Effects of liquid nitrogen quick freezing on polyphenol oxidase and peroxide activities, cell water states and epidermal microstructure of wolfberry

Abstract

Fresh wolfberry has a very short shelf-life and drying is the most common method for preserving wolfberry. For better retention of the quality and nutritional values of wolfberry, effects of liquid nitrogen spray freezing at different temperatures of −60 °C ± 2 °C (NF-60°C), −80 °C ± 2 °C (NF-80°C) and −100 °C ± 2 °C (NF-100°C) on wolfberry were investigated, as compared with air-blast freezing (BF) at −40 °C ± 1 °C and air velocity of 0.75 m/s. Results showed that the freezing time passing the maximum ice crystal formation zone were 450 s, 150 s, 100 s and 70 s for BF, NF-60°C, NF-80°C, and NF-100°C, respectively. Comparing with NF-100°C, NF-80°C samples showed better appearance, lower peroxidase activity, water distribution more similar to fresh samples and lower damage of inner epidermal cell structure. The current study suggested that the freezing characteristics of wolfberry did not become better at the ultralow freezing temperature of −100 °C, and NF-80°C was considered the most appropriate freezing process for the freezing characteristics of the wolfberry. It is hoped that the current results could be useful to the industry for better preserving wolfberry.