Discussion on the whole process of the hottest vac

2022-10-16
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A brief analysis of the whole process of vacuum freeze-drying (Part I)

Abstract: China has many varieties of fruits and vegetables, large planting area, and a great increase in the capacity of cold storage, but the cold storage technology is still based on ordinary cold storage. Fruits and vegetables are consumed after harvest, and the storage quality is not high. In some areas, they are blindly put into production in controlled atmosphere storage, resulting in a waste of investment. This paper introduces the characteristics and adaptability of several advanced refrigeration technologies in the world at present, and puts forward that proper refrigeration methods should be selected according to the variety and fresh-keeping requirements of fruits and vegetables, so as to achieve good fresh-keeping effect and economic benefits

key words: vacuum freeze-drying, fresh-keeping and cold storage technology

China has many varieties of fruits and vegetables, large planting areas, and a great increase in the capacity of the cold storage, but the cold storage technology is still dominated by ordinary cold storage. Fruits and vegetables are consumed after harvest, and the storage quality is not high. In some areas, they are blindly put into production in controlled atmosphere storage, resulting in a waste of investment. This paper introduces the characteristics and adaptability of several advanced refrigeration technologies in the world at present, and puts forward that proper refrigeration methods should be selected according to the variety and fresh-keeping requirements of fruits and vegetables, so as to achieve good fresh-keeping effect and economic benefits

fruit and vegetable preservation and cold storage processing

China is a large agricultural country, with the largest variety of vegetables and the largest planting area in the world. At present, people's diet has changed from full-bodied to nutritious, and economically developed countries have entered a period dominated by health food PPR tube ring stiffness testing machine and so on [1]; The demand and consumption of fresh-keeping fruits and vegetables have increased significantly, and a new round of agricultural restructuring has provided a good development opportunity for fruit and vegetable production. Fruits and vegetables are prone to deterioration and corruption. Without fresh-keeping treatment, the loss rate after harvest is more than 20%; If fruits and vegetables are stored and transported into cities without preservation, a large number of biological waste will be added to the city, resulting in a waste of energy and manpower. Therefore, strengthening the preservation of fruits and vegetables, gradually pre cooling, processing, packaging and preservation of fruits and vegetables after harvest, and realizing the listing of clean vegetables are an important link in the implementation process of "the lack of raw materials of cabbage has always been the bottleneck project of the company's production"; In addition, after China's entry into WTO, subsidies for agricultural products will be abolished. In order for agricultural products to survive and develop in the international agricultural trade liberalization market, the quality of agricultural products must also be improved; In addition to improving seed selection, breeding and planting, it is also necessary to carry out appropriate fresh-keeping processing of agricultural products to ensure the quality of agricultural products after storage and transportation and increase the competitiveness of agricultural products in domestic and international markets

cold storage processing is the physical processing of agricultural products (without any additives and chemicals). Reasonable selection of cold processing technology can maximize the preservation of food nutrients and color, aroma, taste, shape, and meet people's needs for green food. The cold storage of fruits and vegetables in China began in 1968, and the number and total capacity of cold storages have developed greatly in the past three decades. However, at present, ordinary cold storages are still the main ones, and few cold storages apply advanced cold processing technologies such as vacuum precooling, wet precooling, controlled atmosphere storage, reduced pressure and low temperature storage, which affects the fresh-keeping quality and market competitiveness of fruits and vegetables; On the other hand, some regions blindly put the controlled atmosphere cold storage into operation regardless of product characteristics, resulting in a waste of investment [2]. With the rapid development of the national economy, the continuous improvement of people's living standards, and the expanding export trade of fruits and vegetables, there is no doubt that higher and higher requirements are put forward for the cold storage technology of fruits and vegetables. We should absorb foreign advanced technology, and select the appropriate cold storage processing technology according to the characteristics of fruits and vegetables and the fresh-keeping requirements, so as to obtain good economic benefits and improve the fresh-keeping processing quality of fruits and vegetables

first, precooling

fresh fruits and vegetables are still living organisms after harvest, still breathing and evaporating vigorously, decomposing and consuming their own nutrients, and releasing respiratory heat; At the same time, fresh fruits and vegetables will also release a lot of field heat after being harvested from the field, which will rapidly increase the ambient temperature around the fruits and vegetables after picking, accelerate the ripening and aging, and then the freshness and quality will significantly decline. Therefore, the fruits and vegetables must be cooled to the specified temperature in the place of origin of raw materials in the shortest time after harvest, so as to maintain a low life level of fruits and vegetables and delay aging. This cooling process is called precooling. At present, precooling has been taken as the first process of post harvest processing of fruits and vegetables in foreign economically developed countries

