Introduction
What are antibiotics?
How can they retard fish spoilage?
Which antibiotics can be used?
How can they be applied?
How effective are antibiotic ices?
What commercial tests have been made?
Are they effective on herring?
Which antibiotic is best?
Are there any health hazards?
Do other countries use antibiotics?
Is their use permitted in Britain?
What are the advantages of antibiotic ice?
What are the disadvantages?
Should the fishing industry use antibiotics?

Introduction

White fish, such as cod and haddock, when gutted, washed and carefully stored in ice soon after capture, will remain edible for about fifteen days. Because of the limitations of chilled storage in ice, any likely method of extending the shelf life of wet fish needs to be investigated, so that the value to the industry of the method can be assessed.

This note briefly describes the work that has been done in Britain with antibiotics; their effectiveness and the advantages and disadvantages of their use are discussed, in order that the trade may be fully informed about their value as an aid to better quality.

What are antibiotics?

Antibacterial substances are, as the name suggests, compounds that have some effect against growth of bacteria; they may be synthetic chemicals or substances of natural origin. Those that are produced naturally by living organisms, such as moulds, are called antibiotics; they can diffuse into their surroundings to slow down or stop the growth of bacteria, or even kill them. Nowadays, chemists are able to synthesize some of these substances.

Some antibiotics are effective against only a few types of bacteria; others are effective against a wide range of organisms and are known as broad-spectrum antibiotics. These are the most useful for fish preservation. Antibiotics are effective against bacteria even when present in very small concentrations.

How can they retard fish spoilage?

White fish spoils for two quite different reasons. First of all, uncontrolled chemical changes take place after the fish dies through the action of special ferments called enzymes present throughout the tissues, which result in self-digestion of the flesh and a loss of substances that contribute to fresh flavours. Secondly, bacteria which are present on the skin and gills and in the gut of the living fish begin to increase in numbers after the fish dies, and break down the tissues. They produce stale flavours and finally the flesh becomes putrid.

The rate of growth of the bacteria on the dead fish, already slowed down by the use of ice, can be retarded still further by application of antibiotics.

Which antibiotics can be used?

Soon after the spectacular success of penicillin in the fight against infection during the Second World War, scientists began to investigate its possible use for food preservation. Attempts with this and other similar antibiotics were unsuccessful, but in 1950 Canadian research workers showed that newer types then becoming available were more effective in extending the storage life of fish.

Very many antibiotics have been tried out, and two have been found to be particularly effective. They belong to the group known as tetracyclines. They are called chlortetracycline, usually abbreviated to CTC, and oxytetracycline, or OTC.

Chlortetracycline is marketed for medical use in Britain under the trade name Aureomycin. CTC in its pure form is not used for treating food; the antibiotic is combined with other chemicals that are harmless in food but which help to ensure that the antibiotic will remain evenly distributed and active when in solution, particularly in tap water which sometimes contains small quantities of substances that make the antibiotic inactive. Aureomycin prepared in this readily soluble form has the trade name Acronize.

Oxytetracycline is marketed for medical use under the trade name Terramycin; when made as a readily soluble formulation, Terramycin carries the trade name Biostat.

Other antibiotics may have uses in other forms of fish processing outside the scope of this note, for example nisin may possibly be of value in preventing the growth of food-poisoning bacteria in canned fish products.

How can they be applied?

The most effective way of using CTC or OTC for preserving fresh fish is to dissolve a formulation of the antibiotic in water; the solution can then be applied in one of three ways.

(1) The solution can be frozen and the antibiotic ice can then be crushed for distribution among fish in the same way as ordinary ice.

(2) The fish, usually as fillets, can be dipped into an antibiotic solution and then stored in ordinary ice.

(3) The whole fish can be stowed in refrigerated sea water that contains the antibiotic in solution.

How effective are antibiotic ices?

Laboratory experiments: The first experiments with CTC and OTC were made on pure strains of certain bacteria known to be prominent among those causing spoilage of fish.

It having been shown that the growth of these bacteria could be considerably slowed down, the antibiotics were then tried on samples of fish in the laboratory. Canadian and British research work, as well as experiments in other countries, produced results that were closely in agreement with one another. Trained taste panels were used to assess the freshness of fish in all the British experiments, and in addition chemical tests were made and the number of bacteria present at each stage was counted.

It was found that only after about ten days could any difference in appearance and flavour be detected between white fish, such as cod and haddock, stored in antibiotic ice and in ordinary ice. The concentration of antibiotic was five parts per million, that is in every million parts of the solution from which the antibiotic ice was made, there were five parts of CTC or OTC. Ten tonnes of ice would contain about 50 g of antibiotic at this level of concentration.

In the laboratory, cod that had been kept for 23 days in antibiotic ice were found upon examination to be equal in quality to fish kept only 13 to 14 days in ordinary ice; in other words the shelf life of the cod had been extended by about ten days.

