Theoretical Simulation and Experimental Study on Effect of Vacuum Pre-Cooling for Postharvest Leaf Lettuce

The effect of vacuum pre-cooling process on leaf lettuce was a complex process of heat and mass transfers. Based on the physical properties of leaf lettuce in vacuum pre-cooling process, an unsteady computation model was constructed to analyze the factors affecting vacuum pre-cooling. Some factors such as the pre-cooling temperature, pressure and quantity of the spray-applied water were verified throughout the experiment. The study showed that the measured and simulated values were basically the same, and the overall trend was similar. The lower the vacuum pressure, the greater the cooling rate lettuce and water loss rate. In this experiment, the water volume and pre-cooling pressure were the important factors during vacuum pre-cooling. This paper discovered that the quantities of leaf lettuce covered with water were equal to 4.211–5.977% of the total sample mass and the mass loss of the sample was 1.987–2.873%. Under pre-cooling pressure of 600, 1000, and 1500 Pa, the mass loss was 2.758, 2.701 and 1.929%. After that, the results of calculation indicated that the quantities of capture water of the water-catcher was 1.607–2.567 g, and the cooling capacity of the total sample was 3.722–5.946W in vacuum precooling process. The results reveal that the model of leaf lettuce was fitted and it was confirmed by the experimental data.