Seafood is sensitive and spoils quickly. Sometimes just a few hours under inadequate storage conditions are sufficient to render the healthy, nutritious foods inedible, and in extreme cases even a health risk to consumers. In addition to hygiene and time, temperature is the most important factor influencing spoilage processes. Like a lot of fresh foods fish has to be cooled constantly and on its path from the catch to the consumer must not enter the danger zone of between 8 and 68°C, the range in which decomposing bacteria multiply very quickly. As all consumers must know today, the cold chain should never be interrupted during a product’s journey from the producer to the plate. This applies in particular to frozen products which are common standard today in the seafood trade.
These products combine advantages such as preserved freshness, high nutritive value and product-specific flavour with extremely long shelf-life which, in the case of industrially shock frozen products, is at least six months. An important side effect of freezing is that it reliably kills nematodes and other parasites that can occur in some fish species. In addition, lowering the temperature into the freezing range also kills or at least inactivates a lot of microorganisms. This does not apply to all pathogens, however, and some dangerous bacteria can survive freezing so that for safety reasons it is always advisable to heat frozen seafood products thoroughly prior to consumption.
Wherever seafood is processed, stored, traded or prepared it has to be kept cool. Regular controls are necessary to make sure cooling and freezing systems are working correctly and such checks are an elementary part of a company’s HACCP programme which is now mandatory for all seafood processing companies under the new rules of the US Food and Drug Administration (FDA). This applies equally to imported fish products so they too have to have been produced according to the HACCP rules. Rapid cooling or shock freezing is generally the best method for increasing the shelf-life of seafood but this is hardly to be achieved with the required certainty using “normal” refrigerators and freezers such as those normally found in household use. That is why in the European Union particularly powerful, fast cooling equipment (blast chillers, blast freezers) is mandatory in food and catering companies, restaurants and commercial kitchens which have to conform to regulations 852/2004 and 853/2004.
According to the definition of the US-American Department of Health Guidelines the temperature of the food to be chilled must be reduced from 70 to 3°C within 90 minutes during blast chilling in order to guarantee the required safety for consumers. In the case of blast freezing the maximum time allowed to reduce the temperature of the product from 70 to minus 18°C is 240 minutes. Cooling is generally standard for fish products which are to be stored only temporarily up to consumption. In contrast, deep freezing enables considerably longer storage times because at temperatures of -18°C microbial and enzymatic spoilage processes are extremely retarded, particularly since the water in the flesh of the fish which is necessary for this process has solidified to ice. Although fresh products today play an increasingly important role during marketing of seafood, frozen products are also gaining significance because, parallel to traditional trade channels more and more seafood products are today purchased and delivered via the internet. This new trading segment – e-commerce – is primarily based on frozen products.
When a product is cooled particularly quickly using blast chilling or blast freezing the terms “snap chilling” and “snap freezing” are also used. Snap chilling is one of the common methods used during cook-and-chill processes that are common in big kitchens and in the gastronomy sector. Here the meals are in a first stage prepared to just below “cooked” and then cooled abruptly to 3°C which gives them a shelf-life of about five days. The preparation process of cook-and-chill meals is only complete with the “regenerating” step when the food is reheated prior to eating. The high speed of snap freezing, in which the product is cooled down in a very short time to temperatures below -70°C, is usually achieved through immersion in liquid nitrogen.
Not every freezer is suited to every product
There are several distinctive variants of cooling and freezing and the method chosen is primarily dependent on the type of seafood product, it shape, size and packaging, as well as other process-related parameters, such as the time that is available for chilling during the processing chain, or the initial temperature that the product has at the beginning of the cooling process. Which cooling method is used for which purpose does not, however, only depend on the method’s functionality but also on its profitability and the user’s financial scope. Some freezing systems might be unsuitable because they are too big for the available space or they might be too slow for the planned product throughputs. Plate freezers, for example, are not at all suited to freezing whole tuna. Other freezers could be eliminated because they dry out the fish and thus reduce product quality. Cryogenic freezers that work with liquid nitrogen or carbon dioxide are effective but too expensive for some products if they cannot be operated round the clock or if the running costs exceed the realizable revenue.
In principle there are three basic methods for freezing fish and seafood, involving different freezing equipment:
Air blast freezers are often the best solution for rapid shock freezing if the objective is to combine versatility with an economical procedure. These freezing systems are suited to freezing products of different shapes and sizes. Their performance is, however, decisively dependent on the air throughput which extracts the heat from the product. If the air in the freezing chamber is absolutely motionless then natural convection and thus the heat transfer rate will be low which lengthens freezing time considerably. If, on the other hand, the air is kept constantly in motion and circulated using fans (forced convection) the freezing time is reduced to a quarter of the previous value. For most applications of air blast freezers a circulation speed of 5 m/s is quite sufficient. This value is a good compromise between freezing time and costs for the refrigerant and the fan. If, however, the product is to be frozen in less than 30 minutes the air has to circulate in the chamber much faster, mostly at a speed of 10 to 15 m/s. An essential requirement for uniform, rapid freezing is a constant flow of air within the chamber which reaches and envelops all the products equally. This is not always easy, especially when the frozen food is packed very tightly or stacked on pallets which act as barriers to the airflow. Since the flow of air always takes the path of least resistance producers try to make circulation within the chamber as even as possible by using plates and vanes. The freezing plant has to be defrosted regularly and freed from ice which forms from freezing moisture on the walls of the chamber and gradually accumulates on the ribs of the freezer. This impairs heat transfer and lengthens freezing time.
