Contents Index
Why fish goes bad
Where the bacteria come from
The effect of temperature on bacteria
The control of temperature
The advantages of ice and how to use it
What you can do
When you lay fresh fish out on the slab on a summer morning it may have begun to look rather unattractive by midday and be definitely stale by closing time. Why should this be?
Fish goes bad because of bacteria that grow on and in it. Even a small piece of stale fish weighing only an ounce or two will contain millions upon millions of bacteria. These living organisms are so small that a million or more can nestle comfortably on the head of a pin; even the largest kind can be seen only with the aid of a powerful microscope.
If we know something about the natural history of bacteria, we shall be in a better position to control them. This knowledge can save money by saving fish that will otherwise be wasted and also will help you to sell a better article.
Bacteria of one kind or another occur practically everywhere in nature - in the depths of the sea, in lakes and rivers and on land - and without them life on earth would come to an abrupt end. Plants can make complicated substances, such as, for example, fats and proteins, from simple chemicals - water, carbon dioxide from the air, phosphates and nitrates from the soil - and animals use these complex substances, by eating plants for energy and for building up their own bodies. Bacteria break down the complicated substances into simple substances that can be used again. The whole of life is an endless chain, and bacteria form one essential link in it. It is true that a few kinds of bacteria cause disease; there are far, far more, however, that carry out this essential job of providing the raw materials for the great manufacturers of nature - the plants - to build up again.
The bacteria present on fish on the slab are, of course, robbing you. If conditions for their increase are favourable to them, they may make your fish unsaleable or reduce its quality so that your customers will not buy your fish again but will prefer eggs or sausages. But it is worth remembering that these microscopic spoilers are only doing the same job as they would usefully do if the fish on your slab had happened to die in the sea.
The flesh of a healthy fish is sterile but it isn't long after it dies that bacteria begin to attack it. These can come from a number of different sources. There are always some on the skin and in the slime, sometimes in large numbers. Mud on the sea floor, too, is heavily loaded with them; fish in the trawl, which is dragged over the sea floor, are liable to get some of this mud on them and, even if they are well washed, some bacteria will remain.
The intestines, especially intestines full of food, contain countless millions. Inevitably, some excreta are squeezed out of the fish by the pressure of the net and help to increase the numbers of potential despoilers of the catch. More bacteria may be picked up on the catching vessel by contact with surfaces where other fish have been before - decks, shelves, boxes, to mention only a few - or by careless gutting.
The important point to notice here is that any fish, however and wherever it is caught, will have bacteria on the surface and in the guts. However careful the fisherman may be, bacteria will still be present, although by scrupulous attention to his job he can considerably reduce their numbers.
Shelf life of Cod from catching to condemnation
After the fish is stowed on the catching vessel, these bacteria will still go on growing and increasing in numbers, even if it is thoroughly chilled in plenty of ice. This sets a limit to the storage life of wet fish; cod and haddock, for example, reach condemnation level after about 15 days in ice; and herring, which goes bad much more quickly, after about 6 days.
Our distant-water trawlers, fishing in the Arctic in areas that take perhaps 5 days or more to reach, even in a fast modern trawler, cannot land fish less than 5 days old and some of it may be approaching condemnation level - indeed, about 1 per cent of our distant-water catch is in fact condemned at landing.
The numbers of bacteria go on increasing right up to the time that the fish is cooked. Careless handling can, however, further increase their numbers. Surfaces in contact with fish, such as, for example, filleting boards, returnable fish boxes and bench tops may, if not kept clean, add considerably to bacterial numbers. Contamination may occur anywhere along the line from the deck of the catching vessel to your shop.
Fish bacteria are very sensitive to temperature. The speed at which bacteria multiply depends upon the temperature, provided of course they have plenty of food - and fish is an ideal food for fish spoiling bacteria.
In frozen fish stored at a low temperature the bacteria are kept in a state of suspended animation. Immediately it begins to thaw out, however, they become active again and continue the process of bacterial spoilage. Even at temperatures as low as 15°F. however, that is 17 degrees of frost, some kinds can still grow, although only at a very slow rate.* Freezing, if it is to be properly carried out to give a good product, requires special apparatus. Slow freezing is to be avoided at all costs.
*It is not intended to discuss frozen fish here but it may be of interest to know that it does continue to deteriorate even at - 20°F (52 degrees of frost). Deterioration in this case is due to chemical changes, which also go on faster at relatively higher temperatures, even in the frozen fish.
It is not possible, therefore, to keep unfrozen fish at a temperature low enough completely to prevent bacterial activity. Fish begins to freeze at about 30°F and it is therefore clearly necessary to keep its temperature as near to 30°F as possible, without freezing it, if quality is to be maintained as long as possible. The easiest and best way of doing this is to use plenty of ice, which melts at 32°F.
At temperatures not far above that of melting ice, bacteria become much more active and fish consequently stales more quickly. Cod straight out of the sea, if it is kept at 40°F, reaches condemnation level in about 6 days, instead of about 15 days as it does if kept in ice, and at 50°F in something under 3 days. At 60°F, it is approaching inedibility after a day or so. It cannot be too strongly emphasized that temperature is by far the most important factor that decides the speed at which your fish goes bad.
Ice is used at every stage in the handling and transportation offish, from the time the catch is put below on the fishing vessel to the time it leaves the fish merchant at the port. This demonstrates the widely recognised fact that ice is the most convenient way of keeping wet fish cold. In practice enough ice is not always used, so that fish can warm up to temperatures well above 32°F.
