Tímarit Verkfræðingafélags Íslands - 01.12.1967, Page 88

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TlMARIT VPl 1967
unfrozen, i.e. was kept above —1.2°C. In the
light of the new information given in Figure 3,
one may be more optimistic.
‘Superchilling’ of cod appears to have a prom-
ising future, provided that (a) the appearance
of the product is not important (b) the fish are
stowed for no more than 3 weeks and (c) they
are not to be processed further, e.g. by normal
freezing plants or by smoking, but are to be
eaten at once. This may give rise to problems
if large quantities of fish have been superchilled.
In the hght of what has already been said
about species differences among fish, it will be
realised that, for example, lemon sole (þykkva-
lura) or haddock (ysa) would probably keep in
good condition longer than cod under ‘super-
chilling’ conditions.
Rate of íreezing
Generál
The significance of freezing rate has exercised
the minds of workers for many years, without
any consistent conclusions emerging. One diffi-
culty has been that the palatability and appear-
ance of the products are difficult to assess ob-
jectively, and it is almost impossible to compare
the findings of workers in different laboratories,
since definitions of freezing rate used tend to be
vague. Kondrup and Boldt (1960) reviewed a
number of papers on the effect of varying the
freezing rate on the quality of beef and poultry,
and in spite of there being much material to
hand. they had to admit that the overall findings
were inconclusive. The only reasonably certain
outcome from all the work was that the appear-
ance of the product tended to be better after
rapid than after slow freezing.
In the case of fish, it was concluded by Weld
(1927) that quicker-frozen cod (þorskur) be-
came tough and tasteless more slowly on storage
than slower frozen. Reay (1934) reported that
quicker-frozen herrings (sild) were slightly
better, but that freezing rate had little effect on
quality. Notevarp & Heen (1938) found little
difference in the quality of cod frozen in 1 or in
6 hours. Quicker-frozen herring were considered
slightly better in quality immediately after
freezing, but the difference was found to dis-
appear on cold-storage. Banks (1938) reported
that rapid freezing was essential for good ap-
pearance and suitability for processing of her-
rings. Slowly-frozen herring showed a greater
wastage when fed through a splitting machine.
Reav et al. (1950) found that, for various fish,
there was little detectable effect on the texture
unless the freezing time was more than 4 to 10
hours, in different instances. If herring or had-
dock of the best appearance and suitabihty for
smoke-curing were to be obtained, then the time
to cool from 0° to —5°C had to be less than 2
hours. Aalderink & Rowan (1953) found no
difference in the firmness or flavour of hake
(lysingur) frozen in various times from 12 to
36 hours.
It is clear that the differences between fish
frozen at different rates are not great enough
to be obvious on every occasion, but, on the
average, rapid freezing gives a better product,
especiahy where appearance is concerned. Very
slow freezing should be avoided anyway, because
bacterial spoilage will continue during the time
the fish is still unfrozen.
While the effects of freezing rate may not
always be apparent after thawing out, they can
be perceived in the frozen product. Freezing fish
causes the water from the protein gel to solidify
as minute, discrete particles of ice, which are
separated from each other by the dehydrated
protein. The size of these ice particles depends
on the rate of freezing, becoming progressively
smaller as the rate increases (Plank et al., 1916).
Rapidly-frozen fihets, on account of the vast
number of minute ice particles, reflect much
light from their surfaces, and so look white:
indeed, in very rapid freezing they become so
white that they have occasionally been rejected
by commercial processors in the mistaken belief
that they were dehydrated. Slowly-frozen fish,
containing larger continuous bodies of ice, look
darker and more ‘glassy’.
If fish frozen at various rates are thawed out
immediately, the dehydrated protein reabsorbs
all the water from the melted ice, swehs, and
takes on an appearance that cannot be dis-
tinguished histologically from that of fish that
have never been frozen at all.
If, however, there has been cold-storage,
resulting in denaturation, the protein loses its
power to reabsorb the water and remains in the
shape acquired during freezing. This shape would
be governed in the first instance by the freezing
rate, so here may lie the explanation for the
considerable lack of agreement among investiga-
tors — there is probably a better chance of de-
tecting differences in the appearance of thawed
fish frozen at different rates after some denatur-
ation has taken place.