Defrosters for the fish industry

Whether it’s herring from the North Atlantic, shrimps from Asian farms, or hake from the southernmost point of Africa, a lot of seafood products are traded frozen. Before they can be further processed they have to be thawed. In industry, special defrosters are used to do this. They are based on different technologies and for this reason alone there is no “ideal” technique that fulfils all requirements.

Before investing in defrosting systems a company should precisely define its needs.

Every defrosting technology has its advantages and disadvantages and not every system fulfils all requirements. Anyone wanting to invest in industrial defrosting equipment should therefore define their needs exactly beforehand to enable them to find the most suitable system. Whilst some companies make only occasional use of frozen products and can defrost them in batches, others need automatic defrosting units that work continuously to enable a constant supply of raw materials. It is also of significance how much time is available for defrosting. The desired thawing speed does not only determine the size and the performance of the planned system but also, and to a large extent, the choice of defrosting technology. Companies that do not have steam or hot water connections in their production rooms would possibly have to face follow-up costs. A further point for consideration is which hygiene requirements the product will have to fulfil later on. Foods that up to consumption will not be heated again after thawing have to meet much more stringent requirements than products that will undergo thermal treatment. Anyone who invests in an automatic defrosting unit needs more qualified staff than is required for thawing products in a simple water bath. Independent of this, however, defrosting will lead to additional work that will take additional time because the defrosting apparatus naturally has to be cleaned, disinfected and maintained regularly. The necessary financial outlay often soon pays off, however, due to the higher material yield and constant, often even better quality of the defrosted products which is to be expected from using such systems.


Defrosting with still and forced air

The least expensive but also the least effective method is of course to defrost seafood in the air. The ambient temperature should not be too high (0 to max. 6°C), however. For this method, the product’s packaging and any insulating layers have to be removed and the product preferably spread out in just one layer. Because air has a relatively low thermal conductivity the thawing rate is very low. In order to accelerate the process the outer layers of the frozen block that have already started to thaw should be removed from time to time to allow the air direct contact with the inner regions of the block. This method is cheap but hardly suited to defrosting larger quantities of raw materials, particularly since product quality suffers considerably during slow defrosting because through the long period of time spent in the air the fish loses a lost of moisture from the surface and dries out. It is mainly fillets that are affected here. Another problem of air drying is that it is difficult to monitor and regulate the temperature within the product.

Frozen products are often thawed by immersing them in water in stainless steel baths for several hours.

Air blast thawing units in which water saturated air heated to about 40 to 50°C (usually with steam) is blown over the frozen products at a high speed are considerably more suitable for defrosting. Periodically the direction of air flow is reversed so that the seafood is warmed more uniformly. The circulated air is humidified in the recirculation duct by water sprayed from nozzles. This prevents the products from drying out. Such thawing units that work with moving air (forced air thawing, air blast thawing) can cope with considerably larger quantities of seafood, particularly when they work in continuous operation. They allow for much better process monitoring and control. Additionally, they are comparatively economical because the heated air remains within the unit if it is well sealed off and so can be used several times over, with only the inevitable heat losses having to be compensated. In this kind of thawing unit the frozen products can be arranged either on racks or on conveyor belts. Because forced air units have a high thawing capacity which can also be very finely adjusted they are preferably chosen for sensitive seafood products such as frozen shrimps or fillets of high-value fish species. Problems can really only arise through insufficient maintenance of the unit or due to programming errors. If, for example, the thawing time is set too long the product can be over-thawed or even pre-cooked. An ambient temperature of 40 to 50°C would be high enough for that. This can also happen if the products to be thawed are arranged in such a way that the air accumulates in certain zones leading to the development of hot spots.


Water based thawing methods

One of the simplest methods that is used particularly frequently within the fish industry is water based thawing. The spectrum of methods used ranges from simple immersion tanks to continuously operating sprinkler systems which thaw the frozen products with heated water. Water is a good defrosting medium because it conducts heat 24 times better than air. Water based thawing techniques are particularly suited to whole or gutted and headed fishes (h & g), frozen blocks of fish and frozen shrimps. They are less recommendable for thawing fillets because the texture and flavour of the muscle flesh can be negatively influenced by contact with the water.

