Tímarit Verkfræðingafélags Íslands - 01.12.1967, Page 90

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TlMARIT VFl 1967
Investigation of the process in several labora-
tories confirmed the reduction in fluid loss (Tani-
kawa et al., 1963; Ohta & Nishimoto, 1963;
MacCallum et al., 1964; Boyd & Southcott, 1965;
Chalker et al., 1965), although one group (Anon.,
1983) reported that the reduction was only
marked under conditions where fluid loss would
have been low anyway; where it would have
been high under normal conditions, the advan-
tage gained by using polyphosphate was un-
certain. Another report (Dyer et al., 1964)
stated that tripolyphosphate-dipped and undipped
control paired cod fillets stored at —12°C showed
no clear difference in the amounts of fluid exud-
ing after thawing. Only when such fish were
frozen and thawed twice — a procedure that
materially increases fluid loss — could a clear
advantage in using polyphosphate be found
(MacCallum et al., 1964A).
When we examine the other criteria of cold-
storage denaturation, the picture is less encour-
aging. While one report (MacCallum et el., 1964)
mentions an improvement in texture after using
polyphosphate and another (Ohta & Nishimoto,
1963) an inhibition of insolubilisation of the
protein, the remainder state either that there is
no improvement in protein solubility (Dyer et
al., 1964; Anon., 1964) or that polyphosphates
actually increase the denaturation (Tanikawa et
al., 1963; Nikkilá et al., 1964). This is a serious
drawback; as Nikkilá et al. (1964) point out, the
inventor of the commercial process had studied
only fluid exudation, without taking into account
the condition of the proteins, which is of primary
importance with respect to the overall quality
of the fish.
The explanation of these phenomena must at
present be tentative, since there is some incon-
sistency between the various reports. It seems
fairly certain that fluid loss can be inhibited
without improving the extractability of the
protein, and this poses a problem, since fluid
exudation and protein insolubilisation have al-
ways been regarded as different aspects of the
same phenomenon.
A preliminary report appears at present to
hold the key to the puzzle (Anon., 1964). Histo-
logical sections showed that in the muscle near
the surface of the fillet, i.e. the part that had
been in contact with the polyphosphate, the cells
had swollen and the extracellular spaces had
disappeared. Thus the interior of the fillet was
sealed off by a superficial layer of continuous
protein. If this finding is confirmed, it would
account satisfactorily for the phenomena de-
scribed above.
Polyphosphate dips therefore would seem to
confer mainly an economic advantage: they can
save material bought as fish from flowing away
to waste after thawing. They almost certainly
do not inhibit denaturation of the proteins during
storage, and perhaps can best be said to alleviate
the symptoms rather than cure the disease.
Influence of the freshness of fish at the time
of freezing on the quality of the frozen product
It has been found at Torry Research Station
over many years that white fish such as cod
should be frozen not more than three days after
capture in order to obtain a first-class product.
This time limit applies to gutted fish stowed in
crushed ice after death. Changes in flavour dur-
ing this period are unimportant: the main
criterion is that of appearance.
Fillets made from whole fish frozen after three
or more days in ice look ragged, showing much
gaping between the muscle segments, owing to
a weakening of the connective tissue that holds
the fish together. This detracts irom the appear-
ance of the fish, which cannot then be called
first-class.
Herrings (sild) become stale more quickly than
white fish chiefly because the guts are not re-
moved after capture. Because of this, they should
be frozen after not more than 12 to 18 hours in
ice, depending on the condition of the fish (Reay
et al., 1950). Those with full stomachs deterior-
ate more quickly than those partly full or empty.
Other species of fatty fish such as sprat (brisl-
ingur) and mackerel (makrill) should also be
frozen as quickly as possible after death.
Pre-rigor freezing and its consequences
From the foregoing, it is clearly desirable to
freeze the fish as soon as possible after death.
However, a complication arises here: “rigor
mortis”.
At some time after the death of a fish the
muscles undergo an irreversible contraction, and,
since opposing muscles pull against one another,
the fish becomes stiff, the effect being known
as rigor mortis (“the stiffness of death”). In
trawled cod, this stiffness usually begins within
the first 5 hours, and if the fish have become
rigid in a bent position they are difficult to fillet.
After a further period, the muscles relax
again, this being known as the ‘resolution’ of