Two steps of pre-cooling and refrigerating.
Pre-cooling Before refrigerating or freezing, the food is rapidly cooled from the original temperature (for example, if the vegetable is within 24 hours after harvesting or the temperature after slaughter of the livestock) to a predetermined temperature, and the biochemical reaction of the food itself is suppressed in time. And the reproduction of microorganisms. Commonly used cooling methods are: 1 air cooling method, even if the air is mechanically cooled first, then forced ventilation to the tunnel where the food is placed, or with a pressure difference of 1300 ~ 2000Pa, so that cold air enters and passes through the food box with the transom To accelerate the cooling of the product. The temperature, relative humidity and flow rate of the cold air are determined according to the type of food, and generally the food is not frozen, and the relative humidity is usually 90 to 95%. 2 Water cooling method, using water as the heat transfer medium, using a large-capacity water pump to spray cold water close to 0 °C on the product. Since the heat capacity of water is much larger than that of air, heat transfer efficiency is high, and most foods can be cooled in 10 to 15 minutes. When seawater is used to cool fish, the speed is fast, the temperature is uniform, and the preservation period can be extended. 3 ice block cooling method, commonly used for the cooling of fish and shrimp, each ton of ice can absorb 334720kJ of heat when it is melted into water. The smaller the ice cube, the faster the cooling rate. The disadvantage is that the temperature is not uniform, and the water melted by the ice is difficult to manage everywhere, and is now mainly used as a supplement to other cooling methods. 4 indirect cooling method, suitable for liquid foods, such as milk, egg liquid, juice and so on. The heat exchangers used are tube type, sleeve type, scraper type, sheet type and the like. 5 vacuum cooling method, according to the principle of reducing the atmospheric pressure can accelerate the evaporation of water and the use of moisture evaporation latent heat can reduce the temperature of the food, the food is placed in a closed chamber, the pressure is reduced to 660Pa by a vacuum device, because the boiling point of the water at this pressure is 1 °C, the water on the surface of the food evaporates and vaporizes, and each evaporation of 1 kg of water absorbs about 2,460 kJ, resulting in a decrease in the temperature of the food. For every 5 °C drop in temperature, the moisture in the food is reduced by about 1%, that is, 1% by weight. The vacuum cooling method is suitable for leafy vegetables with large surface area, and can also be used for asparagus, mushroom, celery, spore cabbage; fruit with small surface area/volume ratio or waxy surface, moisture is not easy to evaporate, and should not be used.
Refrigeration Storage temperature, relative humidity and flow rate of air are important conditions for refrigeration. In practical applications, the specific conditions of the three are determined by the type of food, the length of the storage period, and whether the food is packaged.
1 Storage temperature, especially the temperature of the food itself, is the most important factor. For fruits, vegetables and shell eggs, it is generally close to freezing point. However, tropical and subtropical fruits and vegetables have their own minimum storage temperatures. For example, the minimum storage temperatures of banana, pineapple, lemon, green tomato, cucumber, and eggplant are 13, 7, 12, 12, 7, and 7 ° C, respectively. Below this temperature, low temperature damage occurs, such as the appearance of disease spots, internal discoloration, The skin rots until it is completely ulcerated.
2 The humidity of the air is too high, which may cause condensed water on the surface of the low-temperature food, which may cause mildew of fruits and vegetables or sticky mold of meat and poultry; if the relative humidity is too low, the water will evaporate quickly, causing the surface of the food to shrink and the shell gas chamber to increase. , weight loss.
3 In the cold storage, ventilation should be forced to bring the air in the circulation away from the breathing heat of the fruits and vegetables, and keep the temperature of each part of the cold storage uniform. The air flow rate is too low to achieve the above purpose, and too high accelerates the evaporation of water in the food, especially when the relative humidity is low. If the food is packaged with a material with a low vapor permeability, the flow rate and relative humidity of the air have no effect on the evaporation of water from fruits and vegetables.