Within the fish industry the simple water bath made of stainless steel is a popular utensil. The frozen product is simply put into tanks filled with water and left there for a few hours, usually overnight. Mostly a hosepipe is installed to distribute water into the tank whereby the fresh water flows into the tanks constantly whilst the old water which is soiled by thawing glazing, slime, fish blood, drip loss etc. flows out via an overflow. With regard to investment costs such immersion tanks are relatively inexpensive which presumably contributes to their wide use within the industry but this supposed advantage is often forfeited as a result of their relatively high water consumption. In the meantime there are already technically more sophisticated systems which are equipped with integrated water heaters, filters and pumps and which use the water several times over. The quantities of soiled water arising in such systems can still be considerable, however. A rule of thumb says that about 3 to 4 litres of water should flow through the system per kilogram of fish to be thawed.

A further problem stems from the fact that water based defrosting plants are difficult to monitor and control. In order to keep the water temperature constant and geared to the products it would have to be possible to change the water throughput rate during the thawing process as required (the thawing process should be stopped at the latest when the product’s core temperature gets close to 3°C). This is particularly relevant if tap water is used for thawing because the temperature of tap water can vary considerably over the course of the year. There are hardly any systems which offer such options, however. Through being in contact with the water over a longer period of time the raw materials can leach and lose quality, particularly product parts that are at the edge of the frozen blocks. Whilst these parts defrost first and then remain immersed in the water for hours the fish at the centre of the block is often still frozen. The long time spent in the water does not only have a detrimental effect on the texture of the muscle flesh but can also lead to mass development of microorganisms. Even if there is only one single contaminated frozen block within a water bath all the others can be infected too and rendered unusable.

Blowing warm humid air over frozen products is an effective way of thawing them.

A more developed variant of water based thawing techniques is a continuously operating sprinkler system in which the frozen fish lies on conveyor belts or in shelf-like frames and is sprayed from above by a water shower. The water is usually heated to 16 to 18°C which improves heat transfer to the frozen products and shortens thawing time. Although these thawing systems have certain advantages over water baths, the basic disadvantages such as high water consumption and the risk of excessive pathogen development remain.


High frequency, vacuum and microwaves

In recent years new defrosting techniques have been developed which have certain advantages over the traditional methods but are considerably more complicated as regards technology and mostly also much more expensive.

The first of these techniques uses the heat that develops when a high-frequency voltage is applied to frozen blocks to thaw them. The frozen blocks are placed between two parallel metal plates across which a high frequency voltage is applied. If the voltage and current frequency are sufficiently high, heat develops in the frozen block and the ice it contains begins to melt. This method is convincing due to its high performance and speed. A disadvantage, however, is the relatively high electricity consumption and the risks that electricity involves. Apart from that, electric defrosting only leads to satisfactory results if certain conditions are fulfilled. The frozen blocks should be of the same thickness throughout and as far as possible of uniform composition (i.e. the frozen fishes or seafood should not be too big and have to be spread evenly within the block.) This technique is thus less suitable for large species that are frozen round and are distributed irregularly in the blocks because here there is the danger of localised overheating within the product. Such inhomogeneous blocks can be dipped briefly into water prior to thawing to achieve a more uniform conduction of electricity in the block. These drawbacks limit the method’s practical applicability noticeably, however, so that it has so far not been used much in the fish industry.

An equally effective and gentle but more technically demanding technique is vacuum thawing. This technique involves the frozen products being placed in an airtight chamber. (The largest systems currently have a capacity of between 10 and 12 tonnes.) A vacuum is then created within the chamber. At the chamber’s base is a reservoir of water which is heated slightly so that it evaporates constantly. The water vapour condenses on the cold surface of the frozen products which absorb the heat energy contained in the water misting and thereby thaw quickly. The system can be regulated relatively well via the vacuum and the evaporation volume. Products that have a large surface area in relation to their shape and thickness are particularly suited to this thawing method. The thicker a product is, of course, the more time it takes to thaw completely.

