When a fish is stored in melting ice its body temperature falls to about minus 0.5 to 0.6°C, or below the significant limit of 0°C when liquid freshwater solidifies to ice. But although white fish such as cod, saithe and other fish species consist of 80% water they do not freeze at these temperatures because salts, proteins and other dissolved substances reduce the freezing point of their body fluids. In the case of cod, for example, the first ice crystals develop in the blood and tissue fluid at a temperature of minus 1°C. This means that it is possible to cool the fish to slightly below 0°C without it freezing completely and thereby losing the valuable quality status of a fresh product. This cooling technique, which is somewhere between deep freezing and “normal” storage on ice, is called superchilling. The word is used in everyday language for almost every cooling technique for which the temperature is slightly below the critical value of 0°C. Strictly speaking, however, the whole fish – both on the outside and in its interior – must have temperatures of nearly -1°C to a maximum of -3°C for it to be “real” superchilling. There are certain differences from fish species to fish species but in general it can be said that at temperatures of -2.0 to -2.2°C about half of the fluids contained in the fish’s body will be frozen.
Only “real” superchilling will bring the desired key benefits of this technique. The lower temperatures in comparison with normal ice storage reduce bacterial growth which strongly retards spoilage and decomposition processes, so the fish keeps longer, and remains edible for longer. Depending on the quality and freshness of the original product superchilling can extend shelf-life by about two to four days. This widens the time slot that exporters and traders have at their disposal for marketing the product. Superchilled products can, for example, be transported over longer distances to the customer without suffering serious freshness loss which allows exporters to choose a more economical means of transport, such as sea freight instead of air freight. Compared to conventionally ice cooled fishes, traders have more time in which they can offer the products in the usual quality.
Just exactly how long, depends largely on the concrete conditions of superchilling. When stored in chip ice white fishes such as cod with a water content of 80 to 82% are edible for about 15 days. If the raw materials are fresh, and cooling measures are carried out correctly, there are practically no serious quality differences during the first 12 days between fish that is stored conventionally on ice and superchilled fish. The superchilling technique shows its real value when storage time exceeds this. At temperatures of about -1.0°C where about 10 to 15% of tissue fluid has solidified to tiny ice crystals shelf-life already rises to about 20 days. At -2.2°C even to 25 days. However, at this stage already a good half of the tissue fluid has frozen to ice. This is considered acceptable because it hardly limits the usability as a fresh product. At -2.8°C, when the fish’s shelf-life rises to 35 days, nearly all of the tissue fluid is frozen. The fish still has a certain elasticity but it is no longer suited to processing forms such as smoking. Drip loss rises considerably during filleting because the ice crystals grow during gradual freezing and pierce the cell membranes like sharp needles. In contrast to shock freezing where due to sudden lowering of the temperature only very few ice crystals develop, during superchilling at a temperature range of around -3°C when the cooling process slowly turns into freezing, a very large number of sharp ice crystals are formed which penetrate the sensitive cells and tissue.
Superchilling makes the tissue structure of the fillets firmer
Superchilling is a very effective, but also very demanding, sensitive and complex cooling technique. To prevent extensive freezing of the product surfaces and the development of ice crystals within the tissue (which can cause structural damages, deterioration of fillet texture, and increased drip loss) processors today try to keep the temperature in the range of -1°C to -1.5°C during superchilling. This temperature range is what could be called a compromise at which the benefits of the technique can be enjoyed to a large extent without the product being exposed to the risk of a possible quality loss.
