Lots of good points, Peter. But the issue may be more of practicality and
dealing with the existing cold chain the way it is. These days of
globalization, continuous re-processing at the lower possible cost, lean
inventories and factory freezers being phased out the fishery products may
spend less time in the freezer hold or main storage freezers than in freezer
containers or on the superstore shelves, where very often the storage
temperatures are within what we may call an 'abusive range' and definitely
being sub-optimum from the point of view of arresting undesirable
biochemical changes. Besides, how we would define 'quality'? For some of us
it may mean to stay as close to the original state as possible but for the
processor or the consumer it may simply mean being fit for the purpose.
Andrew Strak
-----Original Message-----
From: Howgate [mailto:phowgate@rsc.co.uk]
Sent: Thursday, August 09, 2001 5:27 AM
To: kurt_jacobsen@hnfoods.com; seafood@ucdavis.edu
Subject: Re: Freeing and frozen storage
Dear Kurt
Question A. Typically regulations relating to frozen fish require that
fishery products be held below -18ºC, but this is not cold enough for
maintenance of good quality for more than 2-3 months. Fish products should
be held at/below -30ºC if good quality is to be maintained. Product storage
life, assuming good packaging to prevent oxidation and dehydration, is then
more than 12 months. For a seasonal fisheries such as that for mackerel when
products might want to be sold throughout the year, but caught over a short
period, it is necessary to store at -30, particularly so in the case of oily
fish such as mackerel. Common industrial practice in Britain, and from what
I can gather, elsewhere in Europe, is to hold frozen fish at,
nominally, -30ºC, though in actuality this is probably between -28
and -30ºC. Andrew Strak drew attention to the effect of lowering of
temperature on rate of deterioration, but the effect of lowering temperature
might be greater than Andrew quotes - a halving of the rate of deterioration
for each degree Celsius decrease in temperature. There is a not a great deal
of good quantitative information on comparable rates of deterioration of
fish in frozen storage at a range of temperatures in order to calculate the
value of B in the Arrhenius equation, but my own analysis of what data there
are suggests the halving rate is about 6ºC.
The cost of the extra refrigeration to maintain -30 rather than -18 is small
given good store design and operation. There are extra costs of insulation
for a -30 store compared with a -18 store, but these are negligible when
amortized over the life of the store. The extra costs are recouped by the
extra quality of product, if that can quantified. The publication 'Freezing
and refrigerated storage in fisheries', by Johnston, W.A., Nicholson, F.J.,
Roger, A. & Stroud, G.D., FAO Fisheries Technical Paper 340, FAO, Rome,
Italy, 1994, ISBN 92-5-103579-2, gives a breakdown of costs of refrigeration
and cold storage. The absolute costs will be out of date by now, but the
relativities won't have changed much. I can also recommend Kolbe, E. &
Kramer, D., 1993, 'Planning seafood cold storage', Marine Advisory Bulletin
no. 46, Alaska Sea Grant College Program, University of Fairbanks,
Fairbanks, Alaska 99775-5040, USA.
Question B. Though air blast, and plate freezers, operate with refrigerant
temperature of -40ºC, good practices require that the product to be frozen
such that at thermal equilibrium the temperature of the product should not
be above that of the cold store. Roughly, this means a core temperature
halfway between the freezer refrigeration temperature and the store
temperature. The refrigeration system of a cold store is designed to
maintain the design temperature and not as a freeze. Loading warm, compared
with the store temperature, product into a store puts a addition load on the
system which was not designed for.
Peter Howgate
----- Original Message -----
From: Kurt Jacobsen <kurt_jacobsen@hnfoods.com>
To: <seafood@ucdavis.edu>
Sent: Wednesday, August 08, 2001 6:49 PM
> Dear Members:
>
> When Is It Cold Enough ?
>
> I'm interested in learning more about freezing and cold storage
temperatures
> for oily types of fish such as mackerels.
>
> I understand the basic concepts behind freezing product rapidly as it
> relates to the ice crystal formation and cell structure of the meat, and
> that colder is better for both freezing and cold storage temperatures.
>
> My questions are:
>
> A) At what temperature as you go colder, (-30C, -40C, -50C, -60C....?), is
> there no longer a benefit to product quality, (texture, color, flavor, etc
> after thawing), or is there such a temperature point? (ie: is there a
> breakeven point or law of diminishing returns at which going any colder
> would not add any improvement to the quality of the product, and
therefore,
> investing more dollars in refrigeration equipment to go colder for initial
> freezing and/or cold storage would not be necessary).
>
> B) If you blast freeze these products at say -40C, but most cold storages
> maintain a temperature of -20C, what impact does the temperature
differences
> have on the quality of the product?
>
> Regards,
> Kurt Jacobsen
>
>
>
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