Analysis on the advantages and disadvantages of the methods of food thawing
Read： Publisher：Weifang Higher Machinery Co., Ltd Release time：[2019-12-13]Analysis on the advantages and disadvantages of the methods of food thawing
In the processing of meat products, it is necessary to unfreeze the frozen meat products, and the quality of meat products depends not only on the freezing technology, but also on the unfreezing technology. Thawing technology: in the food processing industry, it is called thawing and rewarming, which is the reverse process of freezing.
In terms of time, even if the same temperature difference between freezing and thawing is used as the heat transfer driving force, the thawing process is generally slower than the freezing process. The general heat transfer process is from the outside to the inside, from the outside to the inside. When freezing, the surface of the material first freezes, forming a solidified layer; when thawing, the surface of the food material first melts.
Some physical properties of water and ice at 0 ℃
It can be seen from the above table that the heat capacity (2.120) of ice is only half of that of water (4.2177), but its thermal conductivity is 4 times of that of water, and its thermal conductivity coefficient is 8.6 times of that of water. Therefore, the heat transfer condition in the freezing process is much better than that in the melting process, it is difficult to achieve a high re temperature rate.
1 air thaw
Air thawing is a thawing method with air as the heat transfer medium, but it can be divided into the following types:
1. Still air thawing (low temperature breeze air thawing)
In this process, the frozen food is placed in the cold storage (usually the temperature is about 4 ℃), and the natural convection of low temperature air is used to defrost. Generally, frozen beef can be completely unfrozen in such a warehouse for 4-5 days.
2. Flowing air defrosting device
This process is also carried out in the cold storage, using 0 ~ 5 ℃ and 90% relative humidity of wet air (need to place humidifier), using a cold fan to make the gas flow through the frozen products at a speed of 1 m / s, the thawing time is generally 14 ~ 24 hours, which can be fully or semi thawed.
The above two methods belong to the most common method in air thawing, with the advantages of slow thawing, so that cells can absorb the juice formed by thawing again, which can better restore the original quality of raw materials and reduce the loss of nutrients; but the disadvantages are also very obvious: due to the long time of thawing, there are many opportunities for microbial proliferation, high risk, and inconvenient for planned production.
2 hydrolytic freezing
With water as the heat transfer medium, compared with air defrosting, the defrosting speed is relatively fast, mainly divided into the following ways:
1. Water immersion and thawing
One is low-temperature water thawing (immersing the thawed product in the flowing low-temperature water to thaw it. The thawing time is determined by water temperature and water flow rate.
The other is still water thawing (the thawing product is immersed in still water for thawing. Its thawing speed is related to water temperature, quantity and quantity of thawed products)
2. Spray thawing
The impact force of spray water is used to improve the thawing speed.
In addition to the impact force of spray, there are also spray water quantity and spray water temperature. However, it can only be thawed as a small fish, not for large meat products and fish.
In the above methods, when thawing, the internal ice crystal melts into water. If it can't return to the original cell and be absorbed by the meat, the water will turn into juice drops and flow out. The main reason is the ice crystal in the process of freezing and the mechanical damage of ice crystal growth in the process of freezing storage.
Microwave thawing
It is the process of using microwave to defrost frozen products. Generally, the industrial microwave frequency is 915MHz. Because there are many polar molecular components in the frozen products, such as water, protein and glycogen, which have great absorption capacity for microwave energy, the defrosting speed is very fast.
About juice loss: as mentioned above, solidified layer will be formed on the surface of the material during the freezing process. When thawing with microwave, the inside and outside of the food will be heated at the same time. Because the freezing point of the ice crystal inside the material is low, it will melt first, but at this time, the solidified layer on the surface does not melt, which is equivalent to forming a protective layer on the outside of the food. Therefore, juice loss is also relatively high Less.
