Effect of airflow velocity on cooling kinetic of different layers of pomegranate by forced air precooling

The objective of the present study was to detemine the influence of airflow velocities on precooling process of pomegranate with thin peel (Shahvar variety), to evaluate the cooling rate and temperature distribution in this product in order to design and optimize the precooling system. Forced air cooling was applied for precooling the center (arils) and peel (outer layer) of pomegranate. Airflow velocities were 0.5, 1, and 1.3 m/s and the air temperature was 7.2°C in the air tunnel during the experiments. For calculating cooling rates (half and seven-eighths cooling time), lag factor and cooling coefficient were determined from the experimental data by a regression analysis. Also, cooling heterogeneity was evaluated in the center and outer layer of pomegranate at different airflow velocities. The results showed that with increasing the airflow velocity from 0.5 to 1.3 m/s, cooling rates increased. Furthermore, with enhancing the airflow velocity, half and seven-eighths cooling time decreased up to 19.35 and 21.76% in the center and by 32.95 and 19.63 in the peel, respectively. This finding was due to increasing convective
heat transfer coefficient. Cooling heterogeneity decreased from 0.5 to 1 m/s and then increased at the airflow velocity of 1.3 m/s. This parameter was dependent on airflow velocity, cooling coefficient and lag factor. The influence of airflow velocity was low after 6400s from starting the cooling process both in center and in peel. The overall results demonstrated that application of this method can explain unsteady heat transfer in precooling process of pomegranate and probably can be used for the other similarly shaped fruits.