The effect of freezing method and frozen storage conditions on the microstructure of wild blueberries as observed by cold-stage scanning electron microscopy

Microstructural information about fresh fruit is difficult to obtain using standard electron microscopy methods. Frozen fruit presents additional temperature-related challenges. However, the food industry would benefit from data that show structure/function relationships in fruit and changes that occur during freezing and frozen storage. The feasibility of using cold stage scanning electron microscopy (cryo-SEM) to describe and define the structure/function relationships of quality attributes of frozen wild blueberries was tested. Cryo-SEM was used to evaluate the structure of wild blueberries that had been frozen using three different methods, stored at three different temperatures, and held in frozen storage for 5 months. Micro-structural data were then compared with quality measures, which included changes in percentage drip, damaged berries, and anthocyanin leakage, to explore relationships between structural changes and observed quality changes. Cryo-SEM results indicated that microstructural changes observed in frozen blueberry samples are characteristic of their temperature history. Quality measurements and microstructural data are in agreement that improvement of frozen blueberry quality resulted from faster freezing rates and lower temperatures of storage than are used presently in standard production methods. This study indicates that cryo-SEM can be a valuable tool in assessing frozen blueberry quality and developing optimal conditions for freezing wild blueberries.