As in the household, microwaves are a useful method for defrosting frozen products in industrial plants, too. The energy-rich radiation penetrates the food and causes water molecules in the product’s interior to oscillate (dielectric heating). The resulting heat ensures that the product does not only thaw on the outside but at the same time in its interior, too. However, frozen water is relatively hard to heat using microwaves because the water molecules are fixed within the ice crystal and will thus only oscillate to a limited extent. This is, however, not the main reason why microwaves are relatively rarely used for thawing in industry but rather the danger of causing thermal instabilities in the frozen products. Everywhere where there is a particularly large amount of water in and on the product more radiation is absorbed and heat produced. This increases the risk that the product will be overheated in some areas whilst just a few centimetres away it is perhaps still frozen. Microwave thawing systems are thus difficult to control. They are rarely used for thawing frozen products completely but are rather used to increase their temperature to slightly below zero degrees, usually to about -5°C. In this way it is possible to limit the risk of overheating and still shorten thawing time.

The initial investment in water-based thawing systems is low, but water consumption is relatively high.


Radio frequency defrosting plants and new techniques

In Europe microwave ovens typically work with electromagnetic radiation which has a frequency of 2.455 GHz. In other countries, particularly in the USA, frequencies of about 915 MHz also come into consideration for industrial microwave ovens. The 902-928 MHz range is freely accessible in the USA as so-called ISM frequency band (Industrial, Scientific and Medical band) – ISM bands are those frequency ranges which can be used for appliances in industry, science and medicine and in the household). Both frequency ranges have their advantages and disadvantages. Basically, the lower the frequency the greater the penetration depth but the lower the absorption rate, too. If the frequency is too high the penetration depth is accordingly low so that only the surface is heated.

That is why industrial defrosting systems were developed that operate with radio waves of relatively low frequency. The way they work is similar to systems that are based on microwaves. The product that is to be defrosted is placed between two parallel electrodes which alternately radiate radio waves. In contrast to microwaves the temperature in the product rises relatively evenly and uniformly although differences can occur here, too, if the frozen block is not sufficiently homogeneous. The risk of local overheating rises the closer one gets to 0°C. That is why the thawing process, similar to with microwaves, is usually ended shortly before this limit is reached (at about -2°C) with radio waves, too. One can reckon on about 15 to 45 minutes to defrost a 5 cm thick frozen block. Depending on the construction type, radio frequency defrosting systems are available both for batch defrosting (c. 40-600 kg/h) and for continuous operation (900-3,000 kg/h).

There are also new kinds of defrosting technology that promise good thawing results. These have either been undergoing tests for several years or are already on the market. Among them are climate systems in which water or steam is sprayed finely into the thawing chamber through jets. This function corresponds largely to thawing under vacuum whereby with the new systems a vacuum does not have to be created. Humidity and temperature within the chamber are constantly controlled so that consistent conditions prevail. Sensors monitor the surface and core temperatures of the frozen products and reduce the performance and heat input to the same extent that the temperature of the frozen products approaches thawing point.

It has long been known that extremely high pressures of about 200 MPa are very suitable for freezing. Due to the high pressure the development of ice crystals is avoided. The product cools to temperatures of about -21°C without ice crystals forming. These only arise – but then suddenly and at the same time everywhere – when the high pressure is reduced. Due to this lightning-speed transformation the damages to the tissue are only slight. Now there is considerable hope that the use of high pressures might offer advantages during thawing, too. Partly this has already been proved in experiments. In the case of frozen tuna blocks that were thawed under 50 to 150 MPa in 30 to 60 minutes the drip loss was considerably lower than when defrosting blocks under normal pressures, for example. However, there were undesired side effects, too, such as a rather unattractive discoloration from the typical tuna red to pink. Equally undesired were the effects when thawing surimi. Here, too, there was discoloration, and sometimes the proteins contained in the product even denaturalised. Only the future will tell whether this thawing method is really suited to industrial applications.