Tímarit Verkfræðingafélags Íslands - 01.12.1967, Page 93

TÍMARIT VFI 1967
91
the coils are much colder than the stored fish,
water molecules will leave the fish and condense
on the coils, covering them with ice and reducing
their efficiency. This situation will arise if the
store is repeatedly warmed up by the door being
opened — colder coils will be needed to bring
the temperature down again to the desired level.
Again, if the fish is brought into the store from
the freezer at a higher temperature than the
store or, worse, if wet fish are placed into the
store to freeze there, the store temperature will
rise and rapid drying will occur.
Drying can be kept to a minimum by having
as big an area of cooling coils as possible. The
best way to achieve this is to have a jacketed
store, where the whole wall and ceiling act as
coolers. This reduces the temperature differences
between cooler and fish. Also the doors should
be opened on as few occasions as possible, and
only fish frozen to a low temperature should
be brought in. A refrigerated air-lock, or an air
curtain to minimise the entry of warm air, help
to prevent the fish from drying out. Otherwise
deterioration of the product is serious and rapid.
Summary
Various conditions of freezing and storing fish
are described, and their advantages and dis-
advantages discussed. The lower the tempei'ature
of storage is, the longer a fish will keep. A
temperature of —30°C is recommended for
practical purposes. Fish containing fat do not
keep as long as non-fatty fish, because the fat
becomes rancid during cold-storage and then
has a disagreeable taste. The length of time
that a non-fatty fish can be stored depends on
the species.
It has recently been found that fish can be
successfully stored at about — 2°C for short
periods provided that they are just cooled to
—2°C and not first frozen to low temperatures
and warmed up to —2°C.
Quickly-frozen fish tend to be slightly better
than slowly-frozen fish, but storage temperature
is more important than rate of freezing.
Using liquid air to freeze fish can give a satis-
factory product if the fish is frozen in the
vapour at —30° to —60°C. However, freezing by
direct immersion in the liquid is not recommended
because the fish break up.
Dipping the fish in solutions of polyphosphate
before freezing reduces the amount of fluid
which is lost after thawing, but does not prevent
the other kinds of deterioration from taking
place.
Freezing fish before “rigor mortis” may give
a product with a rather dark flesh, but the
quality is very good. If the fish is frozen whole
(intact) a good product is always obtained, but
if pre-rigor fillets are frozen, they may be tough
to eat after thawing and cooking, so it is not
recommended to freeze fillets in this way.
If fish dry in the cold store, they become
tough and rancid. Drying can be prevented by
wrapping the fish or by covering with a layer
of ice. The speed with which this layer disappears
from the surface depends on the design of the
cold store.
This paper was prepared as part of the
programme of the Ministry of Technology.
The author wishes to thank Hjalti Ein-
arsson for the Summary.
Crown Copyright Reserved.
Útdráttur
Frysting og geymsla á fiski eru ræddar frá
ýmsum hliðum og kostir og ókostir taldir upp.
Því lægri sem geymsluhitinn er, því meira verður
geymsluþolið. Mælt er með —30°C, sem hæfi-
legum geymsluhita. Feitar fisktegundir geym-
ast skemur en magrar tegundir, þar eð fitan
þránar í frosti og óbragð kemur af fiskinum.
Geymsluþol magurra fisktegunda er mismun-
andi eftir tegundum.
Nýlega hefir komið í ljós, að geyma má fisk
nokkurn tíma með því að kæla hann niður í
—2°C, en sami árangur næst ekki, ef fiskur er
frystur og frosni fiskurinn síðan látinn hitna
upp í —2°C.
Hraðfrystur fiskur er lítið eitt betri en hæg-
frystur fiskur, en geymsluhitinn er mikilvæg-
ari en frystihraðinn.
Frysting í fljótandi lofti getur gefið góða vöru,
ef fryst er í úða við —30° til —60°C. Hins
vegar er ekki lagt til að fiski sé sökkt niður í
fljótandi loft, þar eð þá vill hann brotna í sundur.
Ef fiski er difið í upplausn af polyfosfötum
á undan frystingu, minnkar safatapið við upp-
þíðingu, en það hindrar ekki aðra gæðarírnun.
Frysting á fiski fyrir dauðastífni getur haft
þær afleiðingar, að flök verði dökk að lit, en
að öðru leyti eru gæðin mjög góð. Heilfrysting
gefur alltaf góðan árangur, en flök, sem fryst
eru fyrir dauðastífnun, geta verið bragðseig eftir
uppþíðingu og suðu. Það er því ekki lagt til, að
flök séu fryst þannig.