Tests at sea: The results in the laboratory were sufficiently encouraging to justify trying out the antibiotic ice at sea. Pilot-scale trials were made on a research vessel in 1956, using ice containing only three parts per million of CTC, and several tonnes of fish preserved in this way were examined not only by research workers but by representatives of the industry at Hull, Grimsby and Aberdeen.

Extension of shelf life was not so marked as in the laboratory, but cod kept in antibiotic ice still had several days more storage life than their counterparts in ordinary ice.

A further trial was then made on a commercial trawler using ice containing five parts per million of CTC, and the results were sufficiently encouraging to the trade for them to ask permission to use antibiotic ices commercially.

There was no noticeable difference between fish in antibiotic and ordinary ice until after about 12 days; but cod in antibiotic ice for 16 to 18 days remained free of the usual yellow slime and fillets from them were more translucent, and had no trace of ammonia when cooked, whereas the fish in ordinary ice had become unacceptable about three days earlier.

As a result of all these tests, it was concluded that antibiotic ices containing five parts per million of either CTC or OTC could be expected to give in practice for fish such as cod an extension of shelf life of two or three days.

What commercial tests have been made?

The laboratory and pilot-scale trials had shown that the method was of most interest to the distant water ports and both Grimsby and Hull owners in 1963 carried out further commercial trials. Torry assisted the owners to run the trials and to assess the quality of the products.

These large-scale tests confirmed what had been forecast earlier; there was an improvement in appearance and flavour of the earliest caught fish when kept in antibiotic ice. Cod stored in CTC or OTC for 16 days had the appearance and smell of 12-day-old to 13-day-old cod stored in ordinary ice, a gain of three to four days’ shelf life. When cooked, the extension of shelf life based on taste was slightly less marked and was about one to two days.

These trials gave all sections of the industry the opportunity to examine and assess the relative qualities of the treated and untreated fish, and enabled owners and ice manufacturers to find out more about the problems of making and using antibiotic ices.

Are they effective on herring?

Herring stowed in antibiotic ice show no marked difference from herring in ordinary ice until after about 12 days’ storage. Herring can spoil very considerably during this time and the fat will probably have become rancid. Antibiotics, therefore, can do little to assist preservation of iced herring, since by the time they take effect the fish is for other reasons of poor quality.

Which antibiotic is best?

British experiments have shown no marked difference between the effectiveness of antibiotic ices prepared from either CTC or OTC. No other antibiotic tried so far has proved as effective as these two.

Are there any health hazards?

When the tests in 1956 showed that antibiotics might be of value in the fishing industry, a specialist panel of the Food Standards Committee of the Ministry of Agriculture, Fisheries and Food was set up to examine medical and scientific evidence about the effects of antibiotics on fish as food.

After four years’ careful study they reported that it was unlikely that a danger to health would result from the use of tetracyclines in raw fish provided that the amount of antibiotic present in the raw fish when sold did not exceed five parts per million.

Average amounts of CTC or OTC in raw fish that has been stored in antibiotic ice are much lower than the original concentration in the ice, but the distribution of the antibiotic in whole raw fish is not uniform; concentration is highest on the skin of the fish and in the outer tissue. After 24 days in CTC ice, cod has been found to have 14 parts per million of CTC in the skin and two to three parts per million in the flesh immediately beneath the skin; no CTC was detectable an inch in from the skin.

When the fish were cooked by steaming or frying, only occasional minute traces could be detected in the flesh, often less than one part per million. However, the amounts of CTC present in the raw fish seemed to be very little reduced when the fish were baked. Minute traces can be consumed without harm even to those who are sensitive to antibiotics. Many thousands of pounds of fish would have to be eaten before the total amount of antibiotic consumed constituted a therapeutic dose.

Do other countries use antibiotics?

Canada first approved their use on fish in 1956. Since then the U.S.A., Japan and a number of European countries have also permitted their use on raw fish in concentrations up to five parts per million.

Is their use permitted in Britain?

In July 1962, as a result of the report of the Food Standards Committee, the Ministry of Health and the Ministry of Agriculture, Fisheries and Food jointly made the Preservatives in Food Regulations which permit the presence of up to five parts per million of tetracycline in raw fish for human consumption.

In the Regulations, the terms ‘raw fish’ and ‘processing’ are defined as follows: Raw fish means any fish that has not undergone any processing, but does not include crustacea or molluscs. Processing, in relation to food, includes curing by smoking and any treatment or process resulting in a substantial change in the natural state of the food, but does not include boning, paring, grinding, cutting, cleaning and trimming.

By these definitions, fish fillets are not processed, and could therefore justifiably contain tetracyclines in amounts not exceeding five parts per million when sold. It must be emphasized here, however, that interpretation of the Regulations is the prerogative of the Courts. Since smoke-curing comes within the definition of processing, sale of smoked fish made from raw fish containing an antibiotic might be held to contravene these Regulations.

