China is the world's largest fish market , a number of aquaculture production and processing enterprises , in which no smoke the fish market battle, good refrigeration and preservation methods are the key to success . Our conversation focused on two current aspects .
Body: liquid nitrogen is a colorless, transparent liquid odorless, inert , sterile , low viscosity , chemical stability, and the role of any substance can not afford compounds , boiling point 77.3K (-196 ℃), specific gravity of 0.81 kg / l, latent heat of vaporization 47.5 kcal / kg , in the standard state , the vaporization of liquid nitrogen 647 times the volume can be expanded .
Liquid nitrogen to freeze : When the fish into liquid nitrogen freezer , surface temperature first dropped , with more than 200 degree temperature difference to be intense heat exchange cooling of about 5 to 15 degrees per minute (normal freezing 0.5 degrees per minute cooling ) , the temperature difference between inside and outside in a short time to reach equilibrium . In this sense , essentially a super- quick frozen in liquid nitrogen freezing . There experiments show that fish freezing time is generally 10 to 25 minutes. However, a variety of different initial temperature of the fish , the thickness and shape , the time required for freezing and subsequent changes . Figure 4 is a shad fish frozen in liquid nitrogen temperature curve , can be seen, shad fish thickness of 27 mm , the initial temperature of 17 ℃, the growth over time , tend to balance internal and external temperature quickly , after about 22 minutes and 30 seconds is -44 ℃ of a frozen product .
Generally , fish contains 70 to 80% of water, when the temperature drops to 0 ~ -5 ℃ , most of the water to freeze fish to produce a larger ice crystals , which is called the maximum ice generation area . The faster the freezing , by the time zone of the maximum frozen crystal creating shorter the better the quality of the frozen product . So, frozen fish quality and speed of closely related . Quick freezing in liquid nitrogen , with the destruction of equilibrium , heat rapidly diffuses into the surrounding medium to the matrix composition of the internal redistribution , the performance was weak, so that the stable ice crystals initially formed small , so that the permeation of moisture small , analysis weakened, can greatly reduce protein denaturation , frozen in liquid nitrogen and therefore has the greatest reversibility. After the fish after a hard spray of liquid nitrogen , the surface is adhered with white frost particles in liquid nitrogen , fleshy white, organized crime , thawing and cooking taste , still maintains the original morphology and color, flavor , these are based on ammonia or freon as a refrigerant for a slow freeze that fall.
Cooling preservation: We focus on exploring fresh yellow croaker effect under freezing conditions at the bar . Test is to keep croaker 0 ~ ± 1 ℃, sprayed with liquid nitrogen and ice hidden two methods were preserved for six days, and fish identification sensory volatile basic nitrogen ( micro- Kjeldahl nitrogen determination ) the determination of the results see table III.
After observation, the deposited liquid nitrogen croaker maintain the original surface color and the muscles relatively solid , the test gas is generated due to vaporization of liquid nitrogen, nitrogen gas is inert , reducing the oxygen content in the container so that the fish is not easily oxidized during the cooling preservation , color ; also effective in suppressing the growth of bacteria or fungi .
In China, the price of liquid nitrogen around vary widely used in high economic value of aquatic products ( such as shrimp , shrimp , anchovy, whitebait , etc. ) , you can get high-quality aquatic Frozen , both can enrich people's material life can also be exported in exchange for more foreign exchange . Overall, fast frozen in liquid nitrogen , dry consumption is small, increase the amount of the increase caused by freezing , especially in the freezing improve product quality and other aspects of economic income , will be able to compensate for the increased consumption of liquid nitrogen expenses, liquid nitrogen to freeze a certain economic benefits.
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