Gentle techniques for rapid deep-freezing
This article was featured in Eurofish Magazine 1 2025.
Deep freezing is a time-honoured technique that has roots in ancient practices, notably among the Inuit of Greenland who used it to preserve fish. However, it was not until the early 20th century, with the innovative patents of Ottesen and the pioneering work of Clarence Birdseye, that the method gained traction in industrial settings. Today, market analysts project that the global trade in deep-frozen fish and seafood will approach a staggering USD 400 billion by 2028.
Freshly caught fish has a notoriously short shelf life and is not always available everywhere. This limitation makes frozen fish a viable alternative, as it can be stored for months without compromising flavour or nutritional quality. In addition, frozen fish is often pre-portioned and practically boneless, allowing for quick and easy meal preparation. Its consumer-friendly attributes have solidified its status as one of the most popular seafood options; virtually every supermarket boasts an expansive frozen fish section. Additionally, freezing serves a crucial role in food safety—extreme cold effectively eliminates parasites that may lurk within. Not only large nematodes, but also tiny ones that are only visible under a microscope. To protect against parasites, deep-freezing is even mandatory for fish products intended for
raw consumption.
Rapid freezing offers advantages over regular freezing
While freezing is a powerful preservation method, it is important to clarify that it does not sterilize food as high-heat processes do. Instead, freezing slows chemical processes, particularly oxidative and enzymatic activities, and inhibits bacterial growth. However, bacterial decomposition can still occur at lower temperatures where the bacteria themselves can survive. That is why it is so important to maintain the cold chain without any breaks to prevent bacterial activity from resuming and spoiling the food. Clarence Birdseye’s research revealed a critical insight: the speed of freezing significantly impacts the quality of the frozen product. Slowly frozen water forms large ice crystals that can damage cell walls and result in moisture loss or dehydration. In contrast, rapid freezing—often referred to as flash freezing—produces smaller ice crystals that cause less damage. The amount and extent of cell wall damage and thus the moisture loss (‘drip loss’) are inversely proportional to the speed of freezing. This finding has led to the development of fast and efficient industrial flash freezing processes. A clear distinction exists between conventional home freezing and industrial deep freezing.
While a home freezer can take up to 24 hours to freeze fish, industrial methods achieve a temperature of -18°C in less than three hours. This advanced freezing technology preserves the fish’s freshness, texture, and essential nutrients. The selection of industrial flash freezers is expansive, and choosing the right freezing technology is influenced by a combination of financial, functional, and technical considerations. From small laboratory to large-scale applications, everything is possible. Which process and equipment is best suited for a specific application depends on the size, shape, and composition of the raw material, as well as the desired end product. Fish fingers are cut from compact, deep-frozen fillet blocks with defined dimensions and have different requirements than interleaved fillets. These are placed in layers on plastic sheets and deep-frozen, so that they can be removed individually from the packaging later. The same applies to separately deep-frozen IQF (Individual Quick Frozen) fillets, which can be taken out and prepared as needed, without having to thaw the entire package.
What factors influence the choice of deep-freezing technology?
When selecting a device for deep-freezing fish, financial, functional, and technical aspects must be considered. Financial, as the acquisition and operating costs of the technology must be aligned with the company’s profile and turnover. Functional, because the device must be suitable for deep-freezing the raw material. Horizontal plate freezers are not suitable for freezing whole tuna, for example. Additionally, the freezing technology chosen must fit in with other operational processes. It does make a difference whether a continuous flow or batch operation is used. Then, if a deep-freezing device appears to be suitable, further technical prerequisites must be considered. For example, if you decide on a high-performance and versatile cryogenic freezer, you must take into account that you will need a regular supply of liquid nitrogen or liquid carbon dioxide for its operation. Where this is not secured, the acquisition of this type of device makes little sense.
To reduce the risk of breakage of blocks made from very fatty fish products, they are often filled with water to stabilize them before deep-freezing.
