Dinesh
You need to differentiate between cooling fish in ice slurry and storing
fish in ice slurry.
Ice slurry, because it is in complete contact with the surface of the fish,
is a better heat transfer medium than is flake ice and could be the
preferred procedure for cooling large, warm fish such as tuna. However flake
ice also makes good contact with the surface of the fish plus the melt water
from the ice trickles over the fish making good contact with the fish. Also
there comes a point in the cooling when surface heat transfer is not the
limiting factor for heat loss compared with heat diffusion within the fish.
Altogether there is not likely to be a big difference in rates of cooling
between the 2 systems, but you would have to carry out comparative
experiments to determine the size of the difference in your circumstances.
Ice slurries use more ice to cool a given mass of fish than using flake ice
because the water in the slurry has to be cooled down as well as the fish so
is more costly than using flake. It is important when cooling fish in ice
slurries that the mixture is stirred or the water phase recirculated. When
the fish is added to the slurry some of the ice is melted and the container
has layer of water and fish with the ice floating on top of it. If the fish
has not cooled down completely this layer of fish and water will equilibrate
at a temperature above 0degC and the ice floating on top will not cool it
down. This does not happen with fish stowed in ice; assuming there is an
adequate amount of ice all of the fish will cool down to ice temperature,
(actually a little below it, around -0.2degC, to be pedantic). Chilled
Seawater (CSW) systems - a slurry of ice with seawater- are often used on
pelagic trawlers to cool large catches of pelagic fish, but these systems
recirculate the water to ensure mixing. CSW is not the same as the
refrigerated brine storage used on larger tuna boats.
Though ice slurry is effective in rapidly cooling fish when used properly it
is not a good medium or procedure for storing fish. Fish in ice slurry takes
up more space, and weighs more than the same amount of fish in ice. These
are considerations for the space required to store the fish and for
transporting it. Suspension in water alters the appearance of the fish. This
might not be a consideration for tuna destined for canning but is for fish
going to retail sale. Depending how long the fish is stored in the ice
slurry the product can lose flavour due to leaching into the water, and the
fish can absorb water. This might be favourable for the economics of selling
the product, but not for eating quality.
The Codex advice, and other texts on storage of fish, that fish should iced
in containers which allow for drainage is good advice and should be
followed. Fish in ice slurries are typically held in containers with a
drainage hole; just taking the bung out of the hole will allow for drainage.
Peter Howgate
----- Original Message -----
From: "Dinesh Dhammika uduwana" <dduduwana@hotmail.com>
To: <seafood@ucdavis.edu>
Sent: Saturday, June 16, 2007 12:07 PM
Subject: Chilled Storage of Fresh Fish (tuna)
> Dear All in Seafood List,
>
> Looking forward for the expert opinion on what is the best way of storage
> of fresh fish either only ice (flake Ice) or in a ice slurry composites
> of flake ice and water.
>
> code of hygienic practices for the fresh fish (Codex) says the water from
> melting ice should be drained to prevents bacteriological and chemical
> contamination!
>
> so if we stored in a ice slurry how can we facilitate drainage in stored
> in ice slurry!
>
> expecting expert comments soon.
>
> thanking you
>
> Dinesh Dhammika
> Ensis Fisheries Factory
>
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