First, pre-chilled fresh fruits and vegetables are still living organisms after harvest, still undergoing vigorous breathing and evaporation, decomposition and consumption of their own nutrients, and release of respiratory heat; at the same time, fresh fruits and vegetables are released from the field after harvest A lot of field heat, both causing the rapid increase in ambient temperature around the harvested vegetables, accelerate the maturation and aging, with a consequent significant decline in freshness and quality. Therefore, in the shortest time after fruit and vegetable harvest, it must be cooled to the required temperature in the raw material production area so that the fruit and vegetables can maintain low life levels and delay aging. This cooling process is called precooling. At present, in the developed countries of foreign economy, pre-cooling has been used as the first process after fruit and vegetable harvesting. The pre-cooling methods of fruit and vegetable include air pre-cooling, cold water pre-cooling, ice pre-cooling, vacuum pre-cooling, and wet cold pre-cooling. The first three kinds have already been applied in China, and the latter two are relatively late to start and have few applications. Air pre-cooling method is simple, low cost, is an early common cooling method, but the cooling rate is slow, once required 12-24 hours, easy to cause fruit and vegetable surface shrinkage. Cold water precooling equipment is simple, easy to operate, fast cooling, no dry consumption, low cost, the disadvantage is that vegetables are vulnerable to bacterial contamination in cooling water, and cause the loss of water-soluble nutrients. Due to the large amount of heat absorbed by ice (compared to cold water), ice precooling and cooling are fast and food is not wasted. However, the ice making equipment covers a large area and the initial investment is high. It is generally used only for fish processing or refrigerated transport. Fruit vegetables are pre-cooled with ice, the temperature is not easy to adjust, it is easy to cause cold damage, the temperature is not easy to uniform, generally less used [3]. Wet and cold pre-cooling adopts mechanical refrigeration to accumulate cold water to obtain 0.5°C cold water. Then the heat and mass exchange between the air in the reservoir and the cold water are directly performed by the heat exchanger, and a high-humidity air near the freezing point temperature is obtained. Forced ventilation uses this high-humidity air to cool the material, cooling speed is more than 1 times faster than conventional air cooling method [2]; good preservation effect, can provide comprehensive conditions of high humidity, low temperature and favorable storage of fruits and vegetables; can effectively inhibit the biological Respiration of the body and reproduction of microorganisms prevent the materials from losing water and wilting during cold storage, which is beneficial to maintain the freshness of fruits and vegetables. It is suitable for the pre-cooling and storage preservation of most fruits and vegetables, and the adaptability of the fruit and vegetable to the wet cold pre-cooling method is higher than that of air cooling. , Cold water cooling and vacuum cooling method [2]; wet cooling system uses indirect cooling method, with a cold storage function, can reduce the capacity of the refrigerator assembly machine, low power consumption; wet cold preservation warehouse cost is low, the product can follow the entry, easy to use. The wet cooling system is mainly used for the pre-cooling and short-term preservation of fresh fruits and vegetables (1 to 2 weeks). The fish and meat are frozen and preserved slightly. To achieve long-term preservation, other means of sterilization such as ultraviolet radiation must be provided. Vacuum pre-cooling means putting the fruit and vegetables in a vacuum pre-cooling room and vacuuming with a vacuum pump to create a low-pressure environment so that the internal moisture of the fruit and vegetables can be evaporated. Because the latent heat of evaporation is absorbed, the fruit and vegetables themselves are cooled. The vacuum pre-cooling method has fast cooling speed and low water loss. Generally, the fruit and vegetable can be cooled to 3° C. in 20-30 minutes, and the fruit and vegetable itself lose only 1-3% of the water, cooling uniformly, no pollution, and the effect of fresh-keeping is the most. Well, the disadvantage is that the cost is higher, and the cooling effect is better for leafy vegetables, fresh flowers, and loose cells such as strawberry and celery with large surface area, but for fruits and vegetables with small surface area, thick skin and dense tissue, such as cucumber , tomatoes, potatoes, etc., water evaporation is difficult, the cooling rate is slow, processing costs are high [4]. The pre-cooled and chilled fruits and vegetables have the advantages of high freshness, no need for thawing, rehydration, convenient consumption, etc., and are suitable for foods with short or short-term preservation. In order to reduce processing and transportation costs, the traditional preservation methods for domestic sales of fruits and vegetables are: air pre-cooling → high-temperature refrigeration. Due to the large surface area