Introduction
What is superchilling?
Is superchilling really slow freezing?
What is the extension of storage time?
How is superchilling applied?
Which method is best?
What additional refrigeration is required?
Can superchilled fish be processed right away?
How can superchilled fish be used?
Is superchilling a commercial proposition?
This note explains what is meant by superchilling and describes how the technique can be used for increasing the storage life of iced white fish aboard ship.
The advantages and disadvantages of the technique are discussed and compared with conventional chilled stowage in ice. British trials of the method have so far been confined almost entirely to cod, and this note refers mainly to work on that species.
Superchilling means reducing the temperature of fish uniformly to a point slightly below that obtained in melting ice, thereby extending the storage life of the fish.
When fish is kept in melting ice, the temperature of the fish falls to about 31°F; this is because salt, blood and other substances in the mixture of fish and ice depress the temperature a little below the natural melting point of freshwater ice, 32°F. White fish consists of about 80 per cent water, and all of the water remains unfrozen at 31°F.
When the mass of fish and ice is further refrigerated, some of the water in the fish begins to freeze and the temperature falls. In present practice superchilling means reducing the fish temperature to about 28°F, at which point half the water is frozen, and keeping it there. Bacterial action, and hence spoilage, is slowed a little at this lower temperature, so that the fish remain edible longer.
The slow freezing of fish flesh is undesirable because large ice crystals form which can damage the structure of the muscle, resulting in a thawed product that is less attractive in appearance and less palatable than the original fresh fish. The more slowly the water freezes, the larger the ice crystals and the greater the damage.
At the recommended superchilling temperature of 28°F only half the water in the fish is frozen and the number of large ice crystals formed is not critical, but at 27°F three-quarters of the water is slowly frozen and damage to the fish may be excessive. Hence very close control of superchilling temperature is essential if the damaging effects of slow freezing are to be avoided.
Freezing of fish muscle
White fish in crushed ice remains edible for about 15 days. When cod is superchilled to 30°F under laboratory conditions the shelf life is extended to about 20 days and ice formation in the flesh is insignificant. At 28°F the fish remain edible for about 26 days and ice formation is increased, but not to an unacceptable level. At 27°F shelf life may be as long as 35 days, but damage due to ice formation makes the fish unsuitable for filleting or smoking. Thus the temperature of superchilled cod should not be lower than 28°F and, at this temperature, the extension of storage time over conventional iced stowage can be as much as 11 days under ideal conditions, and at least 6 days in commercial practice. There is no difference in quality between chilled and superchilled cod until after about 12 days in ice.
Two methods have so far been tried on board trawlers; each requires that the fish be first stowed in ice and then that the temperature of the mixture be reduced by means of mechanical refrigeration.
Portuguese method
This method has been used aboard a number of Portuguese trawlers working in the warmer parts of the Atlantic, and has also been tried on a German vessel. Each vertical pound division in the fishroom is made of stainless steel and has hollow passages within it, through which refrigerated brine is pumped. The shelves are also of metal and are spaced about 16 inches apart, so that no part of the mixture of fish and ice on each shelf is more than about 8 inches from a cold surface. In addition, refrigerated brine is pumped through pipe grids buried in the insulation on the deckhead, the tank top and the ship’s sides. The brine temperature is accurately controlled. In more recent Portuguese installations the fish have been stowed in boxes between the metal shelves, in an attempt to overcome the difficulty of discharging a mass of partially frozen fish.
Original Portuguese method of superchilling
Cold air method
Superchilling installations on British trawlers have been designed to avoid elaborate and expensive reconstruction of the fish room; cold air is ducted into the fishroom and then blown between shelves or boxes of fish and ice. Since boxed stowage of iced fish has a number of advantages over other methods of stowage (see Advisory Note No. 15), the most favoured method of introducing superchilling is to link it with boxing at sea.
The conventional fishroom structure of vertical pound divisions and portable shelves is dispensed with and the boxes are stacked in open stowage with narrow vertical gaps left between them. The width of the gap is determined by bosses on the sides of the box. The sides of the fish-room are squared off as shown in the sketch; the cold air is blown down the sides of the fishroom behind the vertical lining and then upwards through the false floor to pass between the boxes.
