Tímarit Verkfræðingafélags Íslands - 01.12.1967, Page 87

TlMARIT VFl 1967
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out commercially in Portugal (Scarlatti, 1964).
In this process, newly-caught fish are stowed in
ice in ‘pounds’ and then chilled to temperatures
just below the freezing point by the application
of a moderate amount of refrigeration. The fish
room on the ship has a lining of stainiess steel
with cooling grids buried beneath the surface,
and plate coolers are installed at the sides of the
pounds so that the fish and ice cool more or less
uniformly without severe temperature gradients.
Fish from West African fishing grounds of ac-
ceptable quality have been landed after more
than 30 days under these conditions (Merritt,
1965). Repeating this work under laboratory
conditions, Merritt (1965) found that the usual
‘life’ of cod after catching, 14 days in melting
ice, could be extended to 21 days at —1°C or
26 days at —2°C. At —3°C the damage due to
ice formation in the flesh rendered the cod un-
suitable for filleting and smoking, but the eating
quality was still acceptable after 35 days.
When the writer tested the same material with
the cell fragility method, it was found that very
little denaturation had occurred. It was tempting,
therefore, to suggest that the fish had never
actually frozen: the proteins of cod muscle at
—1.5°C in the supercooled condition (without
ice present) are not denatured (Love & Elerian,
1964). However, Merritt carried out calorimetric
determinations on the cod and showed that after
8 days about half the tissue water was frozen
out, the mean fish temperature by then being
—2°C.
The results were therefore paradoxical: the
conditions were apparently such as to produce
very rapid denaturation, while the tests showed
that this was hardly occurring at all.
The explanation lies in ‘bound water’. In 1963,
Love & Elerian showed that if cod were frozen
at —14 °C and then stored at the same tempera-
ture, they denatured more slowly than those
frozen, say, to —80°C then re-warmed to and
stored at —14°C. As the temperature was low-
ered, more and more ‘bound water’ was removed
from the muscle protein and converted to ice,
and this change of state was not reversed when
the frozen fish were warmed to a higher, still
sub-zero temperature. According to present views
(reviewed by Love, 1966A), fish muscle protein
becomes denatured because freezing concentrates
the tissue salts by recoving water as ice, and
the strong salt solution, probably in conjunction
with free fatty acids, acts on the proteins and
alters them. If this is true, then the irreversible
freezing out of bound water at low temperatures
results in a higher concentration of tissue salts
in fish frozen to —80° then re-warmed to —14°
than in fish merely cooled to —14°C. In the
same way, fish frozen in an air-blast at —30°
and then re-warmed to —2° would contain more
ice, and so more coneentrated tissue salts, than
fish just cooled to —2°C, and would denature
faster.
Figure 3: Effect of freezing at different initial tempera-
tures on the rate of deterioration at —1.6°C in cod
muscle. • — • air-blast frozen at —30°C before storage.
0-0 frozen in polythene bags in a llquid bath at
—3°C. Each point is the mean of 15 determinations.
To test this theory, an experiment was carried
out in September 1965 in the writer’s laboratory
on cod in sealed bags (a) frozen in a liquid bath
at —3°C and stored at —1.6°C and (b) air-blast
frozen at —30°C and then stored at —1.6°C.
The results (Fig. 3) show very clearly that the
air-blast frozen cod denature much more quickly
than the ‘superchilled’ samples.
From this work it is clear that the ‘temper-
ature of maximum denaturation’, already stated
to be —1.5°C for cod, is only true in the case
of fish first frozen to a relatively low temper-
ature, and then warmed up for storage. Under
‘superchilling’ conditions it is probably lower
than this.
Discussing the new process, Partmann (1965)
concluded that ‘superchilling’ would result in a
badly denatured product unless the fish remained