Analysis and Research of Fruits and Vegetables in Vacuum Cooling

After harvesting, fruits and vegetables are affected by environmental temperature and self-breathing effects. Water evaporates a lot and their nutrients continue to be consumed, which leads to rapid wilting, reduced freshness, and greatly shortened shelf life. In order to overcome the above problems, it is necessary to pre-cool the fruits and vegetables before they enter the circulation.

In various precooling methods, the effect of the vacuum cooling method is better. It has the advantages of uniform temperature distribution, fast cooling time, short storage time, and clean hygiene. However, this method also has its disadvantages. As the water evaporates, the appearance of fruits and vegetables will appear as wilting and shrinking, and the freshness and crispness will be greatly affected, and even the food value will be seriously lost. At the same time, the hydrolyzing enzymes in the food The vitality is strengthened, the polymer material is hydrolyzed, and the shelf life of the product is shortened. Therefore, it is necessary to control the water loss rate as low as possible during the cooling process. Therefore, in the process of vacuum cooling, the cooling rate of vegetables with high water spray rate is faster than that of low water spray rate, so that the cooling time is less. Since the relative energy consumption is the product of relative power and time, the relative energy consumption during the entire process of high water spray rate is low.

Specifically, the moisture on the vegetable surface evaporates first, and then the moisture inside the vegetable begins to evaporate and diffuse outward through the porous structure of the vegetable. During the vacuuming process, under the condition of high water spray rate, the moisture content of the surface of the vegetables is larger than that of the low water spray rate due to the high moisture content of the vegetable surface, and the amount of heat taken away from the vegetables is also relatively large. When vegetables fall from similar initial temperatures to similar final temperatures, the amount of water that needs to be evaporated from the interior is relatively small. The vacuum cooling process relies on the evaporation of moisture to remove the latent heat of evaporation so as to reduce the temperature. Since the latent heat of evaporation of water does not change much within a certain range, the downward trend in temperature is similar to the decrease in weight.

When the vacuum is started, the partial pressure of the air in the vacuum-cooled room is relatively large, so the moisture evaporates slowly and the weight loss is relatively small. With the continuous operation of the vacuum pump, the moisture on the surface evaporates, and the weight loss rapidly increases due to the temperature gradient inside and outside the cartridge, resulting in a pressure difference. Under this action, the internal moisture diffuses and evaporates toward the surface. When most of the water on the surface evaporates, the weight loss significantly slows. As the temperature of the cartridge continues to decrease, the pressure difference and weight loss between the interior and the surface decrease accordingly.

Therefore, in the process of vacuum cooling, vegetables with high water spray rate have relatively low energy consumption, rapid cooling rate, and low water loss rate. To reduce the adverse effect of vacuum loss caused by large amount of water loss on the quality of vegetables after cooling, the amount of water spray can be increased, but it must be ensured that the water sprayed on the vegetable surface can be completely evaporated, otherwise it will be counterproductive.

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