Effect of freezing rate on the quality of frozen strawberries (Fragaria x ananassa)

Domestic blast freezers have been designed to achieve fast freezing rates to preserve the properties of food. However, the relationship between the refrigeration system parameters and the food quality is not yet fully understood. The aim of this study was to investigate the effect of the refrigeration system operating conditions not only on the freezing rate but also on the food quality. To this end, an experimental apparatus capable of controlling the air temperature and velocity was designed and constructed. The freezing rate, drip loss and hardness of strawberry samples (Fragaria x ananassa) frozen applying different air temperatures and velocities were measured. Also, a mathematical model was implemented to predict the freezing rate. On comparing the numerical and experimental results, differences within a ±20 % error band were observed. In addition, the experimental results show a nonlinear relation between the operating conditions and the freezing rate, which was explained by the unbalanced convective and conductive thermal resistances during the freezing process. Regarding the quality of the strawberry samples, the hardness increased by 73 % and the drip loss reduced to around 5.5 % of its original value when the operating conditions were changed from -20°C / 0.4 m⋅s−1 (typical of ordinary household freezers) to -53°C / 6.5 m⋅s−1. Finally, a regression analysis was carried out to predict the drip loss as a function of the operating conditions, resulting in an exponential relation with R2 = 0.92.