Analysis of the whole process of the hottest vacuu

Shallow analysis of the whole process of vacuum freeze-drying (Part 2)

Second, quick freezing of food refers to the rapid passage of the maximum ice crystal formation zone within 30 minutes when the food is cooled, and the central temperature reaches below -15 ℃. Quick frozen food is divided into quick-frozen fruits and vegetables 3 In process products of transportation, quick-frozen livestock and poultry food, quick-frozen aquatic food and quick-frozen conditioned food, etc. With the decline of domestic aquatic resources, the processing prospect of quick-frozen aquatic food is not good; Foreign consumers believe that livestock and poultry food is high in protein and fat, and long-term consumption has caused disturbing obesity, and quick-frozen livestock and poultry food has been neglected; The consumption of quick-frozen fruit and vegetable food and quick-frozen conditioned food continued to grow at a high speed. The processing process of quick-frozen food includes: pretreatment → precooling → quick freezing → packaging → low temperature refrigeration

compared with ordinary frozen food, quick-frozen food has fast freezing speed, wide number of ice crystals and small particles (the diameter is required to be 100 μ M below), which is evenly distributed and will not damage the cell tissue. When the food is thawed, the water melted by the ice crystal can be quickly absorbed by the cells without juice loss. Therefore, quick-frozen food can maintain the original freshness, color and nutrients of natural food to the greatest extent. Quick frozen food stored at -18 ℃ has longer storage time and lower long-term cold storage cost than pre cooled refrigerated processed food. It is the most widely used long-term cold storage method in the world at present. It is applicable to foods requiring long-term storage, such as fruits and vegetables exported to Japan [5]. Quick frozen fruits and vegetables must pay attention to the links of refrigeration, transportation, sales and household storage, and always maintain a constant low temperature below -18 ℃, so as to avoid the damage of cell tissue caused by the expansion of ice crystals due to temperature fluctuations. Quick frozen fruits and vegetables should also pay attention to the thawing method, so that the quality of fruits and vegetables will not be reduced during thawing. Using the microwave internal heating method, the thawing speed is fast, which can make the frozen products more evenly heated and heated as a whole, and can have a better processing state than the conventional thawing method [6]

III. vacuum freeze drying

Western Europe mainly imports dehydrated vegetables [5]. Dehydrated vegetables can be directly added to convenience foods as seasoning for convenience foods such as instant noodles and instant rice, as well as raw materials for further deep processing. The processing technology of dehydrated vegetables is as follows: pretreatment → drying → packaging → storage. The drying methods of dehydrated vegetables include hot air drying, far infrared drying, microwave drying, vacuum freeze drying and expansion drying. Vacuum freeze-drying is a drying method that freezes the cut material in a sealed container, turns the water contained into ice, and then heats it under a high vacuum, so that the ice crystals inside the material can be sublimated directly into water vapor for discharge. Compared with other drying methods, the advantages of vacuum freeze-drying technology are less heat consumption, short rehydration time, good reduction property, small changes in nutritional composition, color, taste, shape, etc. of the dried product, avoiding the defects such as discoloration, flavor change, loss of components, and irreducible drying shrinkage caused by traditional dehydration methods, and its product quality is high [7]. At present, dehydrated vegetables are developing towards vacuum freezing dehydrated vegetables in the world. The disadvantages are that the equipment is complex and the processing cost is expensive. However, the products after vacuum freeze-drying increase the production cost of the box, with light weight (losing 95%-97% of water), convenient transportation, lack of water in the dried materials, and bacteria cannot grow and reproduce. After packaging, they can be stored at room temperature for a long time, up to 1 to several years [8]. Their low storage and transportation costs, to a certain extent, compensate for the lack of expensive processing costs, and are one of the best methods in food preservation today. It is applicable to the processing of products requiring long-term storage or long-distance transportation, as well as food for tourism, mountaineering, navigation, aerospace, etc; Foods containing too much fat are not easy to freeze dry [8], and should not be used

