Advantages of Liquid Nitrogen Quick Freezing Combine Gradient Slow Thawing for Quality Preserving of Blueberry

Lina Cheng,Ying Zou,Yujuan Xu,Yuanshan Yu,Jijun Wu,Jing Wen,Jieli Zhu,Gengsheng Xiao,Weijun Wu,Haocheng Liu,Kejing An

doi:10.3390/cryst10050368

Abstract

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.

Highlights

Blueberry, one of the fruits highest in bioactive ingredient contents, such as polyphenol, polysaccharides, and Vitamin C, has a lot of health benefits for human beings including being anti-cancer, improving memory, and protecting the cardiovascular system
The total freezing time of −80 ◦ C group is significantly shorter than that of Cao’ study. This is because the transfer heat speed of liquid nitrogen spray freezing is faster than that of low-temperature refrigerator freezing, which is a type of contact heat transfer
At different freezing temperature, immersion freezing, and refrigerator freezing is about −1.5~−3 ◦ C

Summary

Introduction

One of the fruits highest in bioactive ingredient contents, such as polyphenol, polysaccharides, and Vitamin C, has a lot of health benefits for human beings including being anti-cancer, improving memory, and protecting the cardiovascular system. Fresh blueberries have a short self-life, which limits their commercial and edible value [1]. Blueberries can be preserved with a maximum shelf-life of 2–4 weeks, which relies heavily on cold chain and high humidity storage conditions [2]. Freezing is the foundation and core technology of food preservation. With rapid heat transfer or fast freezing rate, the time of a food product within the maximal crystal forming temperature range during NF process is short, resulting in small and uniformly distributed ice crystals [3,4,5]. Sanchez-Alonso and coworkers [6] observed that NF showed better results in cod quick freezing (quick freezing time, T21 moisture retention) compared with blast freezing and contact small space freezing. Lopkulkiaert and coworkers [7] demonstrated that NF is significantly superior to air-cooling and shelf-contact freezing in terms of freezing rate and moisture retention in a quick-freezing study of Penaeus vannamei

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Conclusion