As with other foods, the main goal of deep-freezing fish and seafood is to quickly remove the heat by exposing it to as low an external temperature as possible. There are several technical options for implementing this principle. For example, deep-freezing can occur through direct contact between the fish product and a severely cooled surface (contact or plate freezer). In so-called ‘blast freezers’, on the other hand, a cold air stream is continuously blown over the fish to enable heat transfer. In cryogenic processes, the fish products are sprayed or briefly immersed in extremely cold liquid nitrogen (-196°C) or liquid carbon dioxide (-78°C), leading to immediate freezing on the surface and subsequent rapid freezing through the whole product.
Plate freezer
The plate freezer from Birdseye was the first piece of equipment to be developed, in which the packaged fish is clamped between two plate-shaped hollow bodies through which a refrigerant with a temperature as low as -40°C flows. These devices are equipped with hydraulic systems that can move the plates together and apart, pressurizing the product. In order to utilize this freezing principle efficiently, the fish products should have a uniform, flat shape (e.g. fillet blocks), which enables full-surface contact and is a prerequisite for rapid heat extraction. The cooling plates can be arranged horizontally (HPF) or vertically (VPF). Horizontal plate freezers are mainly used for freezing packed cartons with rectangular blocks of fish fillets. The plates must be cleaned regularly between freezing cycles and any ice residue removed to ensure close contact between plates and cartons. If this contact is not guaranteed over the entire surface, the freezing time can be three to four times longer.
In vertical plate freezers, fish can also be frozen unpacked. The upright plates form a kind of container into which the fish is gradually loaded from above. This is particularly practical at sea if the fish is to be filleted or otherwise processed later on land. The only limiting factor here is the maximum weight of the blocks, which must be able to be transported without damaging the fish and with reasonable physical effort. Fish such as cod and haddock produce compact blocks with a density of around 800 kg/m³. However, blocks of fatty fish are not as strong and stable as blocks of lean fish. This is why a little water is often added to herring to fill cavities in the block and give it more stability. The slurry also protects the fish from oxidation and prevents it from drying out prematurely. Contact freezing methods also include immersion in ice-cold solutions, which flow around even irregularly shaped products and ensure even heat transfer through close contact. In the traditional Ottesen process, highly concentrated sodium chloride brine solutions were used for this purpose. However, sugar, alcohol, glycol, or glycerine solutions are also generally suitable, as long as they do not impair or distort the product’s typical taste, colour, aroma, or health value.
Cold air freezing
The most commonly used method for deep freezing is heat extraction using air. However, this relatively thin medium does not have good heat transfer rates and therefore has to be constantly kept in motion with fans (forced convection). The higher the air flow rate, the faster and more evenly the fish product freezes. For example, a fish fillet takes about four times as long to freeze in the stagnant air of a cold store as it does in a constant airflow of 5 m/s. Air blast freezers come in various designs suitable for both batch and continuous operation. Depending on the operating principle, a distinction is also made between fluid bed and belt freezing, as well as spiral and tunnel freezing processes. The fish products are usually laid out freely or in flat trays on a conveyor belt that transports them through a freezing tunnel through which ice-cold air continuously flows. For small and light products such as shrimps, the air flow can also be introduced from below through the mesh belt, so that the individual pieces swirl up and literally float on the air bed, where they are deep-frozen separately (IQF, individual quick freezing). This process is called ‘flow freezing’ or fluidized deep-freezing.
Air blast freezers are usually used in a continuous process. The fresh fish is normally fed into the device at one end and comes out frozen at the other end. The use of this type of device is limited, among other things, by the required freezing time, which should not exceed 30 minutes if possible. Otherwise, there are structural and technical limitations because the linear freezing tunnel then becomes very long, which makes the supply and continuity of the cold air flow more difficult and expensive. Spiral freezers are a solution to this problem, as their design allows the device to take up a particularly small footprint. Here, the frozen products lie on an endless freezing belt that winds around a rotating drum and slowly spirals upwards in the ice-cold air.
Vertical plate freezers can be conveniently filled from above. Here, for example, with shaft waste, which is deep-frozen in blocks until it is delivered to the fish meal factory.
