Re: Information request

From: Howgate (phowgate@rsc.co.uk)
Date: Fri Mar 16 2001 - 11:57:11 PST

Next message: Andrew Strak: "RE: Information request"

Previous message: Voranuch Suvanich: "Seafood Basic SCP and HACCP Training Courses"
In reply to: LMCKENZIE: "Information request"
Next in thread: Andrew Strak: "RE: Information request"
Messages sorted by: [ date ] [ thread ] [ subject ] [ author ]

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 products 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.