Refrigeration, also known as low-temperature storage, refers to the method of storing food at 0℃ or slightly higher than the freezing point of food. Cold storage is to inhibit the activity of microorganisms and enzymes, reduce the activity in food matrix, prevent food from spoilage and maintain the freshness and nutritional value of food. Cold storage is the most widely used food storage method with good effect, low price and long storage time.
First, the principle of food refrigeration
The spoilage of food is mainly caused by the biological activities of microorganisms and the biochemical reactions of enzymes in food. Animal food, such as poultry, livestock, fish, etc. In the process of storage, it is easy to be contaminated by bacteria, which multiply rapidly, thus causing food spoilage. However, the reproduction of microorganisms and the exertion of enzyme activity need suitable temperature and water conditions; If the environment is not suitable, microorganisms will stop reproducing or even die, and enzymes will lose their catalytic ability or even be destroyed. Placing animal food at low temperature can inhibit the reproduction of microorganisms and the action of enzymes on food, and can be preserved for a long time without deterioration.
For plant food, the cause of corruption is respiration. Although fruits and vegetables can't continue to grow after picking, they are still an organism, still alive and breathing. Low temperature can weaken the respiration of fruits and vegetables and prolong their storage life. But the temperature should not be too low, otherwise it will cause physiological diseases of plant food and even freeze to death. Therefore, the refrigeration temperature of plant food should be close to its freezing point without freezing plants.
Second, the cold storage construction requirements
1, the thermal insulation layer has good thermal insulation performance. The better the insulation performance of the insulation layer, the smaller the cold consumption and the smaller the fluctuation of storage temperature, which is beneficial to maintain the storage quality, prolong the storage time, reduce the loss of drying shrinkage and save the investment and operation cost of refrigeration equipment.
2, pay attention to the moisture outside the insulation layer. The temperature on both sides of the cold storage envelope is inconsistent, which is easy to form water vapor partial pressure difference. With the heat transfer, the water vapor outside the cold storage will penetrate into the cold storage, so it is necessary to set up a moisture-proof and gas-proof layer. There are three common moisture-proof methods: asphalt moisture-proof, plastic film moisture-proof and metal sandwich plate as moisture-proof layer.
3. A well-installed air extraction system. After food is stored in the cold storage for a certain period of time, it is easy to accumulate gaseous substances such as carbon dioxide and ethylene, so it is necessary to install a good air extraction system to change fresh air frequently, which is beneficial to the storage and preservation of food.
4. Choose a suitable refrigeration system. Cold storage obtains cooling capacity through artificial refrigeration. At present, liquid gasification refrigeration method or vapor refrigeration method is commonly used. Steam refrigeration can be divided into three refrigeration modes: compression, absorption and injection, of which compression is the most widely used. Compression refrigerator is mainly composed of compressor, condenser, regulating valve and evaporator. This is a closed circulation system.
Three, some problems should be paid attention to in the process of cold storage.
1, precooling. Pre-cooling is a pre-cooling method of food before long-distance transportation or refrigeration, and its requirement is to quickly cool the food to be stored to a specified temperature. This is an important measure to maintain the quality of transported food and extend its shelf life. Pre-cooling is usually carried out in cold storage and pre-cooling room. Common precooling methods include natural air cooling, ventilation cooling, vacuum cooling and cold water cooling. Pre-cooled food should be stored quickly at low temperature.
2. Permissible temperature range and storage period. The temperature in the cold storage can't be constant at a certain temperature value, because of the performance of the refrigerator, the size of the storage capacity and the influence of the temperature difference between inside and outside, the storage temperature will fluctuate within a certain range. Generally speaking, the lower the storage temperature of food, the smaller the change, the better. This is beneficial to food preservation and prevents loss and low-temperature physiological diseases.
3. Temperature control. Different foods have different optimal refrigeration temperatures. The temperature of cold storage and the temperature of food after storage are affected by many factors, such as the difference between the temperature of food during storage and the temperature of warehouse, the efficiency and storage capacity of refrigerator, the air circulation of warehouse, the stacking mode, the variety and maturity of food, etc. The stacking should be reasonable when warehousing, and the warehouse temperature should be adjusted according to the actual situation; Before leaving the warehouse, the temperature should be gradually raised to avoid condensation of water droplets on the food surface due to the large temperature difference between inside and outside.
4. Humidity control. Because of a large amount of heat absorption in cold storage, frost is often attached to the evaporator, which melts and flows away, so that the humidity in the cold storage is often lower than the humidity requirement of food storage. The humidity in the cold storage can be adjusted by increasing the evaporator area, reducing frosting, installing spraying equipment or automatic humidifier. In addition, when the relative humidity in the warehouse increases due to the frequent entry and exit of goods, a dehumidifier can be installed to absorb moisture, and the management of cold storage should be strengthened to strictly control the frequent entry and exit of goods and personnel.