The conveyor belt on which the fish products lie has a significant influence on the quality of the frozen end products. It must be flexible, easy to clean, corrosion-resistant, and suitable for direct contact with food. In addition, it must not extend the freezing time too much. In practice, stainless steel mesh belts are often used, but these are quite expensive and can affect the appearance of the finished product if the structures of the belt show on the contact surface of the fillet. Open mesh belts can also make it difficult to remove the frozen products. If fillet tissue sticks to the belt, this can lead to weight loss. Such defects are practically impossible with plastic belts. Although they are often cheaper to buy, they extend the freezing time by around 10%. The freezing time of continuous cold air belt freezers depends not only on the type of conveyor belt, but also on the principle of air circulation. Industrial units work with either cross or row airflow, which affects both the freezing time and the product-specific energy requirements. To ensure high efficiency in the row arrangement, the air flow is aligned so that the coldest air always hits the coldest fish first (counter-current principle).
Technically, it is extremely difficult to direct the cold air stream over the belt onto the fish in such a way that it freezes effectively and saves energy. Air takes the path of least resistance. It avoids the relatively narrow spaces between the products and swirls around in the empty space above. The quality and efficiency of a cold air freezer is therefore also measured by how well the manufacturer succeeds in directing the air stream as precisely as possible onto the fish using baffles and other design details.
Cryogenic process
The cryogenic process is currently the fastest, gentlest, and most quality-preserving, but also the most expensive method of freezing. Here, too, the products are transported on a conveyor belt through a tunnel, at the entrance of which they are greeted not by cold air, but by liquid carbon dioxide (-78°C) or liquid nitrogen (-196°C). These non-toxic gases are sprayed directly onto the unpackaged fish or seafood, absorbing their heat and cooling them to freezing temperatures in an extremely short time. At the moment of spraying, the carbon dioxide turns into dry ice and steam, which heats up by absorbing the heat from the food and is therefore continuously extracted to ensure consistently low temperatures. The use of liquid nitrogen follows the same principle, but due to the extremely low temperature, requires a high degree of control in order to achieve consistent product quality and protect the operating personnel from health hazards. Cold-resistant protective clothing is essential for cryogenic freezing processes to prevent accidents. This is especially true when liquid CO2 is used in confined spaces. Here, measuring devices are essential that immediately sound the alarm if the oxygen content in the room air drops too low and endangers the breathing of the staff.
In modern liquid nitrogen freezers, deep freezing usually takes place in two stages. In the first step, the fish on the stainless steel conveyor belt comes into contact with a countercurrent of nitrogen gas (temperature around -50°C). During this pre-cooling phase, the fish freezes on the surface, and up to 50 percent of the product’s heat is removed. Only then does the product pass under the liquid spray, where the fish freezes completely. It usually remains in the device for a few minutes afterwards to stabilize the process and achieve uniform temperature conditions in the tissue.
The advantages of cryogenic freezing with liquid nitrogen include the speed of the process and the small size of the freezer. Since it does not require compressors, condensers, or coolers, maintenance, and operating costs are comparatively low. However, the liquid nitrogen must be stored in a vacuum-insulated pressure vessel with continuous venting to keep the contents cool and the internal pressure low. Nevertheless, around 0.5 percent of the contents are lost every day during storage, and an estimated 10 percent of the liquid is lost when the storage vessel is filled. It must also be taken into account that liquid nitrogen is relatively expensive and not always available in sufficient quantities everywhere. In general, it can be assumed that cryogenic deep freezing is up to four times more expensive than conventional air blast freezing.
Like liquid nitrogen, liquefied carbon dioxide is sprayed directly onto the product. Unlike nitrogen, however, this coolant has the advantage that up to 80 percent of it can be recovered and liquefied again. This makes the process much more economical, especially for large freezer units. In addition, carbon dioxide can be stored in insulated containers at moderate pressure, which significantly reduces storage losses. Optionally, very large frozen goods, such as whole tuna, can also be completely immersed in the coolant, which greatly accelerates heat dissipation and enables very rapid deep-freezing. Liquid carbon dioxide is particularly suitable for this type of freezing because, unlike other coolants such as NaCl brine, whose viscosity increases significantly as the temperature drops, it flows completely around the product and ensures surface contact. In addition, CO2 is completely harmless to the product and changes the taste and texture only slightly, if at all. With brine, on the other hand, it cannot be ruled out that salt will penetrate the fillet meat due to osmotic effects.
Manfred Klinkhardt