Layered numerical simulation and heat transfer characteristic analysis on cold shock treatment of cucumber

Abstract: Cold shock treatment has been extensively studied as an effective physical treatment method to improve the storage quality of fruits and vegetables because of environmental benefit. To simulate cold shock treatment and study the heat transfer characteristics of cylindrical postharvest fruits and vegetables in those processes, a heat transfer model was proposed based on the structural features of fruits and vegetables in present study. The cucumber tissue was divided into three layers according to the structural characteristics, and thermophysical properties of each layer were also measured. Cold shock treatment of cucumber with 0℃ cold water was conducted in a constant temperature tank designed by the authors. The measured temperatures of the cucumber tissue at the center, two-fifths of the radius and four-fifths of the radius from the center of fruits during water cooling were compared against the results obtained from the simulation by the model. The maximum error between simulated and measured temperatures was 1.713℃, and the mean errors at the center, two-fifths of the radius and four-fifths of the radius form the center of the cucumber were 0.423, 0.377 and 0.842℃, respectively. The comparison analysis showed that the simulation results were in good agreement with the measured values, which indicated the reliability of the model. Using the validated model, the cold shock processes of the cucumbers with chilled air and cold water were simulated, and the effects of flow velocity of medium, fruit size and thermophysical property difference inside the cucumber on the heat transfer process were also analyzed. Results showed that, when cold shock treatment was conducted with cold water at 0℃, the cooling rate of cucumber tissue was faster, and it caused a shorter required duration and larger temperature gradient along the radius of the cucumber, which resulted that the chilling stress experienced by outside tissue was longer than that experienced by inside tissue of the cucumber. Different from cold water, the cooling rate of cucumber tissue conducted with chilling air was slower during cold shock treatment. This type of cooling required longer duration of cold shock treatment, and the temperature gradient was smaller along the radius of the cucumber, which resulted that chilling stresses experienced by all cucumber tissues were similar. Therefore, cooling medium should be selected according to the heat transfer characteristics of fruits and vegetables with different cooling mediums. The cooling rate increased largely with increased airflow speed of chilling air, which increased from 1 m/s to 5 m/s, and the convective heat resistance had great influence on the whole heat transfer process, so air speed should be improved to reduce the required duration of cold shock treatment. When the water velocity increased from 0.2 m/s to 1.8 m/s, the cooling rate of the cucumber increased by small amplitude, especially when it was more than 0.6 m/s, and the heat resistance of inner cucumber had a significant effect on the whole cooling process. The water speed should be reduced to lower the energy consumption of circulating pump. The thermophysical property difference had greater influence when cold water was used than that when cold air was used. The results can provide theoretical reference for optimizing the process parameters in cold shock treatment of postharvest fruits and vegetables.