Advantages of Spray Liquid Nitrogen Quick Freezing and Gradient Thawing for Quality Preservation of Litchi
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Abstract:
In order to explore the freeze-thaw characteristics of litchi and determine its suitable freeze-thaw method, the effects of spray liquid nitrogen quick freezing (-40 ℃/-60 ℃/-80 ℃/-100 ℃) and gradient thawing (-20 ℃/0 ℃/5 ℃/4 ℃/25 ℃) on the freeze-thaw characteristics and quality of litchi were investigated. The obtained results were compared with those of conventional freezer freezing, immersion freezing and ambient temperature thawing. The results show that in the range of -20 ℃~-100 ℃, the lower the temperature, the shorter the time required to pass through the zone of maximum ice crystal formation, and the better the protective effect on frozen litchi. The combination of spray liquid nitrogen quick freezing at -100 ℃ and a gradient thawing process (-20 ℃~-5 ℃~4 ℃~25 ℃) preserved over 85.00% of nutrients and bioactive substances of fresh litchi (polyphenol content, soluble sugar content, Vc content were 3.92 mg/g, 150.69 mg/g, 2.15 mg/g, respectively). This combined treatment caused only 5.05% of water loss and 57.00% of hardness retention rate, and maintained quite well the integrity of cell structures, which suppressed the flow of free water from the vacuoles towards the intercellular spaces between cells and in the cell walls. In summary, spray liquid nitrogen quick freezing combined with gradient thawing is a suitable freeze-thaw method for litchi, which can maintain its quality. The results of this study provide valuable support for the industrial storage and processing of litchi.
