The spoilage of fresh foods is caused by a series of chemical changes caused by various enzymes and microorganisms contaminated by the environment. Both the catalysis of the enzyme and the propagation of the microorganism require an appropriate temperature. For example, if the temperature of the food is lowered, the activities of both will be correspondingly inhibited.
Plant foods Fruits and vegetables remain alive after harvesting, have a certain immunity, and rely on their own stored nutrients to continue to breathe, absorb oxygen, consume sugar, starch, organic acids, etc., emit carbon dioxide, and produce breathing heat. The amount of heat depends on the type of fruit and vegetables and the storage temperature. In a certain temperature range, the heat of breathing is proportional to the storage temperature. For every 10 °C increase in temperature, the activity of the enzyme is enhanced by 2 to 3 times. Conversely, for every 10 °C decrease in temperature, the heat of breathing can be reduced by 50 to 66%. However, the temperature cannot fall below the freezing point. Otherwise, once frozen, the fruits and vegetables will lose their lives. Even if the temperature is raised, they cannot be revived. The respiratory heat of various fruits and vegetables varies greatly. For example, at 0 °C, the respiratory heat of spinach and green beans is 186-214 and 242-271 J/kg·h, respectively, while the potato and onion are only 19-39 and 26J/ respectively. About kg·h; the respiratory heat of peach, pear, orange and apple is generally 20-60J/kg·h, while that of strawberry is as high as 120-167J/kg·h. Fruits and vegetables with low calorie heat are more resistant to storage.
Fruits and vegetables gradually mature and age while undergoing respiration. The newer modified atmosphere storage method uses the concentration of oxygen in the cold storage atmosphere to increase the proportion of carbon dioxide, so as to appropriately inhibit the respiration and improve the storage effect. However, the ratio of oxygen, carbon dioxide and nitrogen must be appropriate, because various fruits and vegetables have permissible limits for low concentrations of oxygen and high concentrations of carbon dioxide, and they vary greatly from each other. Above this critical concentration, they are harmed. As the storage temperature rises, the respiration of fruits and vegetables is strengthened, and more oxygen is required, so the critical concentration of oxygen increases. When the concentration of carbon dioxide in the ambient gas is low, the critical concentration of oxygen is lowered. A gas with a high carbon dioxide content can inhibit the activity of pectinase and can be used to maintain the hardness of the fruit. But no ideal gas composition is suitable for all fruits and vegetables, so the right gas ratio must be found for each product. Ethylene is another important factor affecting the storage life of fruits and vegetables. All fruits produce ethylene during development, and ethylene has a ripening effect on the fruit and promotes the senescence of leafy tissues. In addition to controlling the ratio of oxygen, carbon dioxide and nitrogen, the new type of air-conditioning cold storage also has a device for eliminating ethylene. The low pressure storage method is a further development of the modified atmosphere storage method. The fruits and vegetables were stored in a closed container, and the pressure inside the device was lowered to 10 kPa by a vacuum device. Under this condition, the oxygen concentration is reduced by 90%, and the produced ethylene is eliminated at any time, which is beneficial to delay fruit ripening. Use a properly sized container and use it as a transport. This low-pressure storage method affects its development due to high investment.
Animal foods Unlike plant foods, during cold storage, most of them have no life, and there is no resistance to the invasion of foreign microorganisms. The activity of enzymes is not to maintain life but to cause decomposition and decay. Therefore, meat, poultry, fish, milk, etc., are only for short-term turnover and cannot be stored. The enzymes in cold-blooded animals such as fish are more catalyzed than warm-blooded animals, and all parts of the fish are infected with microorganisms to accelerate their deterioration. The number of bacteria in the milk excreted from the healthy herd is about 200-600/ml. After extrusion, the number of bacteria increases sharply due to contamination of cattle, milking tools, insects, dust, and the like. The main bacteria are lactic acid bacteria, butyric acid bacteria, gas-producing bacteria, pathogenic bacteria, etc., at a suitable temperature, will multiply. Therefore, the extruded milk should be immediately cooled to about 4 ° C and then stored at this temperature for further processing. In animal foods, shelled eggs are alive and are usually stored in a box and stored in a cold storage of -1.5 to 0.5 ° C and a relative humidity of 85 to 88%. During cold storage, samples are taken every 1 to 2 months to determine if storage can continue.
Dry consumption is a major problem in food refrigeration. During the cold storage, the dry consumption caused by evaporation of water is a natural phenomenon. The process is as follows: When the cold air in the cold storage passes through the food, it not only removes heat, but also evaporates the moisture on the surface of the food, thereby increasing the temperature and increasing the humidity. . Thereafter, when this part of the air is circulated to the evaporation tube of the refrigeration unit, the moisture in the air is condensed and frosted on the evaporation tube, returns to dry air, and is blown to the food by the blower, repeating the cooling and evaporation process. This cycle gradually reduces the weight of the food, shrinks the tissue, and darkens the color. The weight of food is reduced, and it suffers economic losses. In severe cases, the quality deteriorates and the value of the commodity is lost. The way to prevent dry consumption is to reduce the vapor pressure difference between ambient air and food and to reduce the evaporation rate of moisture on the surface of the food. Specific measures include: 1 increasing the humidity of the ambient air by spraying or introducing steam; 2 increasing the area of the evaporator in the cold storage to reduce the temperature difference between the evaporator and the cold storage; 3 appropriately reducing the flow rate of the cold air to reduce evaporation of water; 4 Apply wax or other coating to citrus fruits; 5 Put food in plastic bags, boxes or cartons.