A fish’s quality is at its highest directly after the fish has been taken out of the water and from that point onwards it is no longer possible to increase, but only to maintain, the quality of the raw material. That is because, with the fish’s death, spoilage processes begin whose intensity and speed are strongly dependent on temperature. High temperatures accelerate the growth of bacteria, increase enzymatic activity, and intensify chemical processes within the body tissue. Already a slight increase in temperature of between 0 and 4°C can nearly halve the fish’s shelf-life. No other factor has such a strong impact on the durability of seafood products as temperature. For this reason, constant cooling of fish and seafood is particularly vital whenever the products are not eaten immediately in their place of origin but first have to undergo processing, thereby delaying their arrival in the end consumer’s kitchen.
Today, even consumers who live a long way from the coast want to be able to enjoy high-quality fish products. These products should as far as possible be constantly available and have a high convenience level. Such requirements can only be fulfilled, however, if the raw materials that often come from far away places and have to undergo long transportation, do not lose too much of their original quality during transport, processing, storage and final presentation at the retailer’s. This already gives an idea of the necessary spectrum of cooling requirements and of the immense challenges facing producers of refrigeration technology… Because despite long process chains a fish product does not only have to be safe and edible when it arrives at its final destination but should taste good, too.
The cold chain usually begins at sea because, after removal from the water the fish should be cooled as quickly as possible to a temperature of between 0 and 2°C. Fishes are ectothermic animals, having a variable body temperature that is more or less equal to the water temperature of the surrounding environment. This does not mean, however, that fishing in cold seas could reduce the requirements placed on refrigeration systems. They are no less necessary than in the tropics where the fishes often arrive on board at a temperature of 30°C or more. The body temperature of tuna, for example, can be as much as 10 degrees higher than the surrounding temperature if the tuna has been swimming strongly. In order to cool down a large tuna weighing perhaps 100 kg a refrigerant is needed that can remove the heat from the fish as quickly as possible. Although there are various different ways of doing this, ice is still the preferred method used on a lot of fishing vessels. Smaller boats already take ice with them when they leave harbour but larger trawlers have ice machines on board and can produce their own ice as required. The quality of the different kinds of ice determines the applications to which they are best suited. Whether scale ice, flake ice or nugget ice, it is primarily the temperature, shape, consistency and size of the ice pieces which decides what it will be used for. The right ice is the one that can best cool the catch reliably without damaging it in any way. Its impact on the appearance, colouring or flavour of the fishes should be as negligible as possible.
In the case of smaller pelagic species such as herring, mackerel or sprat it is often not sufficient to simply spread a layer of ice on top of the fishes. Already the bottom of the box into which the fishes are placed in a layer measuring at most 15 cms, should be covered with ice. Better still is to mix sufficient ice in with the fishes before the uppermost layer of ice is added. Under such conditions herring, for example, can be stored for nearly a week without any serious quality losses.
The temperature of the ice is of subordinate significance only. Its cooling capacity is above all dependent on its melting point. The larger the surface of an ice particle in relation to its volume the faster it will melt. This means that small pieces of ice are most suitable for cooling fish quickly because they melt fast and so release their cooling capacity immediately. In contrast, larger ice particles are more suitable for the storage of fish because they melt less quickly. A particularly good refrigerant for freshly caught fish is slurry ice, also called slush ice or flow ice. It is made of sea water which at low temperatures becomes a thick, amorphous ice water mixture, or slush, somewhere between a solid and liquid state. Due to these properties it is pumpable which makes transport on board easier. Slurry ice consists of many millions of tiny ice crystals that are hardly larger than 0.25 to 0.5 mm in size. The crystals have no pointed corners and no sharp edges so the fish cannot be damaged in any way. The ice slush surrounds the fishes and cools them quickly because of their close contact, giving no rise to freezer burns.
On the basis of slurry ice, a new cooling method called the RSW (Refrigerated Sea Water) technique was developed. It is already in use on a lot of fishing vessels for storing herring and other pelagic species. Chilled sea water which is kept at a constant temperature of between 0 and -1°C by a refrigerating plant flows up from below through perforated bottom plates in the tanks in which the fishes are stored. The ice cold water not only cools the fishes but also keeps them afloat as on a water bed so that the sensitive animals are not squashed. This storage method maintains their quality for at least one week even if the fishes have a high fat content or have full bellies when caught. The cool water circulates constantly within the system between the tanks and the refrigerating plant and any dirty water is discharged.