Superchilled box stowage
Superchilled shelf stowage
When the ordinary pound structure is retained, the cold air is ducted down the ship’s sides and blown through openings in the lining that coincide with the air space above each shelf of fish and ice. The shelves are spaced at close intervals so that the centre of each layer of fish and ice is not too remote from the cold air.
The Portuguese method has not been tried in Britain mainly because of the costly equipment involved and the extreme difficulty in discharging the partially frozen mass; the hollow metal pound divisions are expensive to make, and the fitting of these, together with the pipe grids required, makes the cost of the installation prohibitive.
Assuming that a trawler already has a conventional pound structure in the fishroom, the method of blowing cold air over individual shelves is the least expensive to install, since most of the existing structure can be made use of; in addition there is no change in the method of stowage and therefore no difficulty in familiarizing the crew with the technique.
The main disadvantage of superchilling a mixture of fish and ice on an open shelf, whether by blowing cold air over the mixture or laying it on a cold surface as in the Portuguese system, is that melt water, trickling through the mass, can freeze again and fuse the contents of a shelf into a solid block; discharge then becomes extremely difficult and the fish may be damaged in the process. In addition, shelfed stowage, with space above each thin layer to permit the passage of cold air, is rather wasteful of stowage space; the stowage factor compares unfavourably with boxed stowage. Boxing at sea lends itself well to superchilling; all the advantages of boxing are retained, together with extended storage life of fish more than 12 days in ice. Where the fish can remain undisturbed in the same boxes during distribution after landing, the lower temperature achieved on board will also help to reduce spoilage on shore. In order to change over to superchilled boxed stowage, any existing pound structure has to be removed, the fishroom has to be squared off at the ship’s sides, and suitable boxes have to be bought or hired. The problem of weight variation in boxes packed at sea can be overcome with practice, as has been shown during extensive boxing trials on trawlers that had previously carried the catch on portable shelves.
Superchilled cod normally show an improvement in keeping quality over ordinarily chilled cod only after about 12 days; superchilling is therefore of most value in keeping the earliest caught fish on a three-week distant water fishing trip. When the catching rate is very high, the length of voyage is reduced; therefore it is reasonable to assume that superchilling will be of more use on a fishing trip where the average catching rate is fairly modest, say about 12 tons of fish a day.
It is recommended that the same amount of ice be used for superchilled stowage as for chilled stowage, that is about 1 ton of ice to every 2 tons of fish. The crushed ice in intimate contact with the fish will cool the fish initially from sea or deck temperature to about 31°F, and the cold air then cools the mixture of fish and ice more slowly from 31°F to 28°F. With a catching rate of 12 tons a day, or ½ ton an hour, the additional refrigeration capacity required to lower the temperature of fish and ice to 28°F would be in the region of 80,000 Btu an hour; extra refrigeration capacity, about 50,000 Btu an hour, must also be provided to compensate for the heat introduced by fans and fan motors.
Since about half the water is frozen in fish superchilled at 28°F, it is not possible to fillet the fish in this condition; some thawing is necessary before any processing can be done.
By blowing warmer air through the fishroom for some hours before the catch is landed, thawing can be started, but at least 24 hours in still air at about 60°F is required after landing to warm the fish sufficiently for filleting.
Superchilled fish in boxes can be thawed quite rapidly by immersing the box and contents in water at about 65°F; if the mass of fish and ice is broken up as soon as possible, it is possible to thaw the fish sufficiently to fillet them in about 2 hours.
Superchilled cod are likely to be anywhere from 12-26 days old on landing; they are not first class raw material, although they will be better quality than conventionally chilled fish of the same age. Superchilled fish that have been 20 days or more in ice will spoil very quickly once they have been thawed. Some of the fillets will be acceptable commercially for the wet fish trade, but few of them will yield a first class smoked product. Fillets that are too ragged and gaping to be sold in that form can sometimes be used as raw material for the manufacture of fish cakes and fish fingers. Fillets from superchilled fish are not fresh enough for quick freezing in retail packs on shore.