Freezing with cold air as the heat exchanger
Air blast freezers can be designed as continuous or discontinuous systems. In the case of continuous systems the goods for freezing are transported through the freezing plant on a conveyor belt and frozen during this passage. The speed and length of the conveyor belt determine the length of time the products spend in the cold, i.e. how much time is available for freezing. Mostly these systems are designed as tunnel freezers, whereby they have to have a certain length. If the required space is not available one can also use other system designs, for example multi-belt plants using serpentine belt systems where the products move back and forth, or so-called spiral freezers, in which the belt winds a spiral path through the cold chamber. With such systems the amount of time that is available for deep freezing the products can be multiplied to durations much higher than that which is possible using tunnels. Continuous freezers are particularly suitable for companies that produce their products in constant amounts in the same rhythm, so that a constant product flow pushes through the freezer.
If the production quantities vary greatly, very different products are produced, or complete pallets and trolleys are to be frozen, then "batch air blast freezers" are often a better choice. Here the goods arrive in batches and are removed in the same batches after freezing. The loading and unloading of the chamber in cycles with changing product types and quantities requires particularly powerful cooling units and an intelligent air flow so that all products – regardless of their location in the chamber – freeze evenly and uniformly. Inadequate air circulation wastes energy and reduces product quality, because the time to complete the freezing process in some areas of the freezing chamber is unnecessarily prolonged until all products are ready.
A special field of application of continuous air blast freezers are fluidized and semi-fluidized freezers. In these freezing plants a strong stream of icy air is blown against the products from below through a perforated plate and swirled so that they literally "float" on this air cushion. The products move as if drifting on a liquid. If they are poured into the fluidized freezer at the filler opening they dance, bounce and slide on the air cushion to the exit at the other end. Since the swirling air current also separates the products during this journey such systems are particularly suitable for individual quick freezing (IQF). However, this method is only suited to freezing small-sized products because the air flow could hardly lift and carry larger objects such as fish fillets. Apart from that, the turbulence in the air would cause the fillets to assume irregular shapes during freezing which would look unattractive and so not be acceptable. For IQF freezing of small shrimps, especially cooked and peeled, fluidized freezers are the preferred method, however.
For freezing fish fillets it is possible to use semi-fluidized freezer, which combine the advantages of conventional tunnel freezers and fluidized freezers. In these systems, the fillets lie on a normal conveyor belt onto which air is blown from below on the way through the freezer. The strength of the air flow is not sufficient to make the fillets float completely but they are constantly partially moved and also momentarily raised. This ensures that they do not freeze onto the conveyor belt or freeze together and so can be removed from the freezer later one by one. Mostly the air flow is only necessary in the front section of the conveyor belt for once the fillets have solidified on the outside the air flow from below can be reduced. When freezing fillets of different size it is often difficult to adjust the amount of air supply precisely however.
Direct contact accelerates the freezing process
In addition to air blast freezers plate freezers are particularly frequent in the fish industry. It makes sense to use them wherever evenly shaped products, such as cartons and fillet blocks, have to be frozen quickly and inexpensively. A standard version of this freezing plant is the horizontal plate freezer (HPF) in which the plates through which the refrigerant flows are arranged horizontally. The plates are usually made of aluminum and can be moved closer together or further apart using hydraulic pressure so that products of different thickness (usually from 30 to 100 mm) can be held firmly with carefully gauged pressure of typically 70-280 mbar. The applied pressure presses the cooling plates close to the frozen products, thereby improving the heat transfer. In addition, this prevents the product from expanding during freezing when the water it contains increases in volume as it freezes to ice. In vertical plate freezers (VPF) the parallel cooling plates are arranged not horizontally but vertically, whereby a series of small chambers is formed which can be up to one metre wide, 50 cm high and between 25 and 130 cm thick. VPF have the advantage that the food for freezing can be filled into the chamber gradually from above. They are particularly suited to freezing whole fish which is why this method is often used on ships. After the catch the fish are layered in the chamber whole or h & g and after freezing are in the form of compact blocks which can be neatly stowed away in the cold rooms of the fishing vessel.
Automatic plate freezers are a special variant of HPF which are usually used in continuous processing lines. In this system, too, the ready packaged products are held firmly between the cooling plates which, however, move like a conveyor belt through the production area during the freezing process. The main advantage of this method is its continuous operation, because there is no downtime, which is otherwise unavoidable during loading and unloading of the plate freezers.
The fastest method for freezing is the liquid nitrogen freezer (LNF) with which most seafood products can be completely frozen in just a few minutes. LNF systems usually operate on the counter flow principle, since the liquid nitrogen is only sprayed in the rear section of the freezing plant. The nitrogen gas which is released in this process has a temperature of about -50 ° C, flows towards the products that are slowly transported towards the rear on a stainless steel conveyor. Freezing begins in the front part of the system due to the nitrogen gas before the freezing process is completed by spraying on liquid nitrogen. Since the process is extremely fast, LNF can be kept relatively small. They do not need any special refrigeration plant, compressors or condensers, which makes them less prone to interference and so inexpensive to maintain. The biggest cost factor is the constant need for liquid nitrogen, which makes the process about four times more expensive than conventional blast freezing with air blast freezers. Freezing with liquefied carbon dioxide works in a similar way. However, this method requires special security measures to ensure that the CO2 gas does not enter the working areas and put staff at risk.
Immersion freezers are also a very efficient freezing method. Here the heat transfer takes place when the products are immersed in a cooling liquid. Liquids can absorb more heat per unit volume than air, and this accelerates the freezing process. However, the choice of liquid is limited because it must not become too viscous upon cooling and should as far as possible not alter the texture and taste of the product. And so in practice, a sodium chloride brine is often used which is how things were done nearly 100 years ago, when freezing of fish products began with the Danish Ottesen Patents.