喷淋预冷工艺参数对荔枝降温特性的影响

Effects of different spray precooling parameters on cooling characteristics of litchi

为掌握喷淋预冷工艺参数对荔枝降温特性的影响,搭建了荔枝喷淋预冷试验平台,以"淮枝"荔枝为试验材料,研究了喷淋温度和喷淋流量对单层荔枝果实以及多层荔枝果实喷淋预冷降温特性的影响。试验结果表明:单层荔枝预冷,喷淋温度越低,冷却系数越大,7/8预冷时间越短,果实温度均匀性越差,选择(5±0.5)℃,能够保持较好预冷均匀性和较快的预冷速度;喷淋流量增大,冷却系数先增大后趋于稳定,7/8预冷时间先缩短后趋于平缓,与喷淋流量呈二次函数关系,果实温度均匀性提高,临界喷淋流量为5.9 L/(s·m^2);多层荔枝堆叠时,果实离喷头越近,冷却系数越大,7/8预冷时间越短,果实温度均匀性越好,相对预冷时间与层数呈二次函数关系,临界预冷层数为4;研究结果为荔枝喷淋预冷装置的设计及单层与多层荔枝预冷应用提供参考。

To investigate the effects of spray precooling parameters on the cooling characteristics of litchi fruit, a spray precooling test platform was established.“Huaizhi” litchi fruit was chosen as raw materials for this study. The effects of spray temperature and spray flow rates on the cooling coefficient, 7/8 cooling time, and cooling uniformity, as well as the characteristics of multilayer litchi spray cooling, were studied. For spray precooling of single layer litchi, the lower the spray temperature was, the bigger the cooling coefficient was, the shorter the 7/8 cooling time was, and the worse the temperature uniformity of the fruit was. When the spraying temperature was less than (5±0.5)℃, the 7/8 precooling time was shortened, and the precooling unevenness increased significantly (P<0.05). The effects of spray temperature to achieve rapid precooling was limited. Therefore, a spray temperature of (5±0.5)℃ could be chosen in the actual precooling process, and it could maintain the precooling uniformity and accelerate precooling rate of litchi. With the increase of pray flow rate, the cooling coefficient initially increased and then stabilized, and the 7/8 precooling time initially shortened and then leveled gently, and the fruit temperature uniformity increased. The 7/8 precooling time had a quadratic function relationship with the spray flow rate. The 7/8 precooling time decreased slowly when the spray flow was higher than 5.9 L/(s·m^2). This was because the contact area between the litchi and cold water increased more slowly than the flow rate. Therefore, the flow rate of litchi spray precooling could be selected to be 5.9 L/(s·m^2), which could improve the precooling efficiency and reduce the energy consumption of the pump. When multilayer litchi fruits were stacked, the closer the spray nozzle was, the bigger the cooling coefficient was, the shorter the 7/8 precooling time was, and the better the temperature uniformity was. The relative precooling time had a quadratic function relationship with the number of layers, and the number of critical precooling layers was 4.1. When the number of litchi stacks was larger than or equal to 4, the relative precooling time varied little. To improve the precooling efficiency, the number of litchi stacks should be bigger than or equal to 4. However, as the number of layers increased, the precooling uniformity gradually deteriorated. When the multilayer litchees were stacked, the cooling rate of each layer was inconsistent, the precooling time was longer, and the precooling final temperature was not coordinated. The whole process of precooling took 14.02 minutes. After precooling, the σ of the middle longitudinal section was 0.14, and the average temperature was 7.15 ℃. In single layer litchi spray precooling, the spray temperature could be selected as (5±0.5)℃, and the spray flow could be selected as 5.9 L/(s·m^2). The parameters of the single layer litchi spray precooling were used to precool the multilayer litchi, and the number of layers of the best stack of litchi was found to be 4. The research results provided reference for the design of litchi spray precooling equipment and single-layer and multilayer litchi precooling applications.