The duration of a fishing trip made by an ordinary wet fishing trawler is usually determined in one of two ways; the ship has to come home either because the fishroom is full, or because the first-caught fish, on landing, will be nearing condemnation level. Nowadays fishrooms are very rarely filled up, and the pattern of three-week trips to distant waters is governed by the fact that white fish remain edible for about 15 days when stowed in ice, and the fishing grounds are about 5 days’ journey from port.
Extension of shelf life means extension of fishing time in proportion to journey time, thus making the trip more likely to be economic. Indefinite extension of shelf life is possible when the fish are quick frozen at sea soon after catching, and kept at a very low temperature in a cold store until landed. But freezing at sea means construction of new special-purpose freezer trawlers at great cost; therefore there is still a need for other techniques that will give short term extension of fishing time without lowering the quality of fish being landed, and that can be used on existing fishing vessels.
The use of antibiotic ice is one possibility, and the pros and cons of this technique are discussed in Advisory Note No. 19; superchilling is another, and its suitability for commercial application is summarized here. White fish such as cod, which would be inedible after about 15 days in ordinary ice, will keep for about a week longer when they are superchilled; this means that a trawler can extend its voyage by a week and still land its fish in no worse condition than its conventional counterpart, or it can return at the normal time with the oldest part of the catch in better condition. This is the most important advantage that superchilling gives, against which must be balanced the drawbacks of the technique. First, superchilling shows no advantage over ordinary icing until the cod have been stowed for about 12 days; indeed, superchilled cod up to about 10 days in ice are if anything inferior to ordinary chilled cod. Therefore the fish-room has to be divided and equipped to provide both chilled and superchilled storage; otherwise the whole of the catch has to be submitted to superchill temperatures unnecessarily and perhaps undesirably. Since the most convenient way of applying superchilling is to cool the fish in boxes, this may mean having two different methods of stowage on board at the same time.
Next, the cost of fitting out part of the fishroom for superchilling about 1,000 boxes must take into account refrigeration plant, fans, dueling, fishroom structural alterations and supply of boxes. On the other hand, once the system has been installed, running cost is low.
The problem of introducing boxed stowage, whether superchilled or not, into vessels that have previously stowed the catch in bulk or on shelves is probably not a serious drawback; certainly crews can adapt themselves to boxing without too much difficulty, and boxed fish can be landed and marketed through existing outlets.
The biggest practical stumbling block is the fact that superchilled fish are partially frozen on discharge and cannot be handled immediately in the same manner as unfrozen fish. Merchants have to buy fish that they cannot use for 24 hours or more unless they have installed special thawing arrangements, and they have to allocate considerable storage space to the superchilled fish until it is ready for processing. The merchant may also have to contend with higher weight losses than normal, due to increased drip from fillets cut from superchilled fish.
The merchant cannot easily inspect the fish in its partially frozen state within a box; it may be necessary to thaw at least a few boxes before sale so that the merchant can buy by sample. The alternative is to thaw all the superchilled catch before sale, but the advantage of having a built-in reserve of cold is then lost.
There is also a problem of maintaining accurate control of the superchilling process on board. It has been explained earlier that fish kept at 28°F are satisfactory, but that fish kept at 27°F can suffer heavy damage to the flesh as a result of freezing too much of the water content too slowly. Accurate temperature control is quite possible, but things can, and do, go wrong on trawlers under arduous working conditions; intelligent supervision of the operation of the system cannot be entirely dispensed with.
Lastly, the effect of superchilling on the quality of distant
water landings as a whole must not be ignored. Superchilling extends the shelf
life of the first-caught fish; a trawler that can now make 11 superchilling
trips of 4 weeks’ duration in a year instead of 15 ordinary trips should
land considerably more fish, but the average quality of the landings may not be
any higher; indeed, if the catching rate is assumed to be uniform, the
proportion of early-caught fish will increase and the general level of quality
may fall. On the other hand, it should be possible, by limiting the length of
the trips, to raise the level of quality of the earliest caught fish. Owners
must compare the financial gains likely to accrue from improved fishing effort
against the possible long-term effect of landing increased quantities of fish of
perhaps indifferent quality. Superchilling could be useful to the industry for
some years as a stopgap technique between unprofitable wet fishing in distant
waters and the further advance of freezing at sea.
