You guys need to remember your food engineering
This is an unsteady state heat transfer so the rate of cooling is a function
of the thermal diffusivity divided by the thickness (half thickness for a
slab)
The change in temperature will follow ln Gamma vs time as:
Ln [(T - Tice)/(Tinitial - Tice) = - (a x^2) t
T is temp at time t
Tinitial is shrimp temp at start
Tiice is ice water temperature
a = thermal diffusivity of shrimp in m^2/sec
X is half thickness of shrimp (assume it is an infinite slab) in meters
Shrimp and meat have about the same thermal diffusivity I don't have my
books but it is around 2 x 10^-7 m^2/sec if I remember
So cooling a 1 cm thick shrimp from 20 C to 1 C in water at 0 C is
Ln (1/20)= -3 = (- 2 x 10^-7/(0.005)^2 t = 0.008 t
t = 3/0.02 = 375 sec =6.25 min
I will try to find the correct alpha value.
Not this assumes the is an excess of ice/ice water so it dos not change
temperature. The slab solution also neglects end effect which will shorten
the time.
For the warm tuna (how warm) using the same thermal properties and with a
big one say 16 cm thick ( so x = 0.08 meters) then for the same ln gamma ,
ie time for centerline to reach 1 C then
T = 27 hours much longer
So the rate of heat transfer differs based on thickness squared, ie double
the thickness and it take 4 times longer.
-- Dr. Ted Labuza Morse Alumni Distinguished Teaching Professor of Food Science and Engineering Department of Food Science and Nutrition Univ. of Minnesota Rm. 136 A 1354 Eckles Ave. St Paul 55108 email - tplabuza@umn.edu ext Msg 6513072985@mobile.mycingular.com Cell 651-307-2985 Office ( 612-624-9701 UM 7 fax 612-625-5272 NFNC 7 Fax 661-483-3302 web http://www.ardilla.umn.edu/Ted_Labuza> From: Chingling Tanco <crt@mida-group.com> > Date: Wed, 20 Jun 2007 04:00:30 +0800 > To: P Howgate <phowgate@clara.co.uk>, Dinesh Dhammika uduwana > <dduduwana@hotmail.com>, <seafood@ucdavis.edu> > Conversation: Chilled Storage of Fresh Fish (tuna) > Subject: Re: Chilled Storage of Fresh Fish (tuna) > > Hi Peter, > > What about if you were cooling shrimp - say 15-20 grams per piece HO or > 10-20 grams headless. Because of the size of the shrimp, heat diffusion > within the animal will not be much of an issue so does this make ice slurry > more effective for cooling shrimp? What about storing shrimp overnight? > > Chingling Tanco > Mida Trade Manila > > > On 6/19/07 10:30 PM, "P Howgate" <phowgate@clara.co.uk> wrote: > >> 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 >>> >>> _________________________________________________________________ >>> Don't just search. Find. Check out the new MSN Search! >>> http://search.msn.click-url.com/go/onm00200636ave/direct/01/ >>> >>> >>> >>> >> >> >
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