Superchilling has other advantages than the extension of shelf-life. If the chilling technique is used correctly the whole fish is chilled right into its interior to a temperature of about -1°C. In this range, just before freezing, the blood and other body fluids are viscous or semi-fluid which makes the flesh firmer, more elastic and more resilient for the subsequent processing stages. The firmer tissue enables particularly smooth cuts and much more attractive fillets that differ noticeably from conventional products – even to the unskilled eye. Such premium quality usually gets higher prices on the markets, too. The firmer flesh also enables a clearer, more exact cut during trimming of the fillet. The yield rises on average by 2 to 3%. Because the fish is already very cold when filleted and the fillets continue to store this cold, product quality remains largely constant throughout processing. On top of that, the process is less expensive because less ice – if any ice at all – is needed for cooling the fish at the processing plant and during transport to the customer. In fact, cooling with ice would even be of disadvantage if the temperature of the superchilled fish is below -1°C because some kinds of ice are warmer and would thus neutralize the benefits of superchilling. Because no, or hardly any, ice is needed this increases the efficiency of transport since more fish can be transported in the insulated polystyrene boxes. Sometimes a bit of dry ice snow will be added to the fillets for longer journeys to keep the temperature low.
Up to now, the superchilling technique has mainly been used for sensitive fish species and their fillets. For example, white fish such as cod or haddock: the shelf-life of these species can be extended measurably at a temperature range of between -1.2°C and -1.4°C. This has been confirmed by scientific experiments that have been carried out, for example, in Iceland and the UK. An Icelandic study which compared the chemical, microbial and sensory properties of cod cooled conventionally with ice and superchilled cod products confirmed for example a two-day extension of shelf-life and freshness in the superchilled products. This result rather surprised the scientists because the differences between the two product groups in pH value, water content, water retention capacity, and bacterial contamination were actually only very slight. The researchers see the reason for this only minor difference in the optimal and constant cooling of the samples in the laboratory that ensured almost equal basic conditions and they announced that they wanted to repeat the test under conditions that more resembled real working conditions.
However, the superchilling technique is also of course suited to other fish species, for example salmon. In Norway, Atlantic salmon from aquaculture have been examined to see what differences there were in superchilled fillet portions that were kept at different temperatures (-1.4°C and -3.6°C) compared to conventionally ice chilled and frozen portions. Here, too, the differences in key values such as drip loss in both superchilled temperature ranges were less marked than would have been theoretically expected. The most outstanding differences were in the textural hardness of the fillets which was significantly higher in the fillets that had been kept at -3.6°C than those that had been kept at -1.4°C. The most important result, however, was that the shelf-life of vacuum packed salmon fillet portions could be more or less doubled with superchilling at both temperatures compared to conventionally ice cooled products.
Raw materials must be absolutely fresh
Although superchilling maintains the freshness and quality of fish and seafood products for a longer period, once freshness or quality has been lost they cannot be restored. A fish in which spoilage processes are fairly advanced will not get better though superchilling. That is why this complex chilling technique is only worthwhile for top-quality products of absolute freshness. Fishes from aquaculture are particularly suited to superchilling, for example, since they can be harvested as required and immediately cooled to the desired temperature. In the case of fishes that have been caught at sea it is usually day fresh catches that are used, i.e. fishes that are landed on the day they are caught. But even catches from fishing vessels that remain at sea for longer can be suited to superchilling if the fish is immediately cooled on board to below 0°C and subsequently stored at this temperature. Different techniques are used here but most of them are based on the same principle. The fish is first packed in ice as usual and then the temperature of the fish and ice mixture reduced further with the help of technical cooling techniques and kept constant at just under 0°C.
On some Portuguese trawlers that operate in warm water regions of the Atlantic the holds have been divided into several sections using vertical stainless steel walls. After the catch, the boxes that are filled with water, ice and fish are placed between these vertical walls, the distance between which is about that of the width of the fish boxes. Water and ice serve as the cooling medium that ensures consistent heat transfer. The stainless steel walls are fitted with U-shaped pipes through which a refrigerant constantly flows. Its temperature is precisely controlled and adjusted using technical refrigeration units. This system makes it possible to keep the fish at the desired value of below 0°C with high accuracy. The catch arrives “superchilled” in the port and can then be further processed as required.