there are five methods of precooling fruits and vegetables: air precooling, cold water precooling, ice precooling, vacuum precooling and wet precooling. The first three have been applied in China, and the latter two started relatively late and are less applied. Air precooling method is simple and low-cost. It is an ordinary cooling method in the early stage, but the cooling speed is slow, and it takes hours at a time, which is easy to cause the surface of fruits and vegetables to dry and shrink. Cold water precooling equipment is simple, easy to operate, fast cooling speed, no dry consumption, low cost. The disadvantage is that fruits and vegetables are easy to be polluted by bacteria in cooling water and cause the loss of water-soluble nutrients. Due to the large heat absorption capacity of ice (compared with cold water), ice precooling cooling speed is fast, and there is no dry consumption of food. However, ice making equipment covers a large area and has high initial investment, which is generally only used for aquatic processing or refrigerated transportation. Fruits and vegetables are pre cooled with ice, so the temperature is not easy to adjust, which is easy to cause cold damage, and the temperature is not easy to be uniform, so it is generally used less [3]

wet cooling precooling is to obtain 0.5 ℃ cold water by means of mechanical refrigeration and accumulated cooling capacity, and then exchange heat and mass in direct contact with the cold water through the heat exchanger to obtain high wet air close to the freezing point temperature, and then use this high wet air to cool materials through forced ventilation. The cooling speed is more than 1 times faster than the conventional air cooling method [2]; The fresh-keeping effect is good, which can provide comprehensive conditions of high humidity and low temperature for fruit and vegetable storage at the same time; It can effectively inhibit the respiration of organisms and the reproduction of microorganisms, prevent materials from losing water and wilting during cold storage, and is conducive to maintaining the freshness of fruits and vegetables; It is suitable for precooling and storage and preservation of most fruits and vegetables. The adaptability of wet cooling precooling method is higher than that of air cooling, cold water cooling and vacuum cooling methods [2]; The wet cooling system adopts indirect cooling method, which has the function of cold storage, which can reduce the installed capacity of the refrigeration unit and reduce the power consumption; The wet cold fresh-keeping warehouse has low cost, and the products can be brought in and out at any time, which is convenient to use. At present, the wet cooling system is mainly used for precooling and short-term fresh-keeping storage of fruits and vegetables (1 ~ 2 weeks), micro freezing fresh-keeping of fish and meat. In order to achieve long-term fresh-keeping, UV irradiation and other sterilization and disinfection AIDS need to be set, including various infusion tubes, infusion tube drip, implants and other short-term utilization means

vacuum precooling is to put fruits and vegetables in the vacuum precooling room and vacuum them with a vacuum pump to create a low-pressure environment, so that the water in the fruits and vegetables can evaporate. Due to the absorption of evaporation latent heat, the fruits and vegetables themselves are cooled. Vacuum precooling method has fast cooling speed and less water loss. Generally, the fruit and vegetables can be cooled to 3 ℃ in minutes, while the fruit and vegetables themselves only lose% of water. The cooling is uniform, pollution-free, and the fresh-keeping effect is the best. The disadvantage is that the cost is high. For leafy vegetables, flowers and cell tissues with large surface area, they are: strawberries with loose resin, celery and other cooling effects are excellent; However, for fruits and vegetables with small surface area, thick epidermis and dense tissue, such as cucumber, tomato and potato, it is difficult to evaporate water, slow cooling speed and high processing cost [4]

the fruits and vegetables processed from pre cooling to cold storage have the advantages of high freshness, no need to thaw and rehydrate, and easy to eat. They are suitable for short-term or short-term fresh-keeping food. In order to reduce the cost of processing, storage and transportation, the traditional fresh-keeping method of fruits and vegetables sold in China is: air precooling → high temperature refrigeration. Due to the large surface area of vegetables, loose surface tissue and long air precooling time, the vegetables are in a strong breathing state for a long time, which is easy to cause the vegetables to wither and turn yellow. For low-value products, it is more economical to use wet air precooling or cold water precooling. Vacuum precooling has good economic and social benefits for famous varieties with high processing value or exported fruits and vegetables. It is the best method of precooling preservation

(to be continued)

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