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Pre cooling is an important part of fresh-keeping technology (IV)

methods and characteristics of pre cooling

there are many ways of pre cooling fruits and vegetables, all of which have their advantages and disadvantages. During precooling, the most suitable precooling methods and facilities should be determined according to the type, quantity and packaging condition of fruits and vegetables

1. Air precooling can be divided into indoor cooling method and forced ventilation cooling method

characteristics of indoor cooling method: the cooling speed is very slow, which generally takes 1 day and night or even longer. However, this method is simple to operate and does not need additional cooling equipment. Cooling and storage are carried out at the same time. However, it is not suitable for perishable products, because the slow cooling speed will affect the storage effect

characteristics of forced ventilation cooling method: special fast cooling device is used to quickly reduce the product temperature through forced air high-speed circulation. However, its disadvantage is that the cooling surface is uneven, the surface temperature of fruits and vegetables is lower than the internal temperature, and the temperature of the lower part of the fruits and vegetables stacked in the storage box is higher than the temperature of the upper part, which cannot quickly bring out the field heat in the fruits and vegetables, and the energy consumption is also high. But its advantage is that it can be basically applied to the pre cooling and fresh-keeping of all organic agricultural products

2. Water cooling

can be divided into spray type and immersion type

characteristics of water cooling method: it is suitable for fruits and vegetables with small surface; Low cost, wet pre cooled products, easy to cause water pollution, should be replaced at intervals. The speed of water cooling involves many factors, such as the temperature and speed of the cooling medium, the volume and thermal conductivity of fruits and vegetables, the form of product accumulation and packaging, etc. Water cooling has the characteristics of faster cooling speed and less water loss than air cooling. The biggest disadvantage is that it promotes the infection of some bacteria, which is easy to cause the decay of fruits and vegetables, especially the products that have been injured by various kinds, and the disease is more serious

3. Differential pressure precooling

the cooling speed of differential pressure precooling is faster. Generally, the fruit temperature can be reduced from about 300C to about 50c in hours, but it is not as good as vacuum precooling in terms of cooling time and processing capacity. And the differential pressure precooling warehouse should be able to cope with a large number of concentrated loads. The fans with high wind pressure automobile front axle bench fatigue life test steps should be arranged according to the specified method, and special perforated cartons should be used

4. Package ice cooling

the package ice cooling method is applicable to those products that will not cause harm in contact with ice [relevant standard]: its effect is relatively limited, and it can only be used as an auxiliary measure of the above-mentioned methods

5. Precooling in cold storage

characteristics of precooling in cold storage: when the product falls to the required low temperature, it needs to be moved to another cold storage, and the product should be bagged, tied, placed and stacked according to the storage requirements. The precooling speed of the cold storage is slow, which generally takes days to cool to the predetermined temperature, and the products must be started up and operated a few days before warehousing to reach the precooling temperature. The energy consumption is large, and the internal temperature of the items is difficult to distribute

6. Vacuum precooling

water boils at 1000C under standard pressure, and the boiling point also decreases with the decrease of pressure. When the pressure drops to 600 PA, the boiling point of water drops to 00C. It can be seen that the transpiration of water is accelerated under vacuum conditions. When the water in fruits and vegetables transpiration outward, the latent heat is released to the outside with the water vapor, so that the product temperature decreases. This method has the characteristics of fast cooling speed, good cooling effect and convenient operation. Because the vacuum precooling method takes the growth heat from the fruit and vegetable body, the cooling effect is uniform. Vacuum precooling is not limited by packaging containers and materials. Cartons and wooden cases can be used, and the cooling speed is almost the same as that of non packaged products. Generally speaking, the cooling speed is: minutes. The cooling speed is fast, and many batches of products can be processed in one day. The unit processing energy consumption is the smallest. Its disadvantages are: high equipment cost and large one-time investment

application of precooling in the preservation of agricultural products

the function of precooling is to bring out the latent heat in the body of agricultural products, which will generally be naturally released after the harvest of agricultural products. This distribution process is long for some agricultural products, but some agricultural products will distribute quickly. After the harvest of agricultural products, plants will change due to the influence of temperature, humidity and respiration, and this change can directly affect the freshness of agricultural products

at present, in the field of agricultural production and circulation in China, some enterprises with relatively strong financial strength, advanced management means and high management level have generally adopted post harvest pre cooling measures. However, in most areas and vast rural production and operation units, due to the influence of funds, information, ideas, planting and management level and other factors, most agricultural products are not pre cooled after harvest, which greatly reduces the time of fresh-keeping and storage of agricultural products after harvest. Not only does the quality of products fail, but many products suffer great losses during storage

according to the data, the market volume of 26 vegetable products in Japan in 1992 was tons, of which the market volume after pre cooling was 6 tons. The pre cooled market volume of celery, lettuce, green stem vegetables and leeks exceeded 50% of the total market volume. Among them, the vegetables that are mainly listed after vacuum precooling are celery, lettuce, cabbage, chrysanthemum, cabbage, spinach, soybeans, cauliflower, cabbage, onion, Yuanjing, etc; Vegetables and fruits that are mainly pre cooled by differential pressure include asparagus, leeks, strawberries, lentils, cucumbers, tomatoes, eggplants, etc; Agricultural products that are mainly listed after strong wind precooling include green stem vegetables, Dutch beans, green peppers, sweet corn, carrots, radishes, taros and other varieties. Practice has found that most strong wind precooling methods of agricultural products are applicable; Leafy vegetables and flower organic agricultural products are more suitable for vacuum precooling; For agricultural products with large volume and thick cortex, the differential pressure precooling method may be more suitable. Studies at home and abroad have shown that the selling price of fruits can be increased by 40% - 50% by delaying their marketing for months through storage and preservation. In terms of fresh-keeping of agricultural products and other post natal industrialization rates, Italy and the Netherlands are 60%, and other Western European countries are 50%, while China's current post natal industrialization rate is less than 20%. If we can reduce the postpartum loss rate of fruits and vegetables to 10%, it is equivalent to an increase of 50million tons of fruits and vegetables

in view of the low level of post-processing in China, the Ministry of agriculture has proposed to increase the post production processing capacity of China's agricultural products to 45% to 55% of the production processing capacity by 2010, specifically referring to a higher continuous operating temperature. With the continuous deepening and development of the research and development of fresh-keeping technology and related equipment in China, the means applied to the postpartum fresh-keeping treatment of agricultural products will be increasingly improved. In the future, more fresh-keeping technologies and equipment will be widely used in the fresh-keeping and storage of fruits, vegetables and flowers, and China's fresh-keeping industry will usher in a new future