Dear List
I can not agree with Dr Peralta, in respect to effect on quality, that '...
the freezing process is as important as the storage temperature'. In my
experience of evaluating many samples of fish frozen in different ways the
rate of freezing other than very slow freezing, over days, has a negligible
effect on quality. Storage temperature has a large effect on quality. An
experienced sensory panel can detect the effect of 2 weeks storage at -18ºC,
for example. The FAO publication Jose refers to is probably the FAO
Fisheries Technical Paper 340, Freezing and refrigerated storage in
fisheries, cited in my message of the 9th. Let me quote from this
publication from the section on freezing methods:
'There is no widely accepted definition of quick freezing.
It is unlikely that even a trained taste panel could detect the difference
between fish frozen in 1h and 8h, but once freezing times begin to extend
beyond 12h the difference may well become apparent. Freezing times of up to
24h or even longer, achieved in some badly designed and operated freezers,
will almost certainly result in an inferior product. Very long freezing
times, for example, due to freezing fish by bulk stacking in a cold store,
may even result in spoilage by bacterial action before the middle of the
stack is sufficiently reduced in temperature.
Since the temperature just below 0ºC is the critical zone for spoilage by
protein denaturation, an early UK definition of quick freezing recommended
that all the fish should be reduced from a temperature of 0ºC to -5ºC in 2h
or less. The fish should then be further reduced in temperature so that its
average temperature at the end of the freezing process is equivalent to the
recommended storage temperature of -30̊C. With normal freezing practice in
the UK, this latter requirement is defined by stating that the warmest part
of the fish is reduced to -20̊C at the completion of freezing. When this
temperature is reached, the coldest parts of the fish will be at, or near,
the refrigerant temperature of say -35̊C and the average temperature will
then be near -30̊C. This is a rather elaborate definition of quick freezing
and it is probably more strict than is necessary to ensure a good quality
product.
The more widely used definitions of quick freezing do not specify a freezing
time or even a freezing rate but merely state that the fish should be frozen
quickly and reduced in the freezer to the intended storage temperature.'
The requirement for good freezing practice is just that the product be
frozen in equipment designed for the freezing of fishery products. This is
quite adequate and precludes just putting products in a cold room to freeze.
I also should mention that thawing is an important step in the process of
utilising frozen fish. Again the thawing process should be fast, for example
in mechanical thawers, for maintenance of good quality.
Peter Howgate
----------------------------------
> ----- Original Message -----
> From: "Jose P. Peralta" <jperalta@gaechuk.gsnu.ac.kr>
> To: <seafood@ucdavis.edu>
> Sent: Friday, January 10, 2003 2:49 AM
> Subject: Frozen fish vs its Quality
>
>
> Dear list:
> This posting somehow deviates from the original subject string, thus,
> re-titled
> differently.
> The original subject string was, " Ice content in frozen fish."
>
> >>What is more damaging to quality is
> that the fish has apparently been stored at -20ººC before processing and
will
> be stored at -18ººC afterwards. These temperatures are too high for
> maintenance of good quality in frozen fish; a better temperature for
storage> would be around -30ººC.
>
> I would agree that storing at -30 C is better. Furthermore, the freezing
> process is as important as the storage temp. The thermal arrest period
> should
> be kept to 2 hours max.
>
> I recall, in one of FAOs reference, it advices frozen fish storage
> around -30 C,
> also.
>
> The international frozen fish industry have been slow in adopting such
> advisory. Nowadays, more efficient refrigeration systems are available.
> Industry practice still use -18 C storage temp.(also practiced in the
> Philippines).
>
> Frozen black tiger prawn, in blocks, shipped from the Phil to Japan at -18
> C.
>
> There is however, an interesting note on frozen fish vs its quality. Raw
> fish
> tuna market in Japan, has been dictating low temp practices of yellowfin
> tuna to
> be either chilled or frozen at -65 C.
>
> I have info gap on the storage of the frozen tuna, thereafter, but I
believe
> it
> is lower than -18 C. Also, please take note, that the tuna will be eaten
> raw,
> eventually.
>
> There is one thing I am certain, though, the consumers in Japan demand to
> have
> the highest fish quality, and they are willing to pay the price for this
> premium.
>
>
> Jose P. Peralta
>
>
>
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