howgate (phowgate@rsc.co.uk)
Sun, 30 Jan 2000 20:44:56 -0000
On 26 January, Kevinn Lyman raised the question of how to tell if fish has
previously been frozen. He specifically asked the question in the context
of cooked fish, but the question is equally valid in the context of raw
fish. I assume the customer was sold raw salmon, but perhaps returned the
cooked product when lodging the complaint. (What raised the customer's
suspicion the fish had previously been frozen?). The problem is not just
one for the seller/customer; it is a problem for food control as well. In
the UK, food regulations require that fish that has been frozen at some
time, but sold in the thawed state, must be labelled as 'previously
frozen'. There is similar regulations in other European states, and I am
not sure that the principle is not incorporated in EU legislation.
Let me discuss methods for examining raw fish first for previous freezing.
If fish is frozen, stored, and thawed under good manufacturing practices -
frozen rapidly, wrapped to prevent dehydration and oxidation, stored below
-30ºC for not too long, and thawed rapidly - then no simple test will
unequivocally show that the thawed material had been frozen at some time.
This is the point Andrew Strak made in his response to Marcie Ver Ploeg.
The tests that have been referred to, other than the Torry meter, are tests
for deterioration during frozen storage.
Captain Jack Donlan's test referred to by Marcie is an example of the
general test for liquid released from the protein gel during frozen
storage. It is not necessary to bend the fillet. Lay the fillet on a board
and press into it with a couple of finger tips and note if any liquid is
released around the finger tips. There will be no liquid, or just a very
thin line, in the case of unfrozen fish. Frozen and immediately thawed fish
will show a very small increase in the expressible liquid, but the
difference from unfrozen fish is very small and even with experience, one
would not like to be dogmatic that a sample had been frozen with just a
thin line of liquid. The amount of expressed liquid increases as the fish
deteriorates in frozen storage; one that 'oozes' liquid will probably be
past the good quality limit and close or past the edibility limit. The
finger pressure test will also detect if the fillet is hard. If so, it has
suffered cold storage damage. An extension of the hardness test is to hold
out a hand vertically and drape the fillet over it. An unfrozen fish will
be completely flexible and the fillet will touch both the back and the palm
of the hand. Again best with thinner fillets and there is some difference
in drape among species. As the fish deteriorates in frozen storage, the
fillet becomes stiffer and does not drape as much, or at all.
Tests based on aggregation of protein again measure storage deterioration.
No aggregation is detected in fish stored under good conditions. One type
of aggregation is induced by formaldehyde, which is formed from TMAO, but I
believe TMAO contents in salmon are low to negligible.
Richard Chivers referred to the Torry meter. This measures electrical
properties of fish muscle - combination of resistance and capacitance. The
original high readings in very fresh, unfrozen, fish depends on the
difference in ionic concentration between intra- and inter- cellular fluid,
and the properties of the cell membranes. During icing the cell membranes
disintegrate, the fluids equilibrate and the meter readings decrease. The
freezing/thawing cycle has the same, and immediate, effect so that the
meter will show a low reading even when other characteristics of the fish
show it is very fresh. The meter then provides an almost unequivocal test
distinguishing between unfrozen and frozen/thawed fish.
In response to George Chang, yes, there is a need for a simple test to
check that 'wet' fish might have previously been frozen. This would be
useful in quality assurance and for enforcement of labelling regulations.
However, I think he has the purpose the wrong way round. A supplier
concerned about achieving high quality in a product might want assurance
that the fish had previously been frozen. Typically the eating quality of
sea frozen fish, of any species, provided it has been frozen and stored
under GMP, is superior in eating quality to the product from fishing boats
using iced storage. Check it if you don't believe me - a useful project for
a student assignment.
A couple of other points. If the fish is whole, that is has the head on,
then inspection of the eye will unambiguously tell if the fish has been
frozen. In the frozen fish, the lens of the eye is a white, opaque ball.
Histological examination of the muscle tissue will reveal traces of the
voids formed by ice crystals in cells in previously frozen fish even in
frozen/immediately thawed fish. While not a spot test as asked for by
George, could be of interest in case of prosecution for mislabelling.
This still leaves the problem of determining previous history from a sample
of cooked muscle. The exudation test can not be used, and protein
aggregation tests would be ambiguous. The Torry meter is not applicable. I
do not know if the histological test can be applied - I've only tried it on
raw fish. Sensory assessment of flavour and texture of the cooked sample by
experienced assessors will detect frozen storage changes, even very small
changes. In my experience, a well trained panel can reveal the difference
in sensory properties between unfrozen and frozen/immediately thawed fish
when results are pooled over a few samples. (Assessors using objective
scalar methods for measuring flavour and texture, with comparison of mean
scores of several fish each of the two treatments.)
Peter Howgate
This archive was generated by hypermail 2.0b3 on Sun Jan 30 2000 - 12:50:43 PST