Unwrapped raw fish that has been stored in antibiotic ice is not required to be marked as such when offered for sale, but if such fish is prepacked for retail sale, the added ingredient would be required, by the Labelling of Food Order, to be declared on the package.

What are the advantages of antibiotic ice?

White fish gutted, washed and stored in antibiotic ice soon after catching will remain edible for 17 or 18 days; advantage can be taken by the distant water fishing fleet of the improved keeping quality either by continuing to fish for an extra day or two to land bigger catches, but yet producing fish no poorer in quality than when using ordinary ice, or by making the same length of voyage as at present but with the earliest caught fish a day or two fresher in appearance and in eating quality.

There is some residual preservative effect on the earliest caught fish, so that even when repacked in ordinary ice during inland distribution, spoilage may for a time continue to be retarded.

What are the disadvantages?

One serious disadvantage is that, in spite of the evidence that antibiotics used in this way on fish introduce no potential danger to health, consumers may have fears about the purity of fish and may not buy it.

Since antibiotics have no noticeable effect on fish until after about 12 days’ storage, when the fish is no longer really fresh, their use might not necessarily always result in the raising of the average quality of fish landed..

The antibiotic content of the ice and of the fish can fall slowly during the period of storage; thus the full effect of the antibiotic is not always gained.

There can be uneven distribution of the antibiotic in the ice after manufacture; however, the addition of suitable substances to the formulation can do much to prevent this. Among the substances used to help maintain the antibiotic in suspension are carrageenin and carboxymethyl cellulose.

The antibiotics may be inactivated when the ice is made from hard water, or when the water contains residual chlorine. Again, special additives in the antibiotic mixture can to a great extent enable the antibiotic to remain active. For instance, the addition of simple harmless substances such as citric acid can neutralize any free chlorine that may be present.

Scrupulous hygiene should ensure that most bacteria remaining in the fishroom of the vessel at the end of a voyage are destroyed, but it is possible that some might remain to be subjected to further small doses of antibiotics during succeeding voyages, each time increasing their resistance until the antibiotic was no longer effective. Whether this particular disadvantage is a serious one can only be proved or disproved in practice over a long period, but good hygiene should prevent resistant strains from developing.

It is possible that some of the care and attention now given to gutting, (handling and stowing fish at sea might be relaxed if crews came to rely too much upon the supposed powers of the antibiotic.

Should the fishing industry use antibiotics?

Only those in the industry, with all the facts before them, can decide whether or not to use antibiotics.

The small, but definite, improvement in quality of the earlier-caught fish has to be weighed economically both against the possible loss of prestige for fish if this comes to be regarded as an adulterated food, and also against the difficulties of manufacture and proper application of antibiotic ices.

The slight increase in shelf life can be used to improve the average quality or to increase the quantity offish landed; it is conceivable that it could do both; only long periods of operation on a large scale would decide this. For instance, vessels catching a fair amount of fish early in the voyage and then encountering many days of light fishing would not be forced to turn for home quite so soon if their oldest fish were safe for an extra day or two; what size of catch might be brought on board in the extra hours on the grounds is impossible to forecast, but anything caught very late in the voyage is the freshest in the fish catch, so that average quality could conceivably go up instead of down.

It could be argued that any method of preservation that enables just one fish to appear on the consumer’s plate in fresher condition than was possible before, without any danger to the consumer, is a step in the right direction. It could also be argued, however, that attempts to extend the life of wet fish in iced stowage should be abandoned in favour of more sophisticated methods of long term preservation such as freezing and cold storage.

It may be that the combined cost of the antibiotic, its incorporation in ice and its proper application may be more than the increase in price of the fish when sold.

Tony Research Station can advise only on quality; properly applied antibiotic ices containing either CTC or OTC will prevent white fish more than about 12 days old from spoiling quite so quickly as fish in ordinary ice; after about 16 days the treated fish will be equal in appearance and flavour to fish kept for 12 to 14 days in ordinary ice.

There are, however, other factors, economic and sociological, to consider in addition to quality and it is for this reason that only the fishing industry itself can answer the question, should the fishing industry use antibiotics?

If you have any enquiries, write, phone or call at either of the addresses below:

The Director,	The Officer-in-Charge,
Torry Research Station,	Humber Laboratory,
P.O. Box 31,	Wassand Street,
135 Abbey Road,	Hull HU3 4AR.
Aberdeen AB9 8DG.
Tel.: 0224 54171	Tel.: 0482 27879

Other Notes in this series, all of which are available free of charge from the above addresses, are:

1. The care of the fishmonger’s fish, by G.H.O. BURGESS.

2. Handling sea-frozen fish, by A. BANKS.

3. The handling of wet fish during distribution.

4. Take care of your catch.

5. Recommendations for the preparation of smoked salmon, by A. McK. BANNERMAN and J. HORNE.

6. Processing lobsters, by J. J. WATERMAN.

7. The protection of wood in fish rooms, by J. J. WATERMAN and J. MOWAT.