Dear Listers
I have been away from my desk for a little while and have returned to see an
active thread on amount of ice to cool fish. The original request from the
Belliveaus was for and 'easy way to determine how much ice should be added
to a known quantity fish to chill it to a determined temperature knowing how
much fish one has and what the temperature is before adding ice.'. The
answer has been given in some responses, but not completely accurately. Ken
Hildebrandt's response of 24 July is correct, and I hope he does not mind my
repeating it in metric units. The formula for the amount of ice to cool a
unit weight of fish at temperature TēC is: T divided by 80 times the
specific heat of the fish. The simplest calculation assumes the specific het
of fish is 1.0, but for accurate calculation you need to allow for the
specific heats of components of the composition of the fish. The specific
heat of the water phase is 1.0, that of the lipid fraction is 0.5, and of
the solids is 0.3. So, a lean fish containing 79% water, 1% lipid, and 20%
solids has a specific heat of 0.855. A fatty fish of 15% lipid, 65% water
and 20% solids has a specific heat of 0.785. Plug the required value in the
formula above.
The results show that somewhere between 5% and 30% of the mass of fish must
be added as ice to cool the fish even in tropical fisheries with fish at
30ēC . But, as many corespondents have pointed out, this not the whole story
and the ice required to cool the fish might be only a small proportion of
the total amount of ice to be used in the distribution chain. For a start
the calculation ignores the container. A water-saturated wooden fish box at
tropical ambient temperature will require a substantial amount ice to cool
it. The amount of ice needed to maintain the temperature of the fish during
the rest of the fishing trip and the return journey depends on the heat flow
into the fish hold and the journey time. It is possible to calculate the
heat flow into the hold, but it would not be a trivial calculation, and
anyway would depend on parameters that would vary with the circumstances of
the voyage, and during the voyage. The aim is to have ice remaining in the
box at the time of display and auction of the fish at the market, but not so
much that buyer believes he/she is getting short weight. It is good practice
not to stow boxes of fish hard against the sides and bulkheads of the hold.
A thick layer of ice should be laid on the sole plate of the hold, the boxes
laid on that leaving a space between the sides and the bulkheads and this
space filled will ice. Cover the stack of boxes with a layer of ice. The
amount of ice to use to allow for the heat leak, and how it is distributed,
is determined by experience. This blanket of ice will absorb heat leaks into
the hold. Under these conditions a ratio of one part ice to 3 parts fish in
the boxes should be sufficient in cold water fisheries to cool the fish and
box, and leave enough to show to the buyer that the fish have been well
chilled. Warm water fisheries may require a ratio of 1:2.
These comments apply to industrially developed fisheries, with well designed
vessels with well insulated fish holds. The conditions in fisheries in
developing countries, or at least less technologically developed fisheries,
are less than ideal for a variety of reasons. But that is another story.
Peter Howgate.
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