A very good note, Peter. Please remember, however, that there is a substantial difference in heat conductivity between water and water ice. If I remember correctly, the heat conductivity of water is appro. 0.6W/mK and that of the ice 2.5W/mK. That's the reason that although 'calorimetrically' the thawing process may be considered as a reverse one of freezing but the speed difference between them is substantial (much slower process for thawing) and that poses some technical challenges. One of them is that the surface of the fish may be 'cooking' while its center is still frozen if your initial water temperature is high and equilibrium temperature for enthalpy calculation set low, in particular for fairly thick fish such as the Pacific Cod. You are correct stating that the frozen blocks tend to float and fuse together early in the process. Therefore to maintain their separation while at the same time agitating the thaw water is critical for the process uniformity and consistency. Personally, I prefer a lower
initial thaw water temperature with also lower equilibrium temperature for the process but that require a higher water to fish ratio and therefore more space. On the other hand it is easier to control and therefore safer for the product.
With respect to the heat damage to the proteins it relates to some autolytical changes taking place in the flesh at elevated temperatures and that pre-caution is more applicable to cold water species than warm water ones.
Andrew Strak
-----Original Message-----
From: Peter Howgate [mailto:phowgate@rsc.co.uk]
Sent: Saturday, March 01, 2003 3:45 AM
To: Gregg Morrow; seafood@ucdavis.edu
Subject: Re: Thawing Techniques
Greg
500 lbs - are these US or imperial lbs? Anyway, it reads like 5 x 50kg blocks typical of sea-frozen fish frozen in vertical plate freezers, a total of, 250kg. The enthalpy change of frozen fish between -20 and +15ºC is 81kcals/kg giving a total heat load of 20 000kcals. This is provided by 610 l of water at 48ºC cooling to 15ºC so thermodynamically the system is feasible in a 1 000l tote. I have a little difficulty with the practice. The blocks, while frozen, will float, but tend to clump preventing good circulation around individual blocks. Are the blocks held in some sort of rack or frame? There will be thermal circulation of water within the tote, though slow and requiring the 5 hours contact time quoted.. However, I don't think this is much longer than is achieved in a circulating water thawer. Perhaps the contents of the tote are stirred occasionally, especially at the start of the thawing.
I don't think this method will be harmful to the flesh. The Torry Advisory Note on thawing, ( www.oneFish.org and use search for TAN25), recommends a maximum water temperature of 18ºC in circulating water thawers. This will be based on experience and experimentation, but I do not know what the criteria were; the recommendation might be a compromise between speed of thawing and microbiological growth in the circulating water. I am not aware of any irreversible protein damage caused by a temperatures above 18ºC referred to by Andrew; some species of fish live well above this temperature. Water thawing affects the appearance of the skin of the fish - washing out colour - but this will not matter if the fish is to be processed as skinless fillets. Water thawing is not recommended for IQF fillets as the fillets become waterlogged. I haven't checked, but I have a feeling that the skin collagen of fish habituated to cold water melts at temperatures in the 20's so there is a possibility of softening of the skin of t
he fish at the surface of blocks immersed in 48ºC water unless the blocks are moved around when first immersed until the water temperature drops to below, perhaps, 30ºC.
The process described might not be the optimum - define optimum! - way of thawing blocks of fish, but is clearly practical if the factory is using it. I don't think there is any significance in the tote's being insulated; probably just what the factory happens to have. Any effects on quality must be within the parameters of the Quality Assurance programme of the company.
Peter Howgate
----- Original Message -----
From: Gregg <mailto:gregg@keyportfoods.com> Morrow
To: seafood@ucdavis.edu
Sent: Friday, February 28, 2003 11:18 PM
Subject: Thawing Techniques
Hello,
I am curious if anyone can give me their opinion or experience with speed thawing Pacific Cod.
I have just returned from a plant in Europe that was taking Block Frozen, H&G Pacific Cod and thawing it by water.
They put in approximately 500 lb of fish into an insulated tote and fill it with 48°C water.
The fish was left to temper for 5 hours and was ready to be processed by machine into fillets.
I am curious if this method is harmful to the flesh.
Would this be considered an optimal way to thaw fish?
What would the negatives be?
Thank you,
Gregg
Gregg Morrow
Keyport Foods, LLC
4024 22nd Ave. West
Seattle, WA 98199
Direct: 206-267-1598
Ph: 206-284-1947
Fax: 206-284-3055
Mobile: 206-660-2007
Home Fax : 206-524-3998
Email: gregg@keyportfoods.com
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