The biggest catch would be in vain if it were not immediately possible to protect the fish from spoilage through appropriate cooling. A simple rule of thumb states that with every hour in which fresh fish is not cooled after the catch it loses a whole day’s shelf-life. And even if this formula is very approximate it is indisputable that nutritional value and freshness, flavour, vitamins and many other ingredients are best preserved through adequate cooling. Of course, this is also true for a lot of other foods such as fruit, vegetables, dairy products, sensitive bread, cakes and pastries, or meat and meat products but it applies in a special way to fish and seafood.
When a fish dies, decomposition and spoilage processes begin immediately because important defence mechanisms that protect the living organism from such damages no longer function. Bacteria, enzymes and chemical processes work together to dissolve body structures and tissues. Billions of bacteria that occur naturally in the fish’s body slime, on their gills and in their intestines begin to multiply rapidly. Some of these bacteria produce enzymes that break down the fish’s protein. This leads to a gradual change in the flavour, colour, and in the smell of the fish, which becomes more unpleasantly “fishy”. Added to this is the effect of the fish’s own enzymes which played a part in numerous processes within the living organism. A lot of them remain active even after the fish’s death when they begin to break down the body substance of the fish. This leads to quality losses as the fish’s flesh becomes increasingly soft. The third reason for spoilage is chemical processes that are partially the result of atmospheric oxygen which reacts, for example, with the fatty acids in the body tissue making them rancid due to oxidation. Chemical degradation processes play a decisive role in the development of unpleasant smells, too.
The intensity and speed of these spoilage processes depends very heavily on temperature. Higher temperatures accelerate bacterial growth, increase enzymatic activity and intensify chemical reactions. Already an increase in temperature of 0 to 4°C can double decomposition rate and approximately halve the shelf-life of a fish. No other factor influences the shelf-life of seafood products as strongly as temperature. This explains why constant cooling on ice is so important during storage of fish.
Ice quality determines its use
No matter whether a fisherman stays at sea for weeks or returns to port the same day, his fish will only maintain optimal quality if immediately after the catch it is cooled to, and stored at, a temperature of about 0°C. The fisherman can take the ice required for this purpose on board when he leaves harbour or he can produce it himself on board his fishing vessel using ice machines. Although the technology used in ice machines varies from manufacturer to manufacturer the basic principle behind the production process is always the same. Water is applied to the interior of a strongly cooled cylinder in the machine and this water freezes immediately and is then scraped off so that it falls into an ice bin below. This is how pourable scale ice is produced, a type of ice which is frequently used on board fishing vessels because it is easy to handle. With temperatures of nearly minus 7 degrees Celsius scale ice is relatively cold. The ice scales are only about 2mm thick, however, so that freezer burn will not develop and the sensitive fish skin is not damaged. Scale ice is comparatively dry and melts more slowly than other kinds of ice. The melt water that arises during this process keeps the fish sufficiently moist and washes any developing microorganisms from the fish skin.
Several basic criteria that are decisive for the quality of ice and its applications have already been mentioned. It goes without saying that the purpose of ice is always to cool the products but not so strongly that they suffer damage. As soon as the temperature falls below a critical level during storage freezer burns can develop which change the appearance, colouring and flavour of the fish and possibly even render it inedible. The ice should maintain the desired temperature during fish storage but at the same time slowly melt so that the fish is gently washed. This rinses bacteria and other germs from the surface of the skin, protects the layer of slime and keeps the fish constantly moist. The temperature of the ice is of less significance for its cooling capacity which mainly depends on the melting point. The larger the surface of a piece of ice compared to its volume the faster it will change back into water, i.e. melt. Following this logic anyone who wants to cool a fish as quickly as possible should choose small-sized ice pieces because they melt relatively fast and thereby release their cooling capacity quickly. If, however, fish is to be stored over a longer period of time it is generally more advantageous to use larger pieces of ice that will melt more slowly.
However, heed must be taken of the fact that the individual pieces of ice will tend to freeze together due to air humidity or the meltwater and then clump together. The colder the ice is and the longer it is used for cooling the more meltwater is formed and the more this effect is to be seen. Scale ice with its extremely cold temperature of -7°C often clumps together in warm surroundings already after just a few hours. This process can be delayed if the ambient air temperature in the storage room is kept low, too, so that as little meltwater as possible develops.
Chip ice and liquid ice made of fresh or salt water
Chip ice hardly freezes or clumps together at all. At only -0.5°C the temperature of chip ice is relatively warm and not sufficient to make meltwater freeze again. As a rule it is only on the surface where the ice is in direct contact with the warmer ambient air that a thin frozen layer will form but beneath this, the ice remains loose and pourable. Chip ice is a kind of multi-purpose ice because, depending on the size of the pieces of ice, it can be used for nearly all cooling purposes. Small chips which melt quickly are perfect for fast cooling of fish, especially of fresh fish fillets that could freeze in a colder ice. Larger chips that do not melt so quickly are well suited for storage and long-term cooling of fish, for example in fresh fish counters or during transport. Nugget ice or cube ice is really chip ice, too, even if some kinds have particular properties. Nuggets, for example can be very hard on the inside but in contrast on the outside watery and soft. This makes the ice looser, the pieces of ice do not “tinkle” but slide past each other in a homogeneous mass. The relatively soft ice is gentle on the products it chills and ensures – for example when cooling drinks – a pleasant mouth feel.
Most kinds of ice are based on freshwater but some can also be made of salt water. The salinity of sea water ensures that this ice is more amorphous, “softer” so to speak. Scale ice that is made of sea water, for example, is always slightly more crumbly, the shards are not as smooth and resistant to abrasion as in the case of scale ice made from freshwater.
