Certainly Peter. I would like also to add that there are several commercial
chilling media based on ice while mixing it with fresh/salt water at various
ratios to suit specific handling needs. And there are also various types of
ice concerning its physical format. The latent heat of ice may be the same
but its practical performance can be substantially different. While exposed
to ambient temperatures, crushed and flake ice may melt at different rates
that sometimes can lead some people to believe that one is 'colder' than the
other. But in reality their surfaces and therefore heat exchange areas are
so different that under conditions of unlimited heat supplies the flake ice
may melt much faster and therefore may be less desirable than other forms of
ice.
Moreover, although the colded the better rule is a pretty universal one with
respect to handling seafoods but in reality the optimum handling methods for
different species do not have to be the same. For example, some hake
species prone to rapid postmortem proteolytical changes require rapid
chilling rates and minimum pressure exerted on their bodies and those
conditions may be addressed best with refrigerated sea water systems
allowing for subzero temperatures, rapid heat exchange and product buoyancy.
In other cases much slower chilling rates are acceptable but the need to
protect the original colour of the fish may preclude to use any water medium
at all.
Besides, ice may be also used to separate one fish from another and in that
way ensure more aerobic storage conditions. On several occasions I have
noticed bilge odour developments in tight pack containers with very little
ice separating the fish although the storage temperatures were right.
Andrew Strak
----- Original Message -----
From: "Howgate" <phowgate@rsc.co.uk>
To: <seafood@ucdavis.edu>
Sent: Monday, July 24, 2000 5:53 PM
Subject: Re: Amount of Ice to Chill Fish - back to the basics!
> 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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