Tímarit Verkfræðingafélags Íslands - 01.12.1967, Blaðsíða 89

TlMARIT VPl 1967
87
Fluctuation in the storage temperature
If the temperature of the store rises a little
and then falls, the smallest ice crystals melt and,
on refreezing, augment the bigger ones. The
result of repeated fluctuation of the storage
temperature is an increase in the size of the ice
particles and a decrease in their number, so that
quickly frozen fish gradually take on the appear-
ance of slowly-frozen ones.
Dyer et al., 1957A, 1957B made a systematic
study of the effects of raising the storage tem-
perature to —9°C for short periods during stor-
age of cod fillets at —18°C, to simulate the
conditions sometimes occurring during rail or
other transport. A significant deterioration was
detected both by the taste panel and by protein
extractabihty, but it appears to have been solely
the result of the high temperature, rather than
to the fluctuation as such.
Temperature fluctuation can therefore be con-
sidered undesirable because the fish are exposed
to higher temperatures. It also encourages dry-
ing of the fish: this will be dealt with more fully
later.
Ultra-rapid freezing in liquid nitrogen
Until recently, the cost of liquid air or its
fractions has precluded its use for commercial
freezing purposes. However, the fall in price
resulting from large-scale manufacture has made
the use of liquefied gases for the freezing of food-
stuffs an attractive proposition, since a daily
delivery of a liquid air fraction, usually nitro-
gen, would eliminate the need for buying and
maintaining costly refrigeration machinery.
In some processes, the liquid nitrogen is
sprayed into a container of food. It immediately
vaporises, and by controlling the flow the food
can be frozen in gas at around —30°C. This
system seems to be excellent, and not open to
any objection from the quality aspect.
However, it is not advisable to freeze fish by
immersing them directly in the liquid nitrogen.
It was shown (Love, 1955) that treatment of
this kind resulted in badly broken fillets that
literally fell to pieces after thawing. The reason
for such extensive mechanical damage was that,
on immersion, a layer of deep-frozen tissue quick-
ly formed on the surfaces of the fillet while the
centre was still at room temperature. Because of
the very low temperature, this layer was ex-
tremely hard and brittle, so when the centre of
the fillet froze, the expansion due to the freezing
of tissue water cracked open the outer ‘shell’
in many places.
Recent papers on the subject have illustrated
this effect with photographs (Torry Research
Station Annual Report, 1962; Lorentzen, 1964),
and it can be seen that such a product is quite
unacceptable on the grounds of appearance. Tests
with liquid media cooled to various temperatures
(T.R.S. Ann. Rept., 1962) have shown that
—75°C defines approximately the limit of
temperature below which mechanical damage will
occur during freezing by direct immersion.
However, when such shattered fillets were
cooked they were judged as acceptable as those
frozen in conventional ways (Moiseeva & Piska-
reva, 1959; Lorentzen, 1964). It has also been
reported (Moiseeva & Piskareva, 1959) that the
protein extractability in 5% sodium chloride
solution is unaltered by deep freezing of this
kind. Be that as is may, in neither of these
accounts was sufficient storage carriea out to
cause much deterioration in the protein. Love
and Elerian (1963) showed that the denaturation
during cold-storage at —14 °C was progressively
hastened as the initial freezing temperature was
lowered, owing to the removal of bound water
from the protein. Further work is needed to find
out just how important this effect is from a
technological viewpoint, but in the meantime
one should sound a note of warning over freezing
fish in liquid nitrogen.
Pre-freezing treatment in pliosphate dips
It has already been stated that one manifesta-
tion of denaturation in frozen fish is the loss of
fluid after thawing. This can represent a consid-
erable economic loss, quite apart from the re-
duced ‘sales appeal’ of a wet and messy product.
A dip in sodium chloride solution before
freezing will reduce fluid loss but such treat-
ment accelerates the oxidative rancidity during
cold-storage, so is not used. Various phosphate
solutions can, however, reduce fluid loss without
having any effect on rancidity development
(Boyd and Southcott, 1965).
A commercial process was recently launched,
with considerable publicity, to ‘improve’ the
quality of frozen fish by dipping them before
freezing in a solution of sodium tripolyphosphate
with sodium chloride, although the only improve-
ment to be described objectively by the early
writers was the reduction in fluid loss after
thawing (Mahon, 1962; Milleville & Leinen, 1962;
Anon., 1962; Mahon & Schneider, 1964).