“Liquid ice“, a suspension of water, tiny ice crystals and more compact ice pieces, is usually made from sea water. The salinity reduces the freezing point so that it is still liquid at temperatures of down to about -2°C. Salt water is thus ideal for the production of an amorphous liquid ice pulp which is particularly suitable for storing cooling energy. Liquid ice, also know under the names slurry ice, flow ice or stream ice, can be pumped through pipes and thereby transported from the ice machine to the location of application. It surrounds the products it has to chill completely, for example each individual fish, enabling maximal surface contact. The small ice crystals soon dissolve leading to an even cooling process. The temperature of the products falls faster than with other types of ice which is why liquid ice is particularly popular for cooling fishes, for example on board fishing vessels after the catch. Because there are no sharp edges in the slurry ice the fishes suffer no mechanical damages and remain fully intact.
Liquid ice is also good to dose and is particularly hygienic because it can be pumped from the production site directly to its destination without having to be carried by hand. By mixing ice and water the temperature can be adapted fairly precisely to the required range. In general, liquid ice contains about 20 to 50% solid ice but higher ice concentrations are also possible. The “Variable State Ice” System from Axima Refrigeration is said to be able to supply liquid ice with an ice content of between 0 and 100% which covers temperature ranges between 0 and -30°C (such low temperatures are, however, only possible with liquid ice in combination with special additives). The application spectrum of liquid ice is thus very broad, depending on whether it is liquid, pasty or nearly dry.
Compact block ice and ultra cold dry ice made of CO2
The opposite of liquid ice is hard block ice which is a compact, completely frozen ice in rectangular blocks usually weighing between 10 and 25 kg. In the days when there were no electric cooling options, block ice was indispensable for cooling perishable products. Large quantities of these ice blocks which had been sawn from the surface ice of local lakes during the winter were often stored throughout the summer in subterranean storage chambers. They were used to cool not only drinks in pubs but also foods in the household. A lot of storage cupboards used to have a small compartment at the top into which it was possible to push a stick or block of ice. The ice man provided further supplies of ice blocks during regular street-by-street deliveries. In a lot of industrialized countries block ice has today moved rather out of focus because compared to a lot of other types of ice it has certain drawbacks. Prior to its usage, for example, it often has to be cut up which can lead to sharp edges on some of the pieces. Block ice does have some convincing advantages, however. It has a good cooling capacity, can be stored for a long time and is easy to transport. In a lot of warmer countries without nation-wide refrigeration facilities block ice is thus still widespread. In developed countries it is sooner used for more special applications such as ice cream counters, ice walls or for ice sculptures for festive dining table decorations.
|Water used for the production of ice should meet the highest hygiene requirements|
A special kind of ice is solid dry ice which is not made of water but of carbon dioxide (CO2). At temperatures above -78°C dry ice immediately becomes gaseous without melting. Due to its strong cooling capacity dry ice has to be carefully dosed if the products should not freeze. The cooling capacity of this ice is nearly three times higher than that of conventional water ice so that smaller quantities are sufficient for achieving the same cooling effect. This means that dry ice is a very useful option whenever a combination of reliable cooling and minimum weight is required, for example during transport of fresh fish by air. It is also an advantage that when dry ice melts no water develops which might cause damages within the aeroplane. Apart from that, the product is not left “floating” in meltwater. Although this means doing without the disinfectant effect on the skin surface of the fish when the meltwater washes over it this is compensated for by the disinfectant effect of the CO2. Another advantage of dry ice is that it is odourless and flavourless and so does not impair the sensory properties of the fish. Dry ice is available in pellets, slices, blocks or as dry ice snow and can thus be very easily dosed for most application fields.
Food hygiene regulations also apply to ice
There is a wide choice of ice machines and cooling systems for producing the different types of ice for their various applications. In addition to standardised systems for “normal” application fields a lot of manufacturers also offer made-to-measure solutions that are tailored to the particular needs and requirements of individual users. Which system a user ultimately chooses depends on numerous criteria. Modern machines for producing ice should not only have a high capacity, be reliable, robust and low-maintenance but should also be profitable and energy efficient because the cooling energy stored in, and later released by, the ice first has to be invested during the production process.
Of similar significance are the hygiene aspects that have to be considered during the production and use of ice. Ice is usually used in the food sector for cooling fish and other products in order to prevent their premature spoilage and the development of harmful bacteria. It would thus be scandalous if bacterial germs were to get into the fish via contact with the very ice that is used to prevent this. According to EU law both ice and water are seen as foods and thus subject to very stringent hygiene regulations. Scrupulous cleanliness of all machines, rooms and apparatus that are used for the production, transport and storage of ice is thus an absolute must for germ-free, hygienically safe ice.
Probably the most prominent source of germs in ice is the water that is used to make it. It should meet the highest hygiene requirements and if possible be of optimum food quality. Despite this, problems can still sometimes arise, particularly if water has lain unused in pipes for a long time, for example in the inlet pipe or in a machine which is switched off. To prevent the penetration and spreading of bacterial germs some manufacturers equip their ice machines with disinfection systems which work with UV light or ozone. This enables noticeable reductions in the risk of bacterial contamination of ice. Germs can also get into the ice through the air, however, for example through germ-contaminated aerosols or inadequately disinfected surfaces of fish counters and packing cases, and from there onto the fish. As is the case with temperature controlled foods surfaces that come into contact with the ice must be regularly and thoroughly cleaned and disinfected.
Dr. Manfred Klinkhardt