During freezing, heat is removed from a product. To thaw the product again, the flow of energy has to be reversed, i.e. heat has to be put back into the product. The two are opposing processes and they differ in some details. Whereas during freezing the prime aim is to reach the target core temperature in the product as quickly as possible ("shock freezing"), a lot more factors have to be considered during thawing. This makes thawing the more sophisticated and more complex process, particularly with regard to the possible influence on, and the predictability of, the final results. Many of the problems that can occur during thawing are directly connected to heat transfer in frozen products. Heat transfer is a relatively slow process and the larger and thicker the product is, the more time it takes. It can be accelerated by increasing the thawing temperature, but that would also damage the product itself. The longer the thawing process lasts, the higher the risk that bacteria and microbes will develop and spread in the product. Longer thawing times also lead to more drip loss which changes the texture of the fish tissue. The media that are used to transfer heat to the product, such as air, water or steam, also play a role here. And under industrial conditions there is often the additional problem of insufficient space for thawing and the fact that the process should not be too labour intensive.
A number of other factors influence the thawing process, too: the composition of the frozen products, their water content, and the proportion of fat, protein and minerals they contain. It also makes a big difference whether the fish will be cooked and eaten immediately after thawing, or whether it is to undergo processing to a convenience product with a correspondingly long shelf-life that will be eaten later.
Already the time required to thaw frozen products often proves to be a bottleneck which can slow down the production processes of manufacturing companies or even bring them to a halt. If, for example, raw material was not thawed in time or not in the quantities needed for subsequent processing stages it is hardly possible to supply the missing quantities in just a short time. To shorten the defrosting time, it is common in many companies to temper some of the ingredients to have them “on hold” for production. During tempering, the core temperature of -18°C, which is normal for frozen products is raised to about -10°C. The product is still frozen, but it can be stored for about a day in the refrigerator and then be thawed as required in a much shorter time. With every degree that the temperature rises, however, the risk of premature spoilage of the product increases too, because certain microbes grow better under higher temperatures. This means that great care must be taken when tempering that the temperature is kept permanently at the desired level and does not increase too strongly. A lot of industrial thawing facilities already have this additional option, and are able to keep the products at exactly the right temperature.
Carefully metered heat supply should protect against local overheating
The thawing methods available today can be roughly divided into two groups. The first group includes all methods in which the thermal energy is supplied from the outside and penetrates through the surface into the frozen product. The medium used can, for example, be heated air or water in the form of an immersion tank or a sprinkler to spray the water onto the frozen fish. A typical feature of this method is that the fish is always thawed from the outside to the inside, whereby thawing speed decreases over the course of time. This is mainly due to the fact that thawed fish is a relatively poor conductor of heat. The thawed tissue layers on the surface constitute a kind of insulation that protects the inner core which is still frozen. The thicker the layer of already thawed tissue is, the more effective is its insulating effect and therefore the longer the thawing time. This effect can be seen clearly in the temperature curve which rises more and more slowly in the course of thawing. For this reason, in the case of block frozen products the outer layers should be removed as soon as this is possible without causing damages.
The thawing methods in the second group are characterized by the fact that here the heat is generated directly within the tissue, i.e. the frozen food thaws in the inside and outside at the same time. This method uses the resistance or the absorbent capacity which the body tissue of frozen fish displays to electric current, radio waves or microwaves. A major advantage of these techniques is the significant reduction in thawing times. The disadvantage, however, is that the energy sources are difficult to control, with the result that local overheating of the fish flesh can occur. Regardless of the thawing method care must be taken to avoid heat accumulation because this will lead to quality losses. Quality suffers already as from a temperature of 20°C, and when the temperature rises above 30°C the first proteins will denaturalize in the fish’s body and the texture of the fillet will change. Although the thawing process should not take too long to keep drip loss as low as possible, it should not be too strongly accelerated either, for example by using high temperatures, because then the delicate fish tissue will suffer. Ideal thawing temperatures for most species are between 5 and at most 10°C. Once the product is thawed, it should be immediately cooled to about 0-2°C and stored until further processing or sale.
The simplest method, which does without complicated technology, is thawing in still air. This involves spreading out frozen fish or frozen fish blocks on appropriate materials in a room and then waiting. It is an extremely slow process that is suitable only for small product quantities because the temperature in the room should not exceed 18°C. By the time a frozen product has completely thawed under such conditions the outer layers have often already dried out and decay processes have begun. Although the method is very cheap it is rather unsuitable for industrial purposes.
Reduce drip loss, prevent development of microbes
Thawing times can be considerably shortened if the air is kept in constant motion, for example by using fans. Industrial machinery manufacturers offer a wide range of air blast systems ranging in size from small one-trolley chambers for a few kilograms to voluminous thawing rooms for several tonnes of frozen goods. Their constructional design is almost always the same or at least similar. The frozen fish is laid out on shelf racks in high trolleys (2 meters in height, for example). The larger the surface of the frozen fish or the frozen blocks and thus the greater the contact with the ambient air, the more quickly the product can thaw. For this reason, the distances between the shelf racks in the trolleys should be approximately twice the thickness of the product to enable the air to circulate evenly throughout. The air is slightly warmed and enriched with moisture to improve the transfer of heat energy to the fish and to prevent its drying out. Sensors monitor the thawing process, especially the temperature curve and humidity. If necessary, the ambient air is cooled. Air blast thawing systems allow "just in time" thawing of frozen products so that they are available at exactly the time they are required. This method is suitable for both batch processes and continuous thawing within processing lines.
