Food quick-freezing is a food preservation method that has rapidly emerged in recent years. It is also an effective method for protecting food color, aroma, taste, shape and nutrient health functions, and it has a unique role in prolonging the storage and preservation time of fruits and vegetables and reducing loss. . As a food preservation method, frozen or quick-freezing can effectively extend the storage period of food, so that it can not only regulate the market supply to meet consumer demand, but also increase the income of the fruit farmer. The requirements for quick freezing and cold storage are different. In terms of temperature, freezing is required to reach a temperature of -18[deg.] C. or lower, and refrigerating is required to be around 0[deg.] C. When the food is in a frozen state, the microorganisms will not carry out the growth and development activities; and the refrigeration will only have a certain inhibitory effect on certain microorganisms. The quick-frozen packaged strawberries, litchi, cherries, red bayberry, apples, pears, grapes, and watermelons are quickly placed in a cold storage at -18°C to -20°C for frozen storage. The shelf life can be up to 1 year or even several years. Can only be stored for a few days, up to several weeks. 1. Quick freezing principle Freezing is the extraction of heat or energy in a product until after the latent heat of the free water molecules in the product is eliminated, the moisture becomes a preservation method for the ice crystal structure, which can effectively inhibit the microorganisms and enzymes. Activity is an important method of frozen preservation in the modern fruit and vegetable processing industry. The moisture in fruits and vegetables is not pure water but a mixture of organic and inorganic substances. These substances include salts, sugars, acids, and more complex organic molecules such as proteins and trace gases. Therefore, fruits and vegetables must be cooled below 0°C to produce ice crystals, and the temperature at which ice crystals begin to appear, the so-called freezing point or freezing point. When the liquid temperature drops to the freezing point, the liquid phase and the crystalline phase are in equilibrium. To make the liquid into a crystalline body, the equilibrium state must be broken. That is, the liquid phase temperature must be reduced to slightly below the ice point. temperature. When the liquid is in a supercooled state, a crystallization center is generated due to a certain stimulating effect. After the stable crystallization center is formed, if the heat is continuously dissipated, the crystal of ice will continuously increase, and the crystal phase changes and releases heat to make water or The temperature of the aqueous solution rises from the subcooled temperature to the freezing point temperature, and the liquid becomes solid, which is called icing. Ice formation includes two processes, namely the formation of nuclei and the increase of nuclei. Crystal nucleus formation is a part of very few water molecules in a certain regular order to form particle-type particles. The size of crystal formation is related to the number of crystal nuclei and the freezing speed. Under fast freezing conditions, crystal nuclei are widely formed inside and outside fruit and vegetable cells. The number of crystal nuclei formed is large, and they are needle-shaped crystals with uniform distribution. This distribution will not damage the cell tissue, and it will easily restore the original condition after thawing, and better maintain the original quality of fruits and vegetables. Thawing after thawing fruits and vegetables will reduce the amount of ice crystals. The increase of ice crystals will damage the cell structure. After thawing, the sap will flow out, the texture will be corrupted, the flavor will disappear, and the quality of the product will be affected. Therefore, in the quick-freezing storage process, temperature fluctuations should be avoided. Otherwise, even quick-frozen products will lose the advantage of quick freezing. Mastering the principle of quick freezing has positive significance for the quick-freezing and preservation of commercial fruits and vegetables. 2. Quick-freeze process The principle of quick-freeze of fruits is basically the same as the principle of quick-freeze of vegetables. The biggest difference is that most fruits do not need to be blanched and most vegetables must be blanched. Taking fruit as an example, the process of quick-freezing and fresh-keeping is as follows: Raw material picking → selection → cleaning → peeling → detune → enucleation → grading → cooling → sugaring or vitamin C → quick freezing → weighing → packaging → quick-freeze storage → transport → sale. 3. Operation points (1) Fruit plus sugar instead of hot water blanching Soak fruits in syrup water with vitamin C or without vitamin C instead of hot water blanching to prevent mold growth. In general, single frozen fruits do not need to be blanched, and some fruits such as peaches and apples often undergo browning during the quick freezing process. Even after sugar soaking, discoloration may still occur after storage for a certain period of time. In order to prevent such browning of fruits and maintain their original color, vitamin C with a mass fraction of 0.1% to 0.5% should be added to the sugar liquid. (2) After the fruit is cooled and drained, the temperature of the commonly used cold air must be -30°C to -40°C, and the wind speed should be 2m/s to 17m/s. During freezing, the cold air is forced at a sufficient speed from the bottom to the top of the belt to force air blow. The strong air flow blows the product up and floats. Like a boiling fluid, the low-temperature cold air can directly contact with the product particles, and the heat exchange is greatly affected. Strengthen the speed of freezing. In the case of fruits, flowbed bed freezers are generally not used because they have the potential to damage fruits. The most commonly used is a granular freezer, a mesh belt freezer. During the freezing process, it must be ensured that the fruit cannot be unevenly stacked. (3) Keeping the circulation at a low temperature The entire circulation should be carried out under freezing conditions below -18°C. 4. Food thawing Frozen foods should be thawed and reconstituted before use. Thawing and freezing are two opposite heat transfer processes. Thawing of non-fluid foods is slower than freezing. Frozen foods should be thawed prior to consumption and should not be thawed. For long-term storage, the faster the thawing process, the better. Thawing can be performed in the refrigerator, at room temperature, in cold or warm water, or thawed by microwave methods. The microwave thawing is uniform and rapid, but the organized components of the processed fruits and vegetables must be uniform to achieve a good thawing effect. 5. Market potential Although quick-frozen fruits and vegetables began to appear in the 1930s, due to good quick-freezing effect and long storage time, they can be supplied at any time, convenient for consumption, and develop rapidly. Especially in the past 20 years, with the improvement of people's living standards, the popularity of cold chain transportation and household refrigerators has made the rapid-freezing of fruits and vegetables more rapid, and the demand in the international market is also increasing. At present, China has become a large exporter of quick frozen fruits and vegetables. It has more than 300 export production companies. Most of the products are sold to European and American countries and Japan, and the annual foreign exchange earning exceeds 200 million U.S. dollars. Japan and the United States import the largest amount from China and increase year by year. The reason why China's quick-frozen fruits and vegetables have strong competitiveness in the international market is because China's labor costs are low and raw materials are cheap. Therefore, China's exports of frozen fruits and vegetables to Japan and the United States are increasing. Fast-frozen fruits and vegetables are also popular in the domestic market, and the consumer market is also rapidly expanding. There is ample market space for both domestic sales and foreign sales.
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