From: LMCKENZIE <lmckenzie@raptis.com.au>
To: Seafood Group (E-mail) <seafood@ucdavis.edu>
Sent: Friday, March 09, 2001 1:11 AM
Subject: Information request

I would appreciate any advice or information available on the tray-packing gas flushing of fish fillets.

Regards, Lisa McKenzie

Peter Howgate Wrote:


Apologies for the delay. I refer to a message of 9 March from Lisa
McKenzie asking for information tray-packing/gas flushing of fish fillets. I assume she had in mind modified atmosphere packaging, MAP, and following are some notes based on my own experience of this topic.

Peter Howgate


MODIFIED ATMOSPHERE PACKAGING

MAP in fish processing is most commonly applied to packs for retail sale.  Typically the containers are made, and the product loaded and sealed, in a multistation machine. A semi rigid tray is formed in the machine from a web of thermoform plastic. The tray is flanged, and is ribbed to enhance its rigidity. The product is loaded into the tray and trays pass in batches to the vacuum and filling station where the air is exhausted and replaced by the gas mixture. The trays, still in the chamber, are sealed by the top film then released. The packs can be labelled with stick-on labels or can be inserted into sleeves printed with the information.

The plastics films need to be selected for the purpose. The lower film needs to be thick enough to give a sufficiently rigid tray, and the upper film strong enough to withstand puncturing during normal handling. Both films must be resistant to diffusion of the gases, particularly the carbon dioxide, CO2, which is critical for extension of storage life, though complete resistance is not required for the few days of storage of MAP packs. Selection of appropriate films is a specialist area of packaging technology and the manufacturer of the packaging machinery will be able to advise.

It has been known for a very long time that CO2 inhibits the growth of spoilage organisms on meat and fish and MAP seeks to take advantage of this fact. The greatest inhibition is given by 100% CO2 atmospheres, but it is not possible to use this concentration in practice. CO2 dissolves in the fish flesh so that the volume of gas in the pack decreases and the packs collapse. High concentrations of CO2 have other undesirable effects on sensory attributes such as excessive drip loss, dulling of appearance and a slight 'sparkling' sensation in the mouth when eating the cooked product. Too low a CO2 concentration to avoid pack collapse and the other effects provides no inhibition of spoilage and the compromise is to use between 40% and 60% CO2, more often 40%. The remainder can be nitrogen or nitrogen and oxygen. It is advisable to pack at a slight overpressure to allow to some extent for solution of CO2 in the fish.

The ratio of gas to fish needs to be around 2-3:1, that is, for a 250g pack, the volume of the tray should be at least 750 ml. This high ratio is required to provide sufficient inert gas to avoid pack collapse and sufficient CO2 to exert an effect. This high ratio of pack volume to product weight is a disadvantage of MAP because of the considerably increased volume of storage and carrying capacity required compared to other forms of packaging.

 It has been suggested that fatty fish should be packed in an oxygen-free atmosphere to reduce development of rancidity, but it is difficult to demonstrate this effect in practice over the typical storage times of MAP products. Contrary to this, it has been suggested that oxygen should be included in the gas mix to reduce the risk of development of botulinum toxin. It is not possible to be certain that fish do not contain spores of Clostridium botulinum and exclusion of oxygen favours growth of the spores and formation of toxin. This hazard has been extensively studied for a variety of gas mixes and fish products and the risk seems to be very small indeed, and no different from other forms of packaging of fishery products.

There is no doubt that CO2 inhibits growth of spoilage bacteria, but the recorded extention of storage life of MAP products depends on the criterion used to judge end of storage life. Almost all reports on studies of storage life use a criterion of unfit for consumption, that is equivalent to about 14 days in ice for air-stored gadoid such as cod. However, MAP product are intended for retail sale in supermarkets and they use a fresher criterion of end of storage life of presence of some fresh flavours and absence of any spoilage flavours, equivalent to about 7-8 days in ice for air-stored gadoids. With this criterion, MAP in CO2 provides no extension of storage life because CO2 has no effect on the biochemical reactions resulting in loss of intrinsic fresh flavours.

Even when unfit-for-consumption criteria are used, any extension of storage  life by CO2 is difficult to achieve in practice. The most important factor is temperature of storage. The effects of CO2 in extending storage lives are greatest when the product is stored at 0oC, and are negligible above about 5oC. Typical ambient temperatures around products during distribution of fish in the chill chain around 2oC, and temperatures of chill display counters in supermarkets are typically around 4oC.  MAP of fish was taken up by several supermarkets in Britain in the mid 1980's, but my informal observations in such outlets in the region of Britain where I live, southern England, is that MAP is hardly used nowadays for fishery products, though used for red meats.


FURTHER READING

There is a large literature on MAP of fishery products in scientific journals; following are reviews.

Betts, G.D. (Ed.). (1996). A code of practice for the manufacture of vacuum and modified atmosphere packaged chilled foods. Campden & Choleywood Food Research Association, Chipping Campden, Gloucestershire, GL 6LD, U.K.

Church, I.J. & Parsons, A.L. (1995). Modified Atmosphere Packaging

Technology: A Review. Journal of the Science of Food and Agriculture, 67, 143-152.

Church, N. (1994). Developments in modified-atmosphere packaging and

related technologies. Trends in Food Science & Technology, 5, 345-352.

Davies, A.R. (1997). Modified-atmosphere packaging of fish and fish products. In: Fish Processing Technology. 2nd edn, Blackie Academical & Professional, London. pp 200-223.

Davis, H.K. (1993). Modified atmosphere packaging of fish. In: Principles and applications of modified atmosphere packaging of foods, Blackie

Academic and Professional, pp 189-228.

Farber, J.M. (1991). Microbiological aspects of modified-atmosphere packaging technology - a review. Journal of Food Protection, 9, 58-70.

Gibson, D.M. & Davis, H.K. (1995). Fish and shellfish products in sous vide and modified atmosphere packs. In: Principles of modified-atmosphere and sous vide product packaging, J.M. Farber & K.L. Dodds (eds) Technomic

Publishing Co. Inc., Lancaster, PA, USA. pp 153-174.

Skura, B.J. (1991). Modified atmosphere packaging of fish and fish products.

In: Modified Atmosphere Packaging of Food, B. Ooraikul & M.E. Stiles, eds, Ellis Horwood, New York, pp 148-168.

Stammen, K., Gerdes, D. & Caporaso, F. (1990). Modified atmosphere packaging of seafood.

CRC Critical Reviews in Food Science and Nutrition, 29, 301-331.