Let me deal with Andrew Strak's and Ted Labuza's messages together.
Firstly Andrew's. The original question concerned freezing and the
temperature of frozen storage, but, yes Andrew, the complete supply chain
for frozen fish has more stages than just the freezing and loading into a
cold store stages. When discussing quality losses in the supply chain it is
worth considering two scenarios. One is a in integrated supply chain were
the complete chain, at least up to display in retail outlets, is under the
control of a single company. This would be the case for the large food
companies making fish products under the own brand names. Examples in Europe
are Birdseye, Ross, Findus, Igloo. These companies have comprehensive
quality assurance of their products throughout the chain from catch to sale.
They are well aware of importance of low temperature during frozen storage
of their products and hold their frozen fish at -30ºC. The products they
make are not those I would often choose, but this is a matter of
presentation rather than quality. When I have tasted them - clearing up the
fish fingers from my grandchildren's plates - the fish portion of the
product is certainly of very good quality. The latter scenario is that which
I might call the disassociated supply chain in which segments are under the
control of different companies. A vessel catches fish and markets it at the
port. There it is bought, processed and frozen. The frozen product might be
passed on for further processing before sale to the consumer. This latter
the chain can have weaknesses of good quality assurance particularly with
regard to temperature of storage. The company that stores the product might
not have contacts with the final consumer and might take the cheapest, or
what it believes is the cheapest, option, that is, storage at - 18? C, and
processors and merchants further along the chain might not look to closely
at how the product had previously been stored providing the price was right.
Bear in mind that the storage life printed on the package is from the time
of packing. Good accountancy practice requires you don't pack the product
until just before despatch!
The part of Ted's message I'd like to discuss is that concerning economics -
it has a bearing on the points just made. There is no doubt that
refrigeration costs to hold product at -30?C are greater than holding
at -18?C under a given set of design parameters, and perhaps I should expand
a little or more on what I mean by "small" and "good store design and
operation".
I would certainly not advocate that a processor already operating a cold
store at -18?C should just turn down the thermostat to -30ºC. Without any
change in insulation, there would be a 25-30% increase in heat load which
would have to be taken up by the refrigeration system. The system might not
be able to get the store down to -30?C, but even if it could it should be
running flat-out which would not be good for the machinery. Even if
insulation were increased - not all that easy or cost effective as a retro
fit - the refrigeration system might still not be capable of
achieving -30?C, and would not be efficient in the energy sense even if it
could. No, I had in mind the situation in which a processor wishes to build
a cold store and is deciding what the design temperature should be. Then
both the building and the refrigeration system could be designed for what I
would recommend as an operating temperature of -30?C.
Yes, construction and operation of a cold store at -30?C will be more
expensive than one at -18?C, but how much more expensive, and is this
expense small? The extra heat gain from operating a low temperature can be
reduced by adding extra insulation, and Ted and I agree that the extra cost
of insulation when installed at the construction phase is insignificant over
the lifetime of the store. The capital cost of the refrigeration system for
the lower temperature will be greater though, again, I would expect extra
expense would be negligible over the lifetime of the store. Some operating
costs will depend on the operating temperature and it is worthwhile trying
to estimate how big a difference operating at -30 rather than -18 would have
on these. The FAO Fisheries Technical Paper 340 I referred to in my previous
message has a breakdown of be refrigeration load for a cold store operating
at -30. (Anyone wishing to look at these figures in detail, on pp 83-84,
should note there are a few editorial errors. The insulation thickness
should be 0.22m rather than 0.25m. The number following the divisions sign
in the line beginning "Heat leak" in section (1) should be 0.22. The number
3500 near the top of page 84 should be 35 000. The calculated values are
correct). The refrigeration load due to heat leak through the surfaces of
the store is temperature dependent and in the model calculation contributes
32% to the total load. This can be reduced by increasing the insulation and
the thickness of 0.22 m of polystyrene might nowadays be considered rather
low. Moving products into and out of the cold store inevitably means some
loss of cold air and replacement by warm air, and refrigeration load from
these air changes again will be temperature dependent. In the model. the air
changes contribute 19% of the total load. Though air changes cannot be
avoided altogether they can be reduced by good control of loading
operations, by installing curtains, and by using hatches rather than moving
products through the main door.
The model allows for product being above the operating temperature of the
store when stowed, in this case at a temperature of -20ºC. This is not good
practice and products should be much closer than that when transferred to
the store. Nothing is to be gained energywise by taking product out of the
freezer too soon. A particular amount of heat has to be extracted from the
product and this is done more efficiently in the freezer than in the cold
store.
Of course adopting good practices to minimise refrigeration load reduces
operation cost for storage at -18 as well as at -30ºC, but will be more
important for the lower temperature.
It is worth considering the costs of frozen storage in the total supply
chain. This has been done for the situation in Norway by Ola Magnussen,
currently a professor at the Institutt for klima- og kulderteknikk,
Trondheim, Norway. He looked at energy consumption for various supply chains
from catching to delivery at inland wholesale markets. In the scenario
involving production of frozen product and transportation in refrigerated
lorries for 600 km, the freezing stage accounted for 1.8% of the total
energy utilization of 14MJ/kg of product, and frozen storage at -30ºC for 1
month for 0.4%. Storage for 12 months would account for 4%. (The major
energy use was in the fishing stage. For a mix of inshore, near water, and
middle water trawling it accounted for 93% of energy utilization in the
scenario just discussed).
This long discussion is in support of my original contention that the extra
costs of storage of fish at -30ºC rather than at -18º are small, especially
relative to the total costs of frozen fish products. The effect on quality
though of storage at the lower temperature is large and the costs should be
borne to assure good quality products to the consumer. Andrew raised the
question of what is meant by quality. This is a topic for another
discussion, especially in relation to storage life, but I go along with one
of his alternatives; for me an aspect of good quality is for the product to
be close to the original quality. In the case of frozen fish this can be
achieved by storage at -30, but not by storage at -18ºC.
Bye, bye. Off on holiday this weekend for two weeks.
Peter Howgate
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