Berries are perishable fruits with high nutritional value. Freezing is an effective way for food preservation. Freezing and thawing methods play key roles in preserving edible values and commodity values of food. This study investigated the effects of spray liquid nitrogen quick freezing (NF_{−20~−100 °C}) and gradient thawing on the physical and functional characteristics of blueberries by using immersion and refrigerator freezing and microwave, ultrasonic, room- and low-temperature, and static-water thawing as comparisons. The results show that NF_{−80 °C} freezing combined with −20~−5~4 °C. gradient thawing can retain more than 95% of polyphenols and other nutritional compounds (including pectin, soluble sugar, and vitamin C) in thawed blueberries compared with fresh blueberries. Besides, this method shows the best results in preserving the hardness, cell structure, and water distribution of blueberries. It is also revealed that the ultralow temperature (−100 °C) freezing does not bring a significant advantage in preserving blueberries. Rapid thawing methods such as microwave and ultrasound thawing are not suitable for blueberries, which might be due to their small size and thin skin. The results suggested that the combination of NF_{−80 °C} freezing and −20~−5~4 °C gradient thawing is the optimal process for blueberry preservation. The outcomes of this study will serve as theoretical guidance for improving the industrial process for freezing and thawing blueberries.

中文翻译：

---

液氮速冻结合梯度缓融技术在蓝莓保鲜中的优势

浆果是易腐烂的水果，具有很高的营养价值。冷冻是保存食物的有效方法。冷冻和解冻方法在保持食品的食用价值和商品价值方面起着关键作用。本研究通过浸泡和冰箱冷冻以及微波，超声波，室温和低温，研究了喷雾液氮速冻（NF _{−20〜−100°C}）和梯度解冻对蓝莓的物理和功能特性的影响，和静态水解冻作比较。结果表明NF _−80°C冷冻并与-20〜-5〜4°C结合。与新鲜蓝莓相比，梯度融化可在融化的蓝莓中保留超过95％的多酚和其他营养化合物（包括果胶，可溶性糖和维生素C）。此外，该方法在保持蓝莓的硬度，细胞结构和水分布方面显示出最佳效果。还显示，超低温（−100°C）冷冻在保存蓝莓方面没有带来明显优势。诸如微波和超声波解冻的快速解冻方法不适用于蓝莓，这可能是由于它们的体积小且皮肤薄。结果表明NF _-80°C的组合冷冻和−20〜−5〜4°C的梯度融化是蓝莓保存的最佳过程。这项研究的结果将为改善蓝莓冷冻和解冻的工业过程提供理论指导。