IV. controlled atmosphere preservation

controlled atmosphere refrigeration is a refrigeration method that manually controls the gas components of the fruit and vegetable storage environment on the basis of refrigeration, so as to achieve a better effect of fruit and vegetable storage. Controlled atmosphere cold storage has been recognized as the most advanced method of fresh-keeping and storage of fruits and vegetables in the world. Compared with ordinary cold storage, its storage period is extended by one time, and the shelf life of fruits and vegetables after leaving the warehouse can be extended to days, which is times [9] of ordinary cold storage, so that fruits and vegetables can maintain the brittleness of freshness, and the nutritional composition, hardness, color, weight, etc. are almost the same as the newly picked state, with excellent storage effect

the products stored in the controlled atmosphere warehouse need to adopt the method of whole in and out of the warehouse. The fruits and vegetables stored in large quantities and for a long time (such as products sold out of season and products used for light migration) should be stored in a special controlled atmosphere refrigerator, which is easy to detect and regulate the gas composition of the storage environment. However, because the cost of air conditioning warehouse is higher than that of ordinary cold storage, He said: "the central government requires the supply side reform to add and subtract, and it needs to add expensive modified atmosphere equipment. At present, the domestic modified atmosphere storage is only used for apple, pear and other fruit varieties that need long-term storage and are difficult to store in ordinary high-temperature warehouses.

for fruits and vegetables that are sold immediately with many varieties, small batches and short-term fresh-keeping storage, they can also be used: pre cooling → vacuum packaging or modified atmosphere fresh-keeping packaging → high-temperature refrigeration (0 ~ 4 ℃) Processing technology; Its small package can be sold directly on shelves (refrigerated cabinets). Vacuum packaging technology and equipment have been relatively mature. The preservation period of modified atmosphere packaging is 2 ~ 2.5 times that of vacuum packaging. In about 20 days, China has developed a variety of modified atmosphere packaging machines and put them on the market [10], and the packaging speed needs to be further improved

modified atmosphere fresh-keeping packaging is also suitable for fresh flowers and chilled meat. In recent years, with the improvement of living standards, the sales volume of frozen meat in the domestic trade market has declined, and cooled meat has become a new consumption requirement. The cooled meat forcibly cooled by cold air adopts vacuum packaging or modified atmosphere fresh-keeping packaging, and its shelf life can reach two to three weeks, which can meet the requirements of storage, transportation and sales inside and outside the province. Compared with frozen meat, it has better fresh-keeping effect and low cost

v. storage and preservation under reduced pressure

storage and preservation under reduced pressure, also known as low-pressure storage and preservation or vacuum storage and preservation, is the development of controlled atmosphere storage. It is to place fruits, vegetables and other fresh foods in closed warehouses (containers), extract the air in the warehouses (containers) during the cooling process, and reduce pressure and oxygen. With the continuous reduction of pressure, some gases harmful to storage (such as ethylene) also decrease or basically disappear, It creates favorable conditions for the long-term storage and preservation of fruits and vegetables. Different from controlled atmosphere storage, reduced pressure storage only changes the total content and density of air in the storage environment, and does not change the air composition and combination ratio. The vacuum precooling and decompression refrigeration processes can be completed simultaneously and continuously in the normal operation of the decompression warehouse, which reduces the storage process links; Since the fruits and vegetables stored in the warehouse do not need airtight packaging, ozone can be easily used in the warehouse for continuous disinfection, sterilization and mold inhibition [11], which meets the four elements of fruit and vegetables storage: low temperature, high humidity, low oxygen and high-efficiency corrosion prevention, and the storage and fresh-keeping period is twice longer than that of ordinary cold storage [12]. Decompression cold storage can not only store fresh fruits and vegetables, meat, aquatic products, flowers, but also store high-grade biological products, archives, films, cultural relics and other items that are not suitable for storage under normal temperature and pressure. It has a wide range of applications and has become a research hotspot of scientists all over the world. At present, the cost of decompression cold storage is basically close to that of controlled atmosphere cold storage

the cold storage technology of fruits and vegetables in China has a history of decades, but most of them are ordinary cold storages. In the future, compared with foreign countries, we should pay attention to the introduction of world advanced cold storage technology, improve the fresh-keeping and storage quality of fruits and vegetables, and improve the market competitiveness of fruits and vegetables; At the same time, there are many kinds of fruits and vegetables, so we should choose the appropriate cold processing technology according to different variety characteristics and storage requirements, in order to obtain the best refrigeration effect and economic benefits; The relevant departments should formulate a set of quality standards for cold storage and preservation in line with international standards as soon as possible; Vigorously promote the combination of agriculture and industry, improve the added value of agricultural products, and develop modern new agriculture

source: China Vacuum