A modified version of thawing in air is possible with vacuum equipment that operates at low temperatures and with high humidity. When the air is evacuated from the airtight unit the moisture condenses, is deposited on the frozen fish product and releases a significant amount of heat which speeds up the thawing process. This method is particularly effective because the condensing water vapour penetrates into even the tiniest gaps and cracks in the frozen fish blocks. These systems are often equipped with sterilization units in order to prevent the development of bacterial organisms. Modern vacuum thawing systems are not only gentle to the products because they transfer the heat evenly onto them and keep them constantly moist, but also very hygienic.
An alternative to thawing in the air is to use water. This can be done by direct immersion, by spraying water onto the frozen fish, or by a combination of the two methods. Water as a medium for transferring heat energy during thawing has several advantages. It is a denser medium than air; it can be heated well, and the thermal energy is stored longer. From an energy point of view it thus makes more sense to run the water in a circuit and use it a number of times over. The water has close contact with the product that is to be thawed because it wraps itself around the fish closely and transfers heat effectively. However, these advantages are counterbalanced by some disadvantages. Fish fillets that are left lying in water for a longer period are washed out and lose flavour and so this thawing method is more suitable for whole fish. It is also problematic that microbes accumulate in the heated water, and these can contaminate all the fish during thawing. This is especially true for systems where the water is used several times.
At its simplest, defrosting in water can be carried out in a tank with a continuous flow of water. Automated thawing systems have their own sensors and control units to enable continuous monitoring of all important process parameters and thus their optimal adjustment. Thawing systems in which frozen blocks are thawed by spraying with water are often constructed so that the outer layers of fish fall off the block onto conveyor belts so that they can be removed immediately. Another advantage is that the fish remain moist and the skin surfaces which are often germ-laden are constantly rinsed. Some thawing systems combine the immersion and spray methods with each other. Here the frozen fish are usually in baskets which circulate alternately through an immersion bath and a kind of shower facility in which water is sprayed from above onto the baskets. This is very effective because the heat is transferred directly and distributed uniformly over the entire surface of the product that is to be thawed.
Modern technologies shorten thawing times
Electric thawing methods are even faster and more efficient. They have hardly been able to assert themselves in industrial practice yet, however, because the technology required is relatively expensive, both to purchase and to operate. An extremely effective way to thaw frozen fish products, and probably the fastest of all, is based on the electrical resistance the frozen blocks display to flowing current. In "dielectric heating" the frozen fish blocks are literally jammed between two parallel metal plates between which current flows. Frozen fish is a poor conductor of electricity but the electric resistance decreases rapidly once the fish is warmer. The basic principle of this thawing method resembles an electric heater in which the filament heats up when electricity passes through it. Due to its high thawing capacity dielectric heating is especially suitable for large quantities of fish or applications that need to respond flexibly to changing demand requirements, for example dining establishments that are suddenly confronted with large numbers of guests. For optimum thawing results the blocks should be packed uniformly thick and homogeneous so that the current can heat them evenly. If the frozen blocks are not evenly packed this can lead to local overheating and the associated quality losses. Continuously operating dielectric thawing systems are able to thaw a tonne or more frozen fish in an hour. However, this advantage is very quickly outweighed by the effort and costs caused by this thawing technique, so that it is rarely used in practice.
Radio frequency defrosting works in a comparable manner only that the heat in the frozen product is not produced by the direct action of current, but by high-frequency radio waves that oscillate at several million cycles per second (up to about 80 MHz). With radio frequency thawing the frozen product passes on a conveyor belt through a tunnel in the upper and lower part of which (above and below the conveyor belt) two plate-like metal electrodes are arranged. They are connected to a radio-frequency generator which produces the high-frequency oscillating radio waves. Their energy sets the dipolar water molecules in the frozen fish in motion, the molecules rub against each other and thereby produce heat throughout the product, not only on the surface but also deep inside, regardless of the thickness, size and shape of the fish, or its thermal conductivity. In this way the fish thaws quickly and evenly. The heating within the product can be controlled by the amount of electric voltage applied to the two electrodes, and the speed of the conveyor belt.
Because thawing with radio waves is relatively fast, the losses through drip loss remain low and microbes can hardly develop. Due to the good control of the process this method can not only be used for defrosting, but also for tempering frozen products. Since radio waves can penetrate non-metallic materials, frozen products can often even be thawed directly in their packaging, for example in cartons, polystyrene boxes or polythene bags. The high speed of continuously operating radio frequency defrosting systems allows frozen raw materials to be included in production lines according to "just-in-time" criteria. Compared to traditional thawing techniques that are based on air or water and often require larger thawing rooms RF systems also require much less space for the same capacity.
Thawing systems that work with microwaves have similar benefits. The basic principle behind them is largely similar to that of the microwave ovens which are to be found in almost every home kitchen. Unlike the domestic devices, however, the industrial systems usually operate continuously, so that frozen products are thawed within just a few minutes. The electromagnetic field of the microwaves is generated by a magnetron like those that are also found in radar systems. This thawing method is particularly suitable for thawing thinner frozen products, because the waves are proportionately absorbed in the upper layers of the frozen products. However, modern industrial systems are designed so that the "outside warm inside cold" effect hardly occurs and the products are thawed evenly. It is particularly important that the electromagnetic field is not too strong so that the product is not overheated or even partially cooked during thawing. When used correctly, microwave thawing delivers perfect results. Taste, smell, texture and colour of the fish flesh are not changed and the nutrients are retained. Added to this, the method is very fast. A standard block of frozen fillets at a temperature of -18°C can be thawed in less than 5 minutes. Continuous thawing systems that work with microwave or radio frequency technology are mostly available with capacities ranging from 100 kg to 12 tonnes per hour.
The question as to which is the best thawing method can hardly be answered objectively because the decision for one or the other technology should be based solely on the actual operational situation, the required performance and necessary flexibility, plus various other factors. Therefore, it would be more correct in this context to speak not of the best, but of the most appropriate, thawing technique.