Dear List
The part of Jeb Wyman's message of 18 March relating to freezing of salmon raises several points. At first I dismissed it as being cranky, but on further consideration there might just be something in the vacuum bit. The rest is very dubious.
First the 'claims of extended cold storage shelf life using the unusual process below'. Extended compared to what process? Who claims it?
I would not expect the initial handling - bleeding, holding for 24 hours in chilled water without gutting - to have any effect on storage life as a frozen product. I agree with another correspondent that gutting immediately after capture would be preferable to holding ungutted because of the possibility of belly-burn, but affected tissue could be trimmed later.
The intriguing process is submitting the fish to vacuum for 1 minute. I can't imagine where the 'significant amount of fluid' is coming from. How much is significant, and what is the nature of the fluid? There could be some oil in this fluid, but there will be still a lot left in the tissues, more than enough to potentially oxidise to give rancid flavours. However, the process should exhaust dissolved air in the tissue. The question then is: will this loss of air (oxygen) materially reduce the rate of lipid oxidation compared to salmon that has not been evacuated, but will subsequent operations allow diffusion of air that will negate any possible effect? Yeeeeees, it is possible that there would be a lower oxygen tension in the vacuumed fish even after the subsequent operations.
I certainly do not go along with the freezing in a 'soaker freezer'. There is certainly a loss in weight of product during blast freezing , but this is small, in the order of 1-3% I think, but I can't lay my hands on the figures. This by no means 'dessicates the product'. The dessication I have seen on frozen fishery products has all occurred in the still air of a cold store. I think somebody has been taken in by a sales pitch from the makers of the soaker freezer. It takes somewhere in the region of 4-5 hours to freeze whole salmon in a blast freezer; it would be at least double that, probably 3 or 4 times, to freeze one in still air. Not good practice.
I do not see the purpose of freezing the salmon, holding for 1 or 2 months in the freezer, (what temperature?), then cutting into steaks and packing. Why not cut into steaks, pack and freeze immediately after the vacuum process? Or, freeze the fish, bandsaw into steaks, then pack and freeze again. The latter process, bandsawing frozen fish into steaks, or frozen fillets into portions, is a quite common process in making overwrapped consumer products, sold frozen or as thawed 'wet' products.
The double glazing and double wrapping with vacuum packing is definitely belt and braces, twice over. Vacuum packing definitely does retard oxidation, and is good practice for small items like steaks and fillets or fillet portions. Bulk oily fish such as mackerel will keep very well in cold storage at -30ºC as glazed blocks for at least a year. I can vouch for that from personal experience. I would look askance at claims for a frozen product still be 'palatable' after 3 years of storage unless the storage temperature was below -30ºC. Who would want to store for 3 years anyway? The storage costs would probably exceed the value of the product.
Peter Howgate
----- Original Message -----
From: Jeb Wyman
To: seafood@ucdavis.edu
Sent: Monday, March 18, 2002 7:53 PM
Subject: extending salmon shelf life
Dear list:
I would be interested in knowing whether there is any validity to the claims of extended cold storage shelf life using the unusual process below and, if so, whether anyone can explain why it might work.
Troll-caught coho salmon are landed, bled, and put in 32-34 degree chill tank for about 24 hours. After removing head, guts, and tail, the fish are subjected to 30 inches of vacuum for about 1 minute. Although the fish may appear spotless, this process nonetheless extracts a significant amount of fluid from the fish. The fish are then carefully laid in a -50 degree "soaker freezer" (i.e., a freezer hold with -50 ambient temperature but without forced air circulation like a blast freezer, which allegely dessicates the product). They are glazed several times. After one or two months, the frozen carcasses are cut with a bandsaw into steaks, re-glazed twice, and double-wrapped in vacuum packaging. This process is supposed to greatly reduce oxidation. Apparently product as much as 36 month old is still palatable.
I am interested in learning whether vacuum extraction of fluids could remove lipids associated with rancification. Any science to support or de-bunk this process?
On a slightly different topic, I would like to know how food quality may or may not be compromised by directly cooking frozen product without a slacking-out period. Thawing food prior to cooking seems to be the norm. If there were no concerns that the food would cook unevenly or inadequately (i.e., a sanitation issue), does cooking frozen product otherwise damage the tissue and affect texture or taste?
Many thanks for any help you can lend me.
Jeb Wyman
Associate editor
Pacific Fishing/FIS
4209 21st Ave W
Seattle, WA 98199
206/216-0111 ext 215
FAX 206/216-0222
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