of ​​vegetables, loose surface tissue, and long pre-cooling of air, the vegetables are in a strong state of respiration for a long time and can easily cause vegetables. Dry and yellow. For low-value products, the more economical method may be to use cold air precooling or cold water precooling. The use of vacuum precooling, which has high economic value and social benefits, is the best method for pre-cooling preservation. Second, the quick-frozen food quick-freezing is when the food cools, within 30 minutes quickly through the largest ice crystals, and the center temperature reached -15 °C below. Frozen foods are divided into frozen fruit and vegetable foods, quick-frozen livestock and poultry foods, frozen aquatic foods, and frozen prepared foods. Fast-frozen aquaculture foods with the decline of domestic aquatic resources, processing prospects are not good; foreign consumers believe that livestock and poultry foods high protein, high fat, long-term consumption caused by the people suffering from obesity, frozen livestock and poultry food is being neglected; and frozen fruit and vegetable foods The consumption of frozen and processed foods continued to grow rapidly. Frozen food processing process includes: pre-treatment → pre-cooling → quick-freezing → packaging → low-temperature refrigeration. Compared with ordinary frozen foods, quick-frozen foods have fast freezing speed, a large number of ice crystals, small particles (diameter below 100 μm), uniform distribution, and no damage to cell tissue. When foods are thawed, the ice crystals melt water quickly by cells. Absorbed without causing loss of juice. Therefore, frozen foods can maintain the original freshness, color and nutrient content of natural foods. After quick-freezing, the food is stored at a low temperature of -18°C, which is longer than the pre-chilled and cold-processed foods, and the long-term refrigerating cost is low. It is currently the most widely used method in the world for long-term cold storage. Applicable to foods that need to be stored for a long time, such as fruits and vegetables exported to Japan [5]. Frozen fruits and vegetables must pay attention to the refrigeration, transportation, sales and home storage, and always maintain a constant temperature below -18°C to prevent the ice crystals from expanding and destroying the cell structure due to temperature fluctuations. Frozen fruits and vegetables should also pay attention to thawing methods, so that fruits and vegetables will not cause deterioration in the quality of thawing, the use of internal microwave heating, thawing speed, frozen products can be more uniform heating and heating, and can be better than conventional thawing method of processing State [6]. Third, vacuum freeze-drying Western Europe is mainly imported dehydrated vegetables [5], dehydrated vegetables can be directly added to convenience foods, as instant noodles, convenient rice and other convenient food catering seasonings, but also as a raw material for further processing. Dehydrated vegetables are processed as follows: pretreatment → drying → packaging → storage. Drying methods of dehydrated vegetables include hot air drying, far-infrared drying, microwave drying, vacuum freeze drying, and puffing drying. Vacuum freeze-drying is a drying method in which a cut material is frozen in a sealed container, the contained water is changed into ice, and then heated under a high degree of vacuum so that the ice crystal inside the material is directly sublimated to discharge water vapor. Compared with other drying methods, the vacuum freeze-drying technology has the advantages of less heat consumption, short rehydration time, and good reduction properties. After drying, the nutrients, color, taste, shape, etc. of the product have little variation, and the traditional dehydration method is avoided. The defects such as discoloration, bad taste, loss of components, and dry shrinkage cannot be reduced, and the product quality is high [7]. At present, dehydration vegetables in the world are developing vacuum-freeze dehydrated vegetables. The disadvantage is that the equipment is more complicated and the processing cost is expensive. However, the products after vacuum freeze-drying are light in weight (losing 95%-97% of water content), convenient transportation, lack of water in dry materials, bacteria cannot grow and multiply, and can be stored at room temperature for a long period of time after being packaged for 1 to several years. [8], its low storage and transportation costs, to a certain extent, compensate for the lack of expensive processing fees, is one of the best methods for food preservation. Applicable to products that require long-term storage or long-distance transportation, as well as tourism, hiking, navigation, aerospace and other foods; foods containing too much fat are not easy to freeze-dry [8], should not be used. IV. Modified Atmosphere Preservation Atmosphere refrigerated storage is a method of refrigerating, which is based on refrigerating, to manually control the gas components of the storage environment of fruits and vegetables to achieve better storage of fruits and vegetables. Atmosphere refrigeration has been recognized as the world's most advanced preservation method for fruits