The changes in frozen fish are chemical in nature and result in toughening and drying of the flesh and the development of undesirable flavors. The rate at which this proceed depends largely on temperature, the slower the rate of change. The rate is particularly high in the region of -5 ° C.

      According to a CIFT report, many consumers do not realize that, when they eat fish which has been frozen and has acquired a tough texture and objectionable flavors, it is often not the fisher or aquaculturalist who is at fault but rather, deficiencies in cold storage and distribution that are to blame.

      Temperature is not the only factor that determines the rate of deterioration but it is the most important. Various other aspects of cold store design and operation, including the use of packaging and water glazing, also affect the rate of deterioration.

Storage Factors

      The need for storage at a steady -30 ° C for fish has been well and truly demonstrated in commercial operations for close to 40 years. (There are a few exceptions. In Japan, for example, tuna is stored at somewhat lower temperatures in order to prevent color changes.)

      It is not sufficient, however merely to specify the temperature of the cold store, container or cabinet for frozen fish because it is the temperature of the product itself that largely determines the rate of deterioration.

      The product should be cooled to the required temperature before storage because, with the high storage rates and low cooling capacities normally employed, cooling in the cold store is too slow.

      Furthermore, a product that has been cooled to the required storage temperature can be badly affected by heat from adjacent, inadequately cooled products. For these reasons good freezing and handling practices before storage are essential.

Freezing & Handling

      There is a fair amount of folklore linked to the speed of freezing and quality. Some of it is concerned with formation of ice crystals and dehydration and much of it is claptrap.

      Generally, as far as eating quality is concerned, the time taken to freeze a product matters little, provided it is not more than a few hours. Of course where is sound reason for "Quick" freezing; it allow orderly and efficient production, apart form the question of quality. By and large, however, deterioration of quality take place in cold storage, handling and distribution, not in the freezer where time is relatively short.

      The main function of the freezer is to cool the product to the required temperature of cold storage. This is crucial but not always done, especially in time glut supplies of fish when maximum output is required, although it is possible to measure product temperature and determine freezing time without much difficulty

      Fish that has not been cooled sufficiently before storage is liable to suffer marked deterioration. The storage of "warm" product is the major cause of dehydration or so-called "Freezer burn" Freezing and cold storage should be viewed, therefore, as quite separate operations. Use of the term "freezer" to describe the cold store is commonplace but should not be accepted; it stems form the outdated practice of freezing and storing in the same room.

      It also follows that, during handling and distribution, every effort should be made to maintain frozen fish at a low temperature until thawing is required. Since the temperatures encountered in distribution and retail are substantially higher than -30 ° C, the time involved at these stages should be kept short. Unfortunately, current practices leave a lot to be desired.

      The seafood industry's dependence on freezing and cold storage makes uniformly good cold storage practices absolutely essential if consistently high quality product are desired. The industry itself can bring about improvement if processors and cold storage operators agreed and adopted the required standards and exerted pressure on distributors and retailers to maintain high standards.

1. Air Defrosting
      Products defrosted by this method should have the temperature at <40 ° F because if the products have the temperatures between 40 ? F and 140 ° F, the bacteria and microorganisms will grow in that range of temperatures.
The advantage of air defrosting is that it does not require sophisticated equipment or advanced technology. In addition, the products to be defrosted by this method can be refrozen without going through a cooking process. However, it requires a long time and large space for air defrosting method which is its disadvantage.

2. Cold Water Defrosting
The cold water defrosting is quicker than air defrosting, an advantage of cold water defrosting method. The disadvantage is the cold water defrosting must be carried out with cares - that is the products must be filled in a package with a perfect condition, no leaks. If the water from outside can get into the products, it will cause sponge in the products which will lower their quality e.g. meat products, chicken, pork, etc. Another disadvantage of cold water defrosting is using water as a medium to transfer a large volume of heat in order to raise the temperature of products. Another important point is that cold water defrosting method will cause approximately 6% of drip loss.

3. Defrosting by Microwave Tempering
      The application of microwave tempering in the defrosting method is a technology that can increase temperatures from -18 ° C to - 4 ° C in a very short time. This technology has been popularly used in the food industry such as chicken and pork industry or seafood industry. The defrosting time takes only 3-5 minutes, which is the advantage of this method.
The disadvantage is mixed compounds disintegrated and unequal density on every spot of the products. For example, some parts of meats are lean, but some are fat which can better absorb energy from microwave, yielding more heat on such a spot. As a result, the temperatures are not even throughout a piece of meat.

Movement Direction of Microwave

      The microwave technology cannot make the temperatures of a product higher than -4 ? C - -3 ? C because parts of the product are cooked, depending on type of product. In addition, the nature of frequency wave of 915 MHz, which is quite high and short wave length, affects the difficulty in directing the wave control, compared with a radio frequency.

4. 27.12 MHz Radio Frequency Defrosting
      The radio frequency defrosting is currently the newest and modern method. The products going through this defrosting method are readily in use and can also be separated from the frozen block With the radio frequency technology, the regularity of electrical fields is better that of microwave. Since the wave length of radio frequency is longer than that of microwave, the direction of radio frequency wave is, therefore, better controlled.

Radio frequency tunnel 105 kW
Production capacity of 750-3,000 kg./hour

Movement direction of radio frequency

Factors Affecting the Decision on Selection of Defrosting Methods
1. Budget
2. Production capacity
3. Space
      However, decision on selecting a method that can be done quickly will have an impact on the quality of products in a positive way. This is because during the defrosting process, the products tend to deteriorate because of chemical and physical reactions and microorganisms.