This note defines modified atmosphere packing, and summarizes the advantages and disadvantages of the method for fish products. Advice is given on the selection of fish for packing, the appropriate gas mixture, and on packaging materials and equipment. The importance of chilled storage is stressed, and some indication is given of storage life and safety of packs.
Modified atmosphere packing, MAP, means replacing the air in a pack of fish with a different mixture of gases, typically some combination of carbon dioxide, nitrogen and oxygen. The proportion of each component gas is fixed when the mixture is introduced, but no further control is exercised during storage, and the composition of the mixture may slowly change.
Modified atmosphere packing is often incorrectly called controlled atmosphere packing, CAP. Controlled atmosphere packing means packing in an atmosphere whose composition is continuously controlled throughout storage; such control is possible in large storage units, but not in small packs.
The storage life of sonic chilled products, notably white fish, can be extended by packing in a modified atmosphere.
The appearance of the pack is attractive and, since the transparent packaging is not in close contact with the contents, the buyer can clearly see the product.
Modified atmosphere packs have the advantages common to most forms of prepacked fish; they are odourless, easy to label and convenient to handle. In addition they are leakproof and robust.
Modified atmosphere packing is relatively expensive, currently about twice the cost of vacuum packing. Continuous production of rigid packs entails the purchase of expensive packaging machinery and the use of expensive thermoformable film.
Modified atmosphere packs are commonly two or three times bulkier than other forms of pack, and therefore are costlier to carry and store.
The walls of a pack may collapse when the enclosed atmosphere contains a high proportion of carbon dioxide, which is highly soluble in fish tissue. As the carbon dioxide dissolves, a partial vacuum is created, the pack may collapse onto the product, and in some instances the contents may be squashed. The problem can be avoided by correct choice of gas mixture.
Unsightly drip may form inside the pack when too high a proportion of carbon dioxide is used. The problem can be minimized by choosing the right gas mixture and by introducing an absorbent paper pad beneath the fish.
Any extension of storage life attributable to modified atmosphere packing may be lost if the additional safeguard of chilled storage is ignored; the packs must be kept at or close to 0°C throughout distribution if the full benefits of a modified atmosphere are to be maintained.
Carbon dioxide retards bacterial spoilage of fish, but too high a proportion in the mixture can induce pack collapse, excessive drip and, in products that are eaten without further cooking, for example brown crab meat, an acidic, sherbet-like flavour. Oxygen can prevent colour changes and bleaching that would otherwise occur in some products. Nitrogen is an inert gas that is used to dilute the mixture. The following mixtures are recommended, based on an assumed ratio of 3 parts gas mixture to 1 part fish by volume in the pack.
For white fish, scampi, shrimp and scallops a mixture of 40 per cent carbon dioxide, 30 per cent nitrogen and 30 per cent oxygen gives the best results. For salmon, trout, fatty fish such as herring and mackerel, and for smoked fish products, a mixture of 60 per cent carbon dioxide and 40 per cent nitrogen is recommended. Smoked salmon packed in this mixture may sometimes show a green discoloration during storage, the extent of the greening being dependent on the strength of the smoke cure; where greening is likely to occur, the mixture recommended for white fish should be used.
The simpler and cheaper machines pack the product in a flexible bag or on a tray inside a bag to form what is termed a pillow pack. At the more expensive end of the available range are sophisticated machines that continuously form packs with rigid bases from rolls of thermoformable plastics film. Some of these machines have a dual purpose and can be converted to vacuum packing when required, although the changeover can take up to 3 hours. Modified atmosphere packs can be produced at a rate of more than thirty a minute on the fastest machines. Packaging films must have a low gas permeability, and be to the machine maker's specification.
Gases can be obtained either ready mixed, or separately for use in machines that mix the gases before packing. Where gases are mixed in the machine, the gas composition in the packs should be measured at the start of a run, and monitored throughout the day, particularly when faults are suspected or adjustments to the gas mix are made.
Only the highest quality fish should be used for modified atmosphere packs, in order to gain the most benefit from any extension of storage life; packing fish in a modified atmosphere is not a means of marketing medium quality or poor quality fish.
White fish should be of a quality equivalent to 1-4 days in ice, and should be free from blemishes and visible parasites. Herring and mackerel should be of a quality equivalent to 1-3 days in ice, and should contain at least 8 per cent fat. Smoked fish products should be made from fish of the same initially high quality. Salmon and trout should be of a quality equivalent to 1-3 days in ice.
