Effect of Glassy State and Liquid Nitrogen Quick Freezing on the Quality Characteristics of Blueberries

The glass transition temperature T_g’ of ‘Northcountry’ blueberries was determined according to its state diagram. Using T_g’ as the freezing end point, blueberries were frozen at −80 ℃ using an ultra-low temperature freezer (RF−80 ℃), or at −80, −100 or −120 ℃ by direct immersion in liquid nitrogen (LN−80 ℃, LN−100 ℃, and LN−120 ℃, respectively). Freezing curves, juice loss, hardness, nutrient contents, cell membrane integrity, enzyme activities and microstructure were measured to investigate the effects of liquid nitrogen quick freezing combined with T_g’ on the quality characteristics of blueberries. The results showed that the T_g’ of blueberries was −52.55 ℃; compared with RF-80 ℃, the quality of liquid nitrogen frozen blueberries was better maintained. The freezing time of liquid nitrogen quick freezing was significantly shortened, the frozen fruit lost less juice after thawing, and the hardness was better maintained. The contents of soluble solids, titratable acid, ascorbic acid and anthocyanin were closer to those of fresh fruit, the relative conductivity and malondialdehyde (MDA) content were lower, the integrity of the cell membrane was less broken, the activities of peroxidase (POD) and polyphenol oxidase (PPO) were lower, and the microstructural compactness and integrity were stronger. The lower the ambient temperature of liquid nitrogen quick freezing, the better the freezing effect, and LN-120 ℃ minimized the quality deterioration of frozen blueberries. In conclusion, liquid nitrogen quick freezing significantly increased the freezing rate, and its combined with T_g’ helped to maintain good quality of frozen blueberries. The results of this study can provide a theoretical basis for the selection of blueberry freezing conditions.