Dear Mr Dodge,
It is true that oxygen play some role in fish spoilage, particularly
to promote rancidity in fatty fish and to oxidise some of the pigments fish
has on its skin. It is also true that if fish is not wet tend to dehydrate,
particularly in dry climates. This speed up the oxidation rate, because
oxygen is put in contact directly with lipids. In some extreme cases, for
instance small pelagic with a high fat content, rancidity could develops
even before fish become spoiled due micro-organisms. Fish meal from fatty
fish can literally burnt out if humidity falls below a certain level.
Probably you are basing your assertion on some small pelagic oily specie of
this type (South African sardine?).
However, ice and water on the surface of fish can not prevent air to
be in contact with fish. There is always an amount of oxygen dissolved in
water, that depends on the temperature and salinity of the water. It is not
a large amount, but enough to make fish to breath in the water through the
gills and spoilage (and sometimes pathogen bacteria) to thrive on fish
surface, and lipids to oxidise.
Reduction of temperature is the responsible for the reduction of
micro-organism growth (normal and contaminant flora) and the reduction of
some enzymatic and chemical reactions, including lipid oxidation. In a large
number of species (particularly lean and medium-fatty species) spoilage due
to micro-organisms is more important than development of rancidity.
It is true that water from melting ice acts as a barrier against
direct contact of fish lipids with air. However, you can no attain the same
effect of reduction of overall fish spoilage using water alone (even chilled
water). Perhaps you could reduce lipid oxidation maintaining a water
barrier, without to chill the fish. Nevertheless, spoilage due to bacteria
will proceed at a higher rate and definitely you will have in the case of
small pelagic histamine formation due to temperature abuse (not to mention
increase in belly burst percentage).
Therefore it is generally accepted that the key factor of ice use is
lowering of temperature, regardless the end effect . There is more than one
effect, and which effect could be more important in a given case would
depend on the specie, environmental conditions and handling conditions. We
are fortunate that ice produce a number of different beneficial effects.
Nevertheless, you are basically right to point out that in some cases some
ice effect could be more important than others, particularly in a tropical
environment. Very often we tend to be too much biased towards microbiology
effects.
Hector M. Lupin
Fish Utilisation and Marketing Service (FIIU)
Fish safety and quality assurance
Tel.: (39)(06)570-56459
Fax. (39)(06)570-55188
e-mail: hector.lupin@fao.org <mailto:hector.lupin@fao.org>
-----Original Message-----
From: SHRIMPKEM@aol.com [mailto:SHRIMPKEM@aol.com]
Sent: Sunday, July 23, 2000 1:18 PM
To: PDIONPDA@aol.com; seafood@ucdavis.edu
Cc: abstrak@accesswave.ca; Lupin, Hector (FIIU);
jperalta@iloilo.net; tplabuza@tc.umn.edu
Subject: Re: Amount of Ice to Chill Fish
Dear Listers:
The key element in preserving fish whether you use ice or
any of the
multitudes of chilling methods is to prevent air from coming
in contact with
the harvested species.
Robin Dodge
Favorite Foods
Hout Bay, South Africa
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