Tímarit Verkfræðingafélags Íslands - 01.12.1967, Blaðsíða 85

TlMARIT VFl 1967
83
storage progresses. The values are read as optical
density of a homogenate of fish muscle, and these
decrease as denaturation proceeds. Besides being
very quick to carry out, the test is less influ-
enced by biological variation than protein solub-
ility, and it is probably the best method in cur-
rent use for investigating the cold-storage of fish
muscle. Many of the conclusions about various
conditions of freezing and storage, to be de-
scribed below, have been reached by this met-
hod.
The influence of storage temperature
The storage life of various products
Figure 1 shows the results of cell fragility
determinations carried out on cod (þorskur)
fillets, air-blast frozen within a day of catching
and stored at different temperatures. The fillets
were wrapped in foil to prevent drying out. At
—14°C the denaturation is virtually complete in
15 weeks, while at —29°C the same amount of
denaturation would take about 9 years. The
relationship between denaturation rate and
temperature is a logarithmic one (Love, 1962),
which explains why fish stored at —29°C dena-
ture so much more slowly than those at —24° C.
TABLE 1
The Storage Life of Fish Products at Various Temperatures as
Estimated by Subjective Tests
—9 "C —18°C —21 "C —25 °C —29 °C
good inedible good good inedible good good inedible
White fish (gutted) .. 1 mo. 4 mo. 3-5 mo.’ 4 mo. 15 mo. 6-8 mo.1 8 mo. more than
4 yrs.
Herring (gutted) 1 mo. 3 mo. 2-3 mo.1 3 mo. 6 mo. 3-5 mo.1 6 mo. more than
1J yrs
Flatfish 4-6 mo.1 7-10 mo.1
Smoked white fish . .. 1 mo. 3 mo. 3J mo. 10 mo. 7 mo. more than
1 yr.
Kippers 3 wks. 2 mo. 2 mo. 5 mo. 4$ mo. more than
9 mo.
Kippers (vacuum more than
packed) 1 yr.
’Figures taken from the Intemational Institute of Ref rigeration Code of Practice, ‘Recommended Oonditions
for Cold Storage of Perishable Foodstuffs’. Other figures are from Reay Banks & Cutting (1950), these being
derived from fish stored in ice for not more than 24 hours between catching and freezing. All but the smoked
fish were 'glazed’ with a layer of ice, packed in woodenboxes lined with parchment paper, and kept within 0.5CC
of the stated temperature. Samples were tasted and compared with fresh fish at regular intervals.
Figure 1: Deterloration of frozen cod stored at different
temperatures, as measured by the cell fragility technique
(a decrease in Ejcm_ signifies deterioration). o-o
—14°, • - • —20°, A — A —24°, ■ — ■ —29°C.
Reproduced by courtesy of the Society of Chemical
Industry.
While it is clear from the Figure that the
product will keep longer as the temperature is
lowered, it may come as a surprise to find that
we at Torry Research Station recommend a
storage temperature as low as —30°C for storing
fish — after all, it is unusual to store fish for
9 years. The point to remember, though, is that
9 years is the time needed to produce a very
tough, unacceptable product. The time for which