries after freeze-thaw, measuring quality indicators such as hardness, elasticity, chewiness, adhesiveness, cohesiveness, and resilience, as well as determining the content of soluble pectin, covalently bound pectin, and ionically bound pectin. The results indicated that the osmotic dehydration treatment with calcium salt and sugar solution effectively reduced wrinkling and texture softening of blueberries after freeze-thaw. The juice loss rate of the calcium propionate and sugar solution-treated group was approximately 25.42% lower than that of the control group, significantly lower than (P < 0.05) the other treatment groups, confirming calcium propionate as the optimal calcium salt additive. Optimization screening of the calcium propionate concentration revealed that at 1.5 g/mL, it significantly reduced juice loss and cell membrane permeability of blueberry fruits after freeze-thaw (P< 0.05) , resulting in a hardness of 1.54 N, elasticity of 0.75 mm, soluble pectin, covalently bound pectin, and ionically bound pectin contents of 1.03, 0.83 and 0.68 mg/g, respectively. This effectively mitigated the softening of blueberry fruits and maintained optimal freeze-thaw quality. The combination of 1.5 g/mL calcium propionate concentration with osmotic dehydration technology enhances the quality of freeze-thawed blueberries, making them suitable for consumption and processing. The results of this experiment showed that 1.5 g/mL calcium propionate combined with 65 g/mL sugar solution could effectively protect the texture parameters of blueberries after freeze-thaw, including hardness, elasticity, chewability, adhesiveness, cohesiveness and resilience, maintain the stability of long chain covalently bound pectin, and inhibit the formation of short chain soluble pectin and loose ion-bound pectin. And minimize the juice loss rate and cell membrane permeability. These results indicate that the osmotic dehydration of 1.5 g/mL calcium propionate combined sugar solution can maximize the effective combination of calcium ions and blueberry pectin, form a "calcium bridge" to inhibit the decomposition of long chain pectin, maintain the integrity of cell wall, reduce the damage of ice crystals to blueberries, and make blueberry fruits have better texture and quality. This study provides technical support and theory to solve the problem of texture softening caused by intolerant freezing of blueberry raw materials, and is conducive to promoting the application of new collaborative freezing technology in frozen storage of processed raw materials. The research findings not only provide a data foundation for addressing the issue of fruit softening caused by long-term freezing in the blueberry processing industry, but also provide a theoretical basis for developing cryoprotectants and modification techniques to enhance the frozen storage quality of fruits and vegetables.