Because it is fairly expensive to convert a fishing vessel to enable the use of this “Portuguese chilling method” some British trawlers use a simpler superchilling technique. This method is based on blowing in ice-cold air. Flat fish boxes are stacked on top of one another after icing so that sufficient gaps and intermediary spaces remain between them for the ice-cold air that is produced by refrigeration sets to flow through and circulate. This simple superchilling technique is perfectly suited to storing fish below 0°C straight after the catch at sea. However, it is considerably more difficult to achieve a uniform homogenous cooling result using the cold-air refrigeration technique. Whilst fishes that are lying on the surface often already freeze in the ice-cold air duct it can be several degrees warmer in the interior of the fish box. This endangers the freshness status of the fishes and often even leads to the fishes at the bottom sticking together and forming blocks when melting ice-water freezes again in the cold air. Heat then has to be used to get the fishes apart again and this, of course, interrupts the superchilled chain.
Superchilling on fishing vessels above all offers the advantage of being able to increase the duration of the fishing trip in accordance with the durability of the fish species. Depending on the on-board storage temperature, for example, cod can have been caught already three weeks prior to landing. Of course, in spite of superchilling, such fishes will have a lower quality and freshness than fishes that have only been stored for a few days on ice. Superchilled catches only enable freshness benefits when they are compared with fishes that are just as old that have been kept on ice the whole time. Superchilled fish whose potential storage time after the catch has already been largely used is hardly utilisable for the fresh market. And such raw materials are not suitable at all for the production of frozen foods such as IQF fillets.
Superchilled cod from the Lofoten Islands for customers throughout Europe
In the meantime some technology producers have even developed mechanical solutions for “superchilling” fresh white fish fillets and loins efficiently prior to packaging. The SuperChiller from Marel, for example, combines the advantages of the well-known blast and contact cooling techniques. Before the 1-4°C fillets pass through the chiller on a Teflon conveyor belt they go through a cooling bath with slurry ice (made of frozen salt water) which has a salinity of between 1 and 2.5%. This prevents the fillets from freezing to their interiors in the minus 8°C air current of the SuperChiller. Through close contact with the Teflon conveyor belt and the icy air current so much heat is removed from the fillets that when they leave the Chiller again after eight to ten minutes their core temperature is approximately -1°C. The superchilling process stabilises the fillet structure so that it will be more resilient to subsequent mechanical stresses that occur during processes such as skinning.
The Norwegian company Aker Seafoods is one of the companies that have for years been offering superchilled white fish fillets. The company’s marketing concept is based on maximum processing of the fish. Their range comprises mainly fresh fillets, loins and portions. Cod accounts for about 55% of superchilled production, a quarter is haddock and the remaining 20% saithe. Fresh products spoil quickly however, and every hour that the fish loses during processing and transport shortens the remaining shelf-life in the counter or shelves at the retailer’s. The superchilling line at the Stamsund processing plant on the Lofoten Islands has enabled Aker to broaden its expertise in the freshness segment. The whole line in which the company invested more than 4 million euros has a daily capacity of 20,000 kg product weight. Aker mainly processes daily catches to superchilled products but partly other raw materials, too. These must not, however, be older than 3 to 5 days. Because the Aker company on the Lofoten Islands is located at the centre of highly productive fishing regions for cod and other white fishes, two thirds of the landings can be processed to superchilled fillets and portions. Only in autumn and winter is the landed cod often not fresh enough for superchilled products because the fish swarms migrate northwards and the trawlers thus have to overcome longer distances to the fishing grounds.
Everything in the company is geared to fast processing. From the pier at which the trawlers land their catches it is only a few metres to the filleting machines. After filleting, the fillets are washed and then immediately cooled in a cooling tunnel to -1 to -1.5°C. The ice-cold fillets can be skinned more sparingly and have less gaping. After skinning, an Intelligent Portion Cutter cuts the tails off the fillets in a pre-defined size. The most important product of the superchilled line is loins which are cut and trimmed by hand to ensure maximum yield. After that the fresh loins and fillets are packed in insulated boxes for transport. The whole process from arrival of the fish to the superchilled fillet takes just under ten minutes. The main buyers of the high-quality products are located in France and nearly half of the production of the Stamsund Aker plant are exported to there. The remainder is divided between Denmark, Sweden, Finland, the Benelux states and Germany.