Tímarit Verkfræðingafélags Íslands - 01.12.1967, Side 86

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TlMARIT VFl 1967
the fish remain indistinguishable from newly-
frozen is only about a year, and if there is some
fluctuation in the temperature of the cold store,
as for instance when the doors are opened, or if
the coating of ice on the surface of the fish dries
off, then the time will be reduced further.
An attempt is made in Table 1 to give the
storage ‘life’ of various marine products as ass-
essed subjectively. It must be remembered that
the times given are very approximate because of
the interaction of variable factors.
It will be seen that fatty fish are always given
a shorter acceptable time of cold-storage than
non-fatty fish, the reason being that it is rarely
possible in actual practice to exclude all air from
the fish, so that some rancidity inevitably deve-
lops and soon renders the product unacceptable.
Wrapping each individual fish in metal foil is
probably the best way to exclude air, but it is
too expensive for most purposes.
The deterioration of fatty fish is the result
of the action of two independent factors: protein
denaturation and fat oxidation. The satisfactory
objective assessment of deterioration in fatty fish
is therefore not at present possible.
Figure 2: Relative rates of deterioration in different
species of North Sea fish, using the extractability of
muscle protein nitrogen in 5% NaCl as the criterion.
The deterioration of non-fatty species, how-
ever, is centred almost exclusively on changes in
the protein, which can be investigated objectively.
Figure 2 shows the decrease of extractability of
muscle protein of several species of fish with
time, and it is at once clear that large differences
exist in the rates of denaturation. The reason for
this is not yet certain, although it is probably
connected with the liberation of free fatty acids
from phospholipid during cold-storage in rela-
tion to the amount of other (‘neutral’ lipid
present (Hanson & Olley, 1964). It is a fas-
cinating subject, but outside the scope of the
present report: reviews are to be found else-
where (Love, 1966A, B). From the practical
angle, it can be seen that the lemon sole (þykkva-
lura) denatures far more slowly than other
species, while the whiting (lysa) denatures the
most quickly. These objective results accord with
general experience in tasting, but are more
accurate. Other unpublished work, using the cell
fragility method, shows that plaice (skarkoli)
and hake (lysingur) denature quite quickly, but
not as quickly as cod (þorskur).
The temperature of maximum denaturation
The rate of denaturation of cold-stored fish
becomes steadily faster as the temperature is
raised, but decreases just below the melting point.
According to various authors reviewed by Love
& Eierian (1964), the temperature of maximum
denaturation rate of different species of fish
ranges from —1.5° to —5°C. Above —1.5° the
proportion of tissue water which is frozen dec-
lines markedly, and so the rate of denaturation
decreases.
'Superchilling’
Cod which are air-blast frozen at —30° and
then stored at —1.5°C are completely denatured
in about 2 weeks. It has therefore been sur-
prising to find a recent resurgence of interest in
keeping fish, albeit for limited periods, at tem-
peratures just below their freezing point.
The intention of Golovkin & Pershina (1961)
in storing fish in this way was to reduce re-
frigeration costs on trans-continental railway
journeys. The fish were frozen to low tempera-
tures and then packed into railway vans with
little extra refrigeration. They arrived at their
destination at —1° to —2.6°C after 1 to 2 weeks,
and on tasting were found to have a good flavour,
since bacterial growth had been stopped. Consid-
erable denaturation must have occurred, because
fluid was lost by the fish after thawing, but the
authors gained their objective since a definite
improvement had been obtained in the flavour of
the fish.
Much more surprising, however, has been the
success of ‘superchilling’, which is being carried