Tímarit Verkfræðingafélags Íslands - 01.12.1967, Page 91

TlMARIT VFl 1967
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rigor mortis. Trawled cod stowed in crushed ice
generally become soft again 30 hours after death.
In former years the question of freezing fish
before rigor mortis (“pre-rigor”) never arose,
because they could not be landed and placed in
the freezer on shore quickly enough. However,
the advent of factory ships which freeze fish
at the fishing ground has resulted in the freezing
of fish which are virtually alive. Two problems
arise in connection with freezing under these
conditions: darkening of the flesh and the possi-
bility of toughening of the texture.
The darkening of the flesh arises simply be-
cause the blood has had insufficient time to
drain away. Fillets cut from a cod immediately
after landing on the deck are red with blood,
and become brown when frozen and thawed. If
the gutted fish are laid in ice for only 12 hours
— maybe less would be sufficient — then fillets
subsequently taken from the body are white.
The dark colour of the muscle of fish frozen
very soon after capture is therefore ummportant,
and is not associated with any undesirable fla-
vour. However, in Great Britain there has some-
times been reluctance on the part of merchants
to buy the fish, since the colour was considered
less attractive than usual.
The toughening of the texture has been a
difficult aspect of pre-rigor freezing to elucidate, ^
because of the lack of agreement among in-
vestigators. Controversy existed as long as forty
years ago — Taylor (1926) was unable to state
definitely whether fish were best frozen before,
during or after rigor mortis, since some investi-
gators had claimed that pre-rigor frozen cod had
a dry and disagreeable taste, whereas others had
failed to confirm this.
The problem now appears to be solved, and it
is perhaps worth pointing out that it was mostly
solved with Icelandic fish caught in Faxaflói in
1961. The Icelandic Department of Fisheries
kindly gave permission for our research ship “Sir
William Hardy” to catch cod inside the statutory
fishing limits, and the experiments that we did
in the laboratory while moored in Reykjavik
harbour enabled us to discover the truth of the
matter, and also the reason for the disagreement
between earlier workers (Love, 1962A).
Pre-rigor cod muscle has a fairly tough
consistency, and this property is reflected in a
low cell fragility reading. Our experiments in
Reykjavik showed that if the pre-rigor fish were
allowed to go into rigor mortis as whole, gutted
fish, being filleted just before sampling, then
as rigor mortis set in the cell fragility readings
steadily increased, until after the resolution of
rigor mortis they stood at the high values
normally associated with good-quality fresh fish.
The muscles of the fish had contracted during
rigor mortis they stood at the high values
length because they were still attached to the
bone, and were also being pulled by opposing
muscles. However, if the fish were all filleted
after catching, so that they went into rigor
mortis as fillets, the contracting muscle, being
loosed from the skeleton, was able to shorten.
Under these conditions the cell fragility values
did not increase as much as before, and the
muscle remained tough to eat on cooking. Figure
4 shows the results of the cell fragility tests.
Protein extractability behaved in the same way,
being low in pre-rigor fish, rising to normal
‘fresh’ values in fish undergoing rigor mortis as
whole gutted fish, but remaining low in those
undergoing rigor as fillets.
Period of
rigor
HOURS AFTFR DEATH
Figure 4: Changes in the texture that occur when cod
muscle passes into and through rigor mortis as whole
fish (solid circles) and as fillets which are free to shrink
(hollow triangles). A low E^cm (cell fragility value)
represents a firm or tough product when eaten after
cooking. Reproduced by courtesy of the Society of
Chemical Industry.
In the case of cod muscle rapidly frozen before
rigor mortis had occurred and then thawed with-
out storage, it was found that the size of the
sample and the rate of thawing governed the
texture after cooking. Thin strips of muscle
thawed in a few minutes at room temperature
shrank to less than half their previous length,
exuded much fluid and underwent extensive
denaturation. If they were thawed slowly at 0°C
they did not shrink, nor did they become tough.
Small fillets thawed in air at about 18°C did not
shrink or denature. If they were rapidly thawed