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.

The aim of this work was to analyze the effect of different osmotic dehydration (OD) processes (sucrose, trehalose, glucose, and lactose) as a pretreatment for grape before exposure to freezing. The osmotic treatment had less effect on blueberry moisture content among different sugars. Impregnation with sugars decreases the freezing time of grapes, compared to untreated grapes. Trehalose and lactose had a remarkable effect on shortening the time for grapes to pass the zone of maximum crystallization of ice. It was concluded that drip loss and electrolyte permeability of cell membranes decreased and the soluble solids content increased after OD treatment. In addition, the firmness and L* values of trehalose‐treated grapes were significantly higher than those of other treatments (p < .05). In summary, osmosis with a carbohydrate solution not only increased the freezing rate but improved quality characteristics of grapes after freezing and thawing. Practical applications Grapes are subject to freezing damage during storage, including significant water loss, berry softening, off flavor occurrence, which reduce the commodity and consumption of grapes. OD can improve the freezing rate by reducing the moisture content in the raw material and reducing the impact of freezing on the quality of fruits and vegetables, which may be beneficial to the process of freezing grapes.

The changes on food quality during thawing process has attracted significant attention of the frozen food industry. The effects of immersion thawing on the quality changes of blueberries were investigated. Blueberries were stored in freezing temperature for 3 days after quick freezing, and then, they were thawed using four methods: water immersion thawing (WIT), sugar immersion thawing (SIT), ultrasound‐assisted water immersion thawing (UAWIT), and ultrasound‐assisted sugar immersion thawing (UASIT). Later, texture profile, physiochemical characteristics, and sensory evaluation analysis were conducted to evaluate quality changes of blueberry after thawing. The thawing procedure employed significantly affected the quality of fruits. The shortest duration for total thawing was observed for the UASIT process. The anthocyanins and total solid content were better retained when blueberries were thawed using SIT and UASIT processes. There was no benefit of using UASIT with respect to protecting the blueberry tissue structure. However, the consumer acceptability of blueberries was not significantly difference among the four thawing conditions. Each method had advantages and disadvantages, and we should choose the suitable means according to consumer and production demand. Practical application This work was aimed at determining if there were any changes on the quality of blueberries when subjected to various thawing treatments. The sugar immersion thawing could be beneficial to nutrient retention in the freezing and thawing processing of fruits.