Tímarit Verkfræðingafélags Íslands - 01.12.1967, Síða 92

90
TlMARIT VFl 1967
by placing them in water at 30°C, they shrank
and became tough. Whole fish frozen before rigor
mortis were never denatured under any condition
of thawing. Being attached to the bone, the
muscle could not shrink.
These phenomena depend on the presence or
absence of adenosine triphosphate (ATP), the
chemical substance which supplies the energy
for muscle contraction. Fish frozen before rigor
mortis contain enough ATP to cause contraction;
if small pieces are quickly thawed, they contract
and shrink, and, as we have seen, the shrinkage
causes denaturation. Larger pieces thaw more
slowly, and under these conditions the ATP de-
composes while there is still ice in the tissue.
The ice in the middle of a fillet which is thawing
slowly holds the shape constant and does not
permit the muscle to shrink. By the time it has
melted, the ATP in the surface layer, which
melted first, has disappeared, so that shrinkage
cannot occur.
Moran (1929) observed that little strips of
frog muscle frozen pre-rigor did not shorten if
they were thawed slowly. Marsh & Thompson
(1958) prevented strips of pre-rigor frozen sheep
muscle from contracting by storing them at
—3.5°C for 4 days before thawing out, and
Tanaka & Tanaka (1956) even managed to stop
fluid exudation in pre-rigor frozen whale muscle
by keeping it at —2°C for 5 days before thawing.
These treatments allowed the ATP to decompose
while maintaining a constant length through the
presence of the ice.
From these observations we can draw several
conclusions of technological importance. It is
clear that any treatment that allows shrinkage
of pre-rigor muscle causes fluid exudation, de-
naturation of the proteins as measured by other
criteria and an undesirable texture (“like
rubber”). If, for example, fillets were frozen
pre-rigor and were cooked by being placed in a
pan of hot fat while in the frozen state, they
would thaw quickly enough to undergo shrinkage
and toughening. Freezing pre-rigor fillets is in
fact never recommended, because although slow
thawing would yield a product that was not
denatured, there is a likelihood that at least
some fillets would enter rigor mortis before
freezing, since the pre-rigor period is short and
variable. Since they would not be attached to
the bone the fillets would be free to contract
and thus acquire a tough texture. It is however
always safe to freeze pre-rigor fish if they are
whole and not filleted, provided there are no
objections to a slightly darker colour.
Acceleration of rigor
Fish in rigor mortis, being fixed in a particular
shape, cannot easily be filleted by hand, nor can
they be successfully fiHeted by machine. Both
before and after rigor mortis the fish is soft
and can readly be filleted. Since it is not recom-
mended to fillet before rigor, then clearly any-
thing which could speed the onset and resolution
of rigor would improve the efficiency of proc-
essing.
Work carried out at Torry by N. R. Jones has
shown that both the onset and resolution of rigor
mortis can be hastened by raising the tempera-
ture. However, at temperatures above about 17°C
the connective tissue breaks down to such an
extent that the fillets gape badly, and fall to
pieces completely at temperatures a few degrees
higher. The temperature at which the fillets
start to gape depends on the species of fish —
warm-water species such as the skipjack can
remain intact at temperatures higher than 17 °C.
However, it appears that the acceleration of the
rigor mortis processes is a somewhat risky and
uncertain procedure.
Drying of frozen fish and cold-store design
Frozen fish placed unwrapped in a cold-store
soon exhibits spots or patches of a light colour,
which soon spread over the whole fish and pene-
trate into the muscle. This effect is a sign of
drying, and it brings with it complete denatur-
ation of the proteins (the flesh cannot success-
fuUy be re-hydrated) and in the case of fatty
fish it enables rapid oxidation of the fat to occur,
since the surface of the fish is now porous to
gases. It should always be avoided, since affected
fish quickly become unfit to eat.
It is prevented by wrapping in impervious
material, or by spraying the fish, after freezing,
with cold water, so as to form a continuous
layer of ice all round, which will give protection
for a while, besides being cheáper than a foil or
plastic film wrap.
The ice layer will evaporate in time, but good
cold-store practice and design will slow down
the process. The lower the cold-storage tempera-
ture is, the longer the glaze will last; it can last
for several months at —30°C. Another important
factor governing the rate of drying of the fish
is the temperature of the fish in relation to the
temperature of the cooling coils in the store. If