Tímarit Verkfræðingafélags Íslands - 01.12.1967, Síða 84

82
TlMARIT VFÍ 1967
Changes in frozen fish during storage
By K. M. Love, D. Sc., Ph. D.
Torry Research Station, Aberdeen, Scotland.
Introduction
As soon as a fish dies, it begins to deteriorate.
Bacteria on the outside of the skin break down
the protective surface coverings and gain access
to the flesh, while the enzymes, which the fish
was producing to digest its food, begin to break
down the gut wall and belly cavity. Even if the
fish are gutted immediately and then packed
in crushed ice to slow down the bacterial growth,
they cease to be acceptable as items of food after
about two weeks because of their unpleasant
odour and taste.
When fish are frozen under the right condi-
tions they will remain fit to eat for much longer
periods. This has been realised for many years
— a process for the artificial freezing of fish was
patented as long ago as 1861 (Taylor, 1926) —
but they must be kept below —5°C if bacterial
growth is to be stopped completely (Leim, 1931).
However, frozen fish do undergo slow de-
terioration of a different sort, becoming gradu-
ally tougher to eat, exuding more fluid on thaw-
ing, and altering in appearance and flavour. These
phenomena are mostly the results of changes
in the actomyosin (Dyer et al., 1950), the main
‘structural’ protein of fish muscle. Alterations of
this type will be referred to as ‘denaturation’ in
the present account.
In addition to suffering protein denaturation,
fatty species of fish may also become rancid.
Unlike denaturation of the protein, the change in
the fat does not develop spontaneously at low
temperatures, but is the result of the action of
oxygen from the air. It may therefore be virtu-
ally prevented by glazing with a coating of ice,
or by vacuum-packing.
The purpose of research on frozen fish is to
slow down these undesirable changes and so
yield an acceptable product over a long period
as cheaply as possible. There have been some
interesting new developments m the field, such
as the use of polyphosphate dips before freezing,
‘superchilling’, and an increasing use of liquid
nitrogen for refrigeration purposes. These will
be reviewed in the following pages, in addition
to the more familiar studies on the effect of
freezing rate and storage temperature.
Measurement of denaturation
The first requirement of profitable research
into denaturation is to be able to measure it, so
that fish frozen and stored under different condi-
tions may be compared.
For many years this requirement was not
satisfactorily met. Taste panels are relatively
insensitive (Love, 1966). Measurement of the
amount of drip exuding in a given time after
thawing, popular with the early workers, is also
too inaccurate to be of much use as a research
tool. Eight other varied techniques intended to
measure denaturation by utilizing various physi-
cal or chemical phenomena have recently been
reviewed (Love, 1966A), but none can be used
successfully as a basis of research.
Reay (1933) showed that while fish muscle
would dissolve almost completely if ground up in
5% sodium chloride solution, the proportion de-
creased after cold storage and thawing. This dis-
covery has formed the basis of nearly all sub-
sequent work, and has been developed by Dyer
and his collaborators into a reasonably re-
producible method for measuring protein dena-
turation in frozen fish. Much valuable informa-
tion has been obtained, and it was only the dis-
covery by Ironside & Love (1958) that the
results were sometimes influenced by the nutri-
tional state of the fish, coupled with the some-
what slow nature of the method, that led to a
further search for an improved method.
The method which was evolved, the 'cell fra-
gility’ method (Love & Mackay, 1962), depends
on the fact that the individual cells of which the
fish muscle is composed become more resistant
to mechanical damage (after thawing) as cold-