Fish should be handled hygienically and kept chilled from the time of capture or harvesting until they are packed; whole fish and fillets should be kept in ice while awaiting processing, and smoked products should be held in a chillroom at 0°C. Ideally an air blast chiller should be provided in the processing line, either before or after the packing machine, since the fish may warm significantly during the packing operation.
Layering of products within a pack should be avoided; a single fillet or portion is more fully exposed to the action of the gases. Layering is unavoidable when packing sliced smoked salmon, but the product does not gain the full benefit of the modified atmosphere.
Wet fish products that are likely to exude drip can be laid on a pad of absorbent paper inside the pack.
Packs with faulty seals can be detected by pressing them with the hands; faulty packs will collapse.
Packs should be clearly labelled according to existing regulations, and should be marked with a sell-by or consume-by date.
Storage life will depend on the species of fish used, its initial quality and fat content, the nature of the finished product, temperature of storage and, in a modified atmosphere, the gas mixture.
Temperature of storage is of paramount importance in deriving the most benefit from a modified atmosphere; packs should be stored at a temperature as near to 0°C as possible, and never above 5°C. Any benefit from a modified atmosphere will be much reduced when storage temperature rises above 5°C.
White fish fillets benefit most from packing in a modified atmosphere; for example cod fillets of high initial quality, packed in the recommended gas mixture, and with a ratio of gas to product in the pack of 3:1 by volume, will keep up to 50 per cent longer at 0°C than when stored under vacuum or unwrapped. Raw shell-on scampi and shrimp keep up to 30 per cent longer at 0°C in a modified atmosphere pack than in other types of pack, and the onset of black spot is inhibited. The storage life of herring, mackerel, salmon, trout, and smoked fish products is not extended in a modified atmosphere at 0°C.
Fishery products in the UK have a good record of safety with regard to food poisoning, and products in modified atmosphere packs are no exception. Some concern has been expressed about smoked products that are eaten without further cooking, for example smoked salmon and smoked mackerel, but the risk of an outbreak of botulism or of scombrotoxin poisoning from these products is no greater when packed in the recommended modified atmosphere than when packed in any other form.
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If you have any enquiries, write, 'phone, or call at the address below:
The Director,Other recent Notes in this series, which are available free of charge in the UK from the above address are:
Torry Research Station,
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135 Abbey Road,
Aberdeen AB9 8DG
Tel. 0224 877071
61 Gaping of fillets, by R. M. LOVE.Earlier notes in the series, most of which are still available, are summarized in:
62 The freezing time of fish, by F. J. NICHOLSON.
63 Fishing ports in the UK, by J. J. WATERMAN.
64 Fish silage, by I. TATTERSON and M. L. WINDSOR.
65 Fishworking machinery, by S. MAIR.
66 Handling and processing mackerel, by J. N. KEAY.
67 The haddock, by J. J. WATERMAN.
68 Icemaking plant, by J. GRAHAM.
69 Cook-freeze fish products, by J. N. KEAY.
70 Advice for the fish industry; who does what, by J. J. WATERMAN.
71 Processing cod; the influence of season and fishing ground, by R. M. LOVE.
72 Reducing odour in fish meal production.
73 Stowage of fish in chilled sea water, by J. H. KELMAN.
74 Handling and processing rainbow trout, by A. MILLS.
75 Freezing small pelagic fish, by I. MCDONALD.
76 Dark colour in white fish flesh, by R. M. LOVE.
77 Squid, by G. D. STROUD.
78 Health hazards of handling industrial fish, by A. WARD.
79 Minced fish, by J. N. KEAY.
80 Round worms in fish, by R. WOOTTEN and D. C. CANN.
81 Handling and processing blue whiting.
82 Hot smoking of fish, by A. MCK. BANNERMAN.
83 Fish smoking: a dictionary, by J. J. WATERMAN.
84 Handling and processing oysters, by G. D. STROUD.
85 Chilled and frozen fish: a dictionary, by J. J. WATERMAN.
86 Shopping for fish: advice on quality, by A. CRAIG.
87 Composition and quality offish: a dictionary, by J. J. WATERMAN.
88 Packing fish in a modified atmosphere, by D. C. CANN.
60 Key to Advisory Notes 1-59, by J. J. WATERMAN.
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