Effects of combined dehydration treatment with different biological-source cryoprotectants on the freeze-thawed quality of blueberries

Blueberry fruits have been highly susceptible to decay and deterioration after harvest. Only a very small proportion can be used for fresh consumption. The majority must be frozen for storage and subsequent processing at present. However, the commonly used direct freezing of blueberries after harvest can reduce the utilization rates for subsequent processing. Some pre-treatments (addition of cryoprotectants and osmotic dehydration) before freezing have great potential to improve the quality of blueberries after thawing. The aim of this paper was to investigate the effect of Lactobacillus plantarum PC11 fermentative bacterial metabolites, dextran, and winter wheat protein combination sugar solution maceration treatments on the freeze-thaw quality of blueberries. According to previous experiments and pre-tests, the concentrations were determined and used in the test. Five groups were established with direct freezing and sugar solution impregnation (65 g/mL) as the control group. The experimental groups were PC11 fermentation bacterial metabolites (10 g/mL), dextran (1 g/mL), and winter wheat proteins (0.1 g/mL) in combination with sugar-liquid impregnation pre-treatment and then freezing. A comparative analysis was performed on the blueberry antioxidant capacity (free radical scavenging capacity, total antioxidant capacity), phytochemical content (total phenolics, ascorbic acid, total anthocyanins, and proanthocyanidins), textural and conformation (adhesion, hardness, chewiness, and elasticity), freezing parameters (subcooling point, freezing point, time to pass through the zone of the maximum ice crystal formation, time difference between subcooling and freezing point), juice loss rate, color difference, cosmetic qualities and cell membrane permeability. Three biogenic freezing acids were also used in the test. The results showed that the treatment with the acids was effective in protecting the quality of blueberries after freezing and thawing. The treatment group with the addition of PC11 fermentation bacterial metabolite (10 g/mL) was significantly different (P<0.05) from the rest groups; The indices of blueberries after freezing and thawing were effectively maintained, where the color difference value was 6.38, which was the closest to that of fresh samples. The shape of fruits was also much closer to the round shape of fresh blueberries, with relatively few wrinkles and browning; The hardness of 1.22 N increased by 9%, compared with the sugar impregnation group. The adhesion, chewability, and elasticity were 0.89 N, 0.56 N, and 0.65 mm, respectively; The anthocyanin and proanthocyanidin content shared a significant difference (P<0.05), with the content of 2.59 and 0.54 mg/g. The total retention of anthocyanin increased by 1.78 times, compared with the sugar impregnation group; The lowest juice loss rate was 6.32%, which was significantly lower than that of the sugar solution impregnation group (P<0.05, 1.36%); The lowest supercooling and freezing points were −7.63 °C and −3.47 °C, respectively. Compared with the sugar solution impregnation, the time to pass through the maximum ice crystal formation zone was shortened by 348.5 s, indicating a high freezing rate. It inferred that the PC11 fermentation bacterial metabolites belonged to the microbial fermentation metabolism, with the lower molecular weights to enter the cell by osmosis for the antifreeze effects. In conclusion, the combination of cryoprotectant impregnation pretreatment and freeze-thawing can be expected to improve the quality of color, texture, and active substance content of freeze-thawed blueberries. The pretreatment conditions and parameters were also optimized to successfully improve the nutrient retention and quality of blueberries. The finding can provide a strong reference for the frozen storage of blueberry during raw materials processing.