Insight into the relationships between microstructure development and quality changes in frozen stored raspberries

Abstract

This study focuses on frozen raspberries, a product highly susceptible to freeze-induced damage yet widely used in the food industry, by examining the combined effects of freezing and frozen storage conditions on their microstructure and quality. Two freezing rates (fast and slow) and three storage temperatures (−5 °C, −12 °C, and −18 °C) were evaluated over a six-month storage period. Prior to freezing, the raspberries’ cellular structure and thermophysical properties were characterized. Throughout freezing and storage, microstructural changes were monitored using X-ray micro-computed tomography. After thawing, texture and drip loss were measured to assess quality deterioration. The results demonstrated that fast freezing better preserves raspberry microstructure, inducing fewer structural changes compared to slow freezing. Conversely, higher storage temperatures promoted recrystallization, resulting in increased structural degradation. The growth of larger ice crystals caused more extensive damage to cell walls and vacuoles, leading to greater drip loss, reduced turgidity, and diminished firmness upon thawing. Importantly, the study highlights a synergistic effect between freezing rate and storage temperature on microstructural evolution. Ice crystals in slowly frozen raspberries were more prone to recrystallization regardless of storage temperature, whereas fast freezing combined with lower storage temperatures effectively maintained smaller ice crystal sizes and better structural integrity. Overall, these findings underscore the critical role of controlling both freezing and storage parameters to minimize microstructural alterations and ensure optimal quality of frozen raspberries throughout the cold chain, for both consumers and industry stakeholders.