and vegetables. Compared with ordinary cold storage, its storage period is extended by 1 time, and the swing period of fruits and vegetables after delivery can be extended to 21-28 days, which is 3-4 times that of ordinary cold storage [9], so that the freshness of fruits and vegetables is kept brittle and nutrition is maintained. The composition and hardness, color, weight, etc., are almost the same as those of the new picking, and have excellent storage effect. The products stored in the air-conditioning warehouse need to adopt the way of entering and leaving the warehouse. The large-scale, long-term storage of fruits and vegetables (such as the products sold in the off-season and the products used for dulling) are stored in a special air-conditioned refrigerator. Gas components are detected and regulated in the storage environment. However, due to the higher cost of cold storage than that of ordinary cold storage, it is necessary to increase the cost of complete set of atmospheric conditioning equipment, the current domestic atmosphere storage is only used for apples, pears and other long-term storage and difficult to store in the ordinary high temperature storage kiwi and other fruit varieties . For variety, small batch, short-term preservation and storage of fruits and vegetables can also be used: pre-cooling → vacuum packaging or modified atmosphere packaging → high temperature refrigeration (0 ~ 4 °C) processing; its small package can be directly swing (refrigeration Cabinet) sales. Vacuum packaging technology and equipment are relatively mature, and the fresh-keeping period of modified atmosphere fresh-keeping packaging is 2 to 2.5 times that of vacuum packaging. In 20 days or so, China has developed a variety of modified atmosphere packaging machines on the market [10], and has to wait in terms of packaging speed. Further improve. Modified atmosphere packaging also applies to flowers and cooling meat. In recent years, with the improvement of living standards, the sales volume of frozen meat in the domestic trade market has declined. Cooled meat has become a new consumer demand. Cooled meat cooled by cold air is vacuum packed or air-conditioned and fresh-keeping packaged. Its shelf life is up to In two to three weeks, it can meet the requirements for storage and transportation within and outside the province, and has better preservation effect and lower cost than frozen meat. Fifth, vacuum storage, fresh-keeping, vacuum storage, preservation, also known as low-pressure storage or vacuum storage preservation, is the development of controlled atmosphere storage, is the fruit and vegetables and other live food in a sealed warehouse (container), in the process of cooling The air in the storage (container) is pumped out and decompressed and reduced in oxygen. As the pressure continues to decrease, some gases harmful to storage (such as ethylene) also decrease or disappear, creating a long-term storage and preservation of fruits and vegetables. Favorable conditions. In contrast to controlled atmosphere storage, decompression storage only changes the total air content and density in the storage environment and does not change the air composition and the combination ratio. The vacuum depressurization warehouse can complete the vacuum pre-cooling and decompression refrigeration process at the same time and in the normal operation, which reduces the storage process. Since the stored fruit vegetables in the warehouse do not need to be airtightly packaged, the ozone can be conveniently disinfected continuously in the warehouse. Bacteria and mold [11] have met the four major elements of storage in fruits and vegetables: low temperature, high humidity, low oxygen, and high-efficient anti-corrosion. The storage and fresh-keeping period is 1-2 times longer than ordinary cold storage [12]. The decompressed refrigerator can not only store fresh fruit and vegetables, meat, aquatic products, and flowers but also high-grade biological products and archives, film, cultural relics, etc., which are not suitable for storage under normal temperature and pressure. It has a wide range of applications and has become a hot topic for scientists around the world. At present, the decompression warehouse cost is basically similar to that of the air-conditioning warehouse. China's fruit and vegetable refrigerating technology has a history of several decades, but most of them are ordinary cold storages. In the future, attention should be paid to the introduction of world-advanced cold storage technologies to improve the quality of fruit and vegetable preservation and improve the market competitiveness of fruits and vegetables. At the same time, there is a wide variety of fruits and vegetables. Depending on the characteristics of different varieties and storage requirements, appropriate cold processing techniques should be selected to obtain the best refrigeration effects and economic benefits; relevant departments should formulate a set of quality standards for cold storage and preservation in line with international standards; vigorously promote agriculture and industry. Combine to increase the added value of agricultural products to develop modern new-type agriculture
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