Freezing gives fish products a long shelf-life
After immediate cooling, the second important method for preserving fishes is freezing. The time between catching and freezing should be kept as short as possible and the fish should be kept cool throughout this time. Raw materials that are of a poor quality at the time of freezing will naturally be no better when they are thawed. Not every fish species is equally suited to freezing. Pressure sensitive raw materials such as sardines or sprat can, for example, suffer damages when the water between the fishes expands during freezing. Freezing can take place both on board the fishing vessel (frozen at sea) which is generally seen as particularly high quality, and on land (land frozen). The prerequisite for the latter is, however, that the fishing trips are not too long and that the raw materials are as fresh as possible when they are landed. Fish products are frozen either individually (IQF – individually quick frozen) or in blocks (block frozen). The blocks should not be too thick so that they freeze through well. Where fatty fishes such as herring or mackerel are concerned the blocks are glazed with freshwater ice after freezing to prevent contact with ambient oxygen and thus oxidation of the fats (rancidity). If the blocks are to be stored for longer periods they are often additionally wrapped in plastic.
The catch can even be directly filleted and the fillets frozen on board factory vessels. This is quite rare today, however, because it involves a lot of work, the technique is relatively expensive and fillet yield is not as high as when the fish is filleted on land. In place of this, the fishes are often frozen whole (round or h&g) and then filleted at a later point in time on land. In China a whole industry has in the meantime established itself for this purpose. Chinese companies buy the frozen fishes from all over the world, thaw them for a short period in their large factories, hand-fillet them and then re-freeze them to double frozen products.
During shrimp processing the raw materials themselves are often re-iced during the production process so that product quality does not suffer.
During cryogenic shock freezing with CO2 the product freezes extremely fast and energy consumption is comparatively low.
The range of technical systems for freezing is almost overwhelming for it seems possible to fulfil almost any user demands or wishes with regard to size, capacity and output of the freezing units. Roughly, the systems can be divided into tunnel, plate and spiral freezers. In tunnel freezers the products are frozen as they pass slowly through the freezer on a linear, usually several metres long, conveyor belt. The principle behind the spiral freezer is similar only that the conveyor does not travel in a horizontal line but in an endless spiral which winds itself slowly upwards. This means that these freezers take up much less space to produce the same output. In plate freezers, which are mainly used for freezing blocks of fish and other flat products the products are fixed between two plates which transfer the cold. Whilst tunnel and spiral freezers enable continuous work processes plate freezers can only work in batches.
Industrially produced frozen products are as a rule shock frozen at speeds of up to five centimetres per hour. This prevents drip loss when part of the cell water is released after thawing whereby not only moisture but also a lot of the aroma and quality of the product is lost.
The efficiency and output of freezing systems is largely dependent on the refrigerant they run on. In addition to fluorocarbons (freons) more and more technical gases such as carbon dioxide (R744) or nitrogen are being used which have a very high ‘cold energy’. Carbon dioxide, for example, as dry-ice snow has a temperature of about -78°C and liquid nitrogen is even colder at nearly -196°C. These cryogenic properties make them highly efficient as refrigerants and they are very flexible in their usages. Compared to “normal” refrigerants, CO2 and liquid nitrogen display the same freezing power but consume less energy so that there is much to be said in favour of these technical gases on climate protection grounds. The traditional refrigerant R134a, for example, contributes nearly 1,500 more towards the greenhouse effect than carbon dioxide. High freezing speeds also reduce bacterial formation in the products, which helps towards supporting not only HACCP standards but also the valid international hygiene and safety standards.
In the fish industry freezing is not only used for preserving products but also for crust freezing to enhance machine slicing properties, or to render the products more suitable for further processing or packaging.
The process of supercooling or superchilling is located in between normal cooling and freezing. The products are cooled to just before their freezing point whereby most temperatures lie below 0°C, but not so that they actually freeze, or become hard. This enables a longer shelf-life – in the case of fresh fish fillets up to two days, for example – than that of normally cooled fillets. A prerequisite for this processing, however, is that the raw materials are absolutely fresh.
Temperature controlled transport by air, over land, or at sea
It is, of course, also important to maintain the correct temperatures after the production of chilled and frozen fish products, too, i.e. during their further storage and transportation. Most processing companies today have their own cooling and freezing capacities that serve as a buffer between production and delivery. If, however, a company produces more than it can store there are storage companies in a lot of locations that have specialised in cold and cool warehousing. Irrespective of whether the products have been cooled, frozen or shock frozen they can then be kept under the correct temperature conditions in such facilities. A lot of storehouses additionally offer their customers a wide range of other services from computer controlled management of the stored goods to logistics services such as repacking and packing, order picking and temperature-controlled transport of the products.
