Effects of freezing conditions on quality changes in blueberries

BACKGROUND Freezing preservation is one of the most effective methods used to maintain the flavour and nutritional value of fruit. This research studied the effects of different freezing conditions, −20 °C, −40 °C, −80 °C, and immersion in liquid nitrogen, on quality changes of freeze–thawed blueberries. The water distribution estimates of blueberries were measured based on low‐field nuclear magnetic resonance (LF‐NMR) analysis. The pectin content, drip loss, and fruit texture were also detected to evaluate quality changes in samples. RESULTS The freezing curves of blueberry showed super‐cooling points at −20 °C and − 40 °C, whereas super‐cooling points were not observed at −80 °C or in liquid nitrogen. After freeze–thaw treatment, the relaxation time of the cell wall water (T21), cytoplasm water and extracellular space (T22), and vacuole water (T23) were significantly shortened compared to fresh samples, which suggested a lower liquidity. Although the freezing speed for samples immersed in liquid nitrogen was faster than other treatments, samples treated at −80 °C showed better quality regarding vacuole water holding, drip loss, and original pectin content retention. CONCLUSION This study contributed to understanding how freezing temperature affects the qualities of blueberries. The super‐fast freezing rate might injure fruit, and an appropriate freezing rate could better preserve blueberries. © 2018 Society of Chemical Industry