During the last few years, Torry Research Station has been studying the temperatures offish from the time it is landed to the time it is sold in the shops. It was found that although the temperature of fish unloaded from distant-water trawlers in summer time was usually within a degree or so of that of melting ice, it began rising from then on until it was filleted and packed in ice for despatch.
The temperatures of fish arriving in London from the ports indicated that, although the ice was obviously having an effect, not enough was being used and in a number of cases it was being used in the wrong places, for example, only at the ends of a box of fillets where it could have no significant cooling effect on the middle. Only 1/3rd of the temperatures measured (about 3800) were below 34°F; 4/5ths were below 39°F.
The temperature of wet fish in fish shops were found to be much higher than this. Over 2000 temperatures of fish in boxes, as delivered, were measured. Only 2/5ths were under 39°F; almost another 2/5ths were above 45°F.
These temperatures are low, however, compared with those of fish on the slab. When the air temperature in the shade outside was between 60-69°F it was found that in the morning nearly 2/5ths of the 500 or so temperatures of fish that were taken in different shops were between 55° and 64°F (the maximum). In the afternoon 3/5ths fell within this range.
Many firms in the industry are now, as a result of this work, more aware of this problem of keeping fish really cool that they have ever been before. Serious attempts are being made to get fish in a cooler, and therefore fresher, condition to you. It is to be feared, however, that unless you can do something to keep down fish temperatures, much of this good work will be undone.
There are various ways in which fish can be cooled and kept cool but the most efficient and easiest way of all is to use plenty of ice in contact with it. There are various refrigerated devices for keeping fish cool in the shop - chill stores and refrigerated display cabinets, for example - but these are not designed to cool fish, only to keep already cool fish cool.
Why should ice be best for cooling fish down? The reason is that it cools fish down more quickly than a refrigerator and it doesn't cool it too far - the fish is never frozen. What in fact you are doing when you cool fish down is to take heat away from it and we are really considering, therefore, the best way of removing heat quickly.
If we put a saucepan full of crushed ice, with a thermometer in the middle of it, on to a lighted gas ring, we shall find that the temperature will remain at 32°F until all the ice has disappeared. A lot of heat is needed to melt all the ice - it takes 144 times as much heat to turn 1 lb ice at 32°F into 1 lb water at 32°F, as it does to warm the one pound of water at 32°F up to 33°F. When we use ice to cool fish, we are turning this stored cold to good use. As soon as ice is put on the warm fish, heat from the fish flows out into the ice and melts it; heat keeps on flowing until there is no difference in temperature between the two. Any further melting of ice that occurs after this is due to heat from other sources - the warm surrounding air, for example.
In a chill set at 32°F heat has to flow from the fish to the cold air instead of to ice. This warmer air has then to be cooled down by contact with the cooling coils.
It doesn't take much heat to warm up air - it takes 10,000 times as much heat to melt a cubic inch of crushed ice as it does to warm up a cubic inch of air from 32°F to 33°F. Also, and this is important, if cooling in air is to be rapid, a hurricane of cold air must be blown over the fish. Ordinary chill stores sometimes have a small fan fitted into them but sometimes they do not have even this.
Another disadvantage of a chill store used without ice is that the fish becomes dried. Moisture from the fish evaporates into the air and is then frozen on to the cooling coils.
The air in some parts of your chill store will be colder than in others. Fish placed near the cooling coils may, in the course of time, actually be frozen even though you have set the thermostat at 33°F. The reason for this is that the sensitive part of the thermostat can only control the temperature in the part of the chill where it is placed. It may be working perfectly satisfactorily and turning off the compressor whenever the temperature at the spot where it is placed drops below 33°F; air that is at 33°F by the time it reaches the thermostat may, however, have to leave the cooling coils at a considerably lower temperature. Slow freezing offish is very bad and spoils both the taste and the texture so that it becomes tough and stringy.
Refrigerated slabs suffer from similar defects if used to cool fish. Cooling takes place only from one side of the fish, that is, that in contact with the slab. If the thermostat setting is too low, however, the fish may be frozen on to it. If the thermostat is set correctly and if the fish is already ice cold, of course, the slab can be quite satisfactory, but only for keeping the fish cold.
These Dos and Don'ts are given to indicate some of the things you can do to serve fresher fish more attractively.
1. Do open up boxes of fish and fillets as soon as they arrive to make sure that some ice remains. Icing should be at top and bottom of boxes not merely at the ends.
2. Don't leave boxes of fish lying about the shop or preparation room. Either re-ice them as soon as they arrive or remove the fish and put on a well-iced display.
3. Do make sure that your display on the slab has plenty of ice. The liberal use of ice does not make your display less attractive. It will keep the fish fresher, discourage flies, and allow you to serve your customers a better quality article.
4. Don't use chills or refrigerated cabinets or slabs to cool fish down - they are not designed for this job and do it very inefficiently.
5. Do make sure that all surfaces likely to come into contact with fish are kept clean. Advice on how to clean is given in the Advisory Note No. 45 "Cleaning in the fish industry".
6. Don't leave offal on the premises for more than 24 hours in summer - it will encourage flies. Advice on how to deal with the fly problem is given in Advisory Note No. 23 "Control of Flies in Fishmongers Shops".
7. Do order frequently. Fish, even in ice, remains fresh for only a limited period. If you have any doubts about the freshness of any of your stock, don't sell it. It will pay you not to.
These Dos and Don'ts have been written to give you an indication of what you can do and are not a complete list.
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