Numerical simulation and optimization of quick freezing process of strawberry by dry ice spray

Quick-frozen fruits and vegetables have a high demand on the processing device for rapid freezing, due to the ever-increasing requirements for the high quality and nutritional value to the taste of consumers in food production. The purpose of this study is to use the sublimation process of dry ice particles, further to form the superfast freezing in low temperature environment, in order to adequately keep the original color, flavor, and high quality nutrition of frozen products, shorten the freezing time, and improve the freezing efficiency. Taking the strawberry as the research material, natural dry ice particles were sprayed into a quick-freezing room, concurrently skimmed the surface of strawberry, and fully contacted with the strawberry to exchange the heat. Therefore, a new innovated platform was designed, including the fabrication of dry ice pellet, spraying system, a quick-freezing room, and a feasible pallet. A thermomechanical model of strawberry in the quick-freezing room was established, to simulate the distribution of temperature, velocity, and pressure during the dry ice spraying using the Comsol multiphysics software. The inlet radius of the dry ice jet was set under the conditions of 18, 20, 23, 25 and 30 mm, and the injection velocity was set as 0.10, 0.15, 0.20, 0.25, 0.30, 0.40, 0.50 m/s, thereby to determine the time when the core temperature and surface temperature of strawberry met the requirements of quick freezing. The solid fraction of dry ice in different time was compared-when the injection velocities of dry ice were 0.20 and 0.30 m/s. The results showed that the freezing time of strawberry decreased, while the freezing efficiency increased-with the increase of the inlet radius and the spray speed of dry ice. When the inlet radius was 25 mm and the flow velocity was 0.30 m/s, the core temperature of all strawberries reached -18℃ at 350s, indicating the uniform distribution of temperature, and the shortest duration between the strawberry surface to -35 ℃ and the strawberry core to -18 ℃. In this case, it was expected to effectively prevent the surface cracking caused by the excessive difference of internal and external temperature in a strawberry. Meanwhile, the solid fraction of exported dry ice was less than others, indicating the quick freezing of strawberry realized with the maximum efficiency. Compared with liquid nitrogen spraying, the zone of the maximum ice crystal formation and the freezing time of dry frozen strawberry reduced by 63.9% and 41.7%, respectively, indicating the excellent cooling performance using dry ice spray. In addition, the quick-frozen quality of strawberry was significantly improved, due to the less freezing time, the improved freezing efficiency, and the reduced energy consumption of system operation, meanwhile, to better preserve the strawberry juice, original color, flavor, and nutrition. An experiment was also used to verify the optimal parameters, where the experimental and simulation errors were 3.70% and 6.03%, respectively, when the surface temperature and core temperature reached the standard value. Specifically, the physical and chemical properties of strawberries were better than the standard after being quick-frozen. The finding can lay a sound foundation for further development of energy-saving and environment-friendly dry frozen devices for fruits and vegetables.