Re: Sodium content of tuna

From: John C. Wekell (john