Within international food trade fresh chilled seafood products are usually transported by air. Frozen products, on the other hand, are more frequently carried in containers which are then taken from continent to continent by shipping lines or from country to country in trucks as this is less expensive. In cargo service, seafood products are usually cooled using dry ice. This type of ice contains enough ‘cold energy’ for longer flights, too, and does not leave any pools. In special cargo containers that are mostly cooled using dry ice frozen products can even be transported for up to 72 hours at a temperature of minus 20°C. The larger ISO containers can be insulated containers that are cooled via two openings on the front side (cold air flows into the container through the bottom opening and leaves the container as warmer air through the upper opening) or integral containers that are equipped with their own cooling unit. These units run on electricity whereby at sea the current is generated by the ship. During transport over land in trucks or via rail, clip-on diesel generators produce the necessary electricity.
Containers are only one of many options for transporting chilled and frozen products over land at national and international level. Depending on the quantity, product type, and distance there are various different possibilities. If a company has only occasional need for refrigerated transport they can use refrigerated trailers or cool boxes, for example. Their separate power supply makes these transport solutions into self-sufficient systems which are very versatile. If they can be connected to a central power network they can even be used as an additional small cold storage depot. Because transport vehicles for chilled and frozen products are often put together with the help of extensions to random truck chassis each customer can have his refrigerated vehicle fitted according to his own individual concepts and ideas. Refrigerated vehicle builders have come up with a wealth of ideas to fulfil their customers’ requirements with regard to safety, hygiene and flexibility. The days are long gone when there was only one vehicle model that had to fulfil all purposes. Vehicles manufactured on a modular system, modern plastics and the possibilities of extra fittings leave hardly any wish unfulfilled. With two separate chambers within the vehicle it is in the meantime even possible to transport chilled fresh and frozen products at the same time. Carrying companies that have specialised in the transport of temperature sensitive foods can have themselves certified according to IFS standards for logistics companies in order to confirm for consumers their reliability, flexibility, good service and guaranteed safety.
Presentation of temperature sensitive products at the retailer’s
If all the players in the cold chain act responsibly the products will arrive at the retailer’s in good quality. And it is there that the demands placed on cooling and freezing systems are particularly high because not only the retailer himself but also the consumers have direct access to the products. Attractive product presentation at the retailer’s today entails the customers being able to pick up and handle, to examine and if necessary put back numerous temperature sensitive products. Despite this, the products must not suffer so much damage that they are no longer edible or enjoyable. This is a huge challenge to retailers, but also and in particular to the manufacturers of sales counters, shelves and freezers. Sales furniture should be attractive and its overall design and appropriate lighting should encourage people to buy what they see. At the same time it should be of such ergonomic design that the sales staff have good working conditions, and of course it has to help maintain the quality of the presented products, many of which are temperature sensitive.
It is probably almost impossible to anywhere nearly describe the full range and variety of refrigerated counters, shelves and freezers that are available today. They differ not only in their sizes and dimensions but also in their technical details and design features. Most of them enable temperature sensitive products – irrespective of the size of the presentation area – to be displayed at the required temperature. They either have a special cooling unit or are designed for presentation in an attractive ice bed. Fresh fish counters are generally made of acid resistant materials which can easily be cleaned and disinfected. The requirements of refrigerated wall shelves from which customers can remove products themselves are even higher. They often work with a chilled air flow which surrounds and protects the products. Modern refrigerated counters and shelves even have a complex air circulation system which can be regulated as required. Infra red sensors measure the temperature contact-free on the product surface and adjust the temperature of the air flow accordingly. In this way it is possible to maintain product quality and at the same time save energy because the products are only cooled as much as necessary. This principle also proves its worth in refrigerated wall cabinets which can quickly be brought back to the necessary temperature even if the doors are opened frequently.
What applies to refrigerated display units is even more applicable to refrigeration technology. There are freezers in all sizes and models. They are mostly plug-in systems which just have to be set up: horizontally or vertically, open or with glass doors, with panorama panes for a better view of the products or as island solutions which are accessible from all sides. Modern attractive design is combined with a generous amount of space. Ecological refrigeration technology works with the refrigerant propane and is free from CFCs and fluorocarbons as well as being low on maintenance. Automatic defrosting is frequently standard to prevent excess ice development, to remove frost and thus to save energy. During the defrosting phases the product temperature has to be maintained, too, however, at constantly below minus 15°C as legally required.
If the customer now carries his fresh chilled or frozen products back home correctly, for example in insulated bags with freezer packs and places them immediately into the fridge or freezer when he arrives home then logistics within the “temperature controlled” chain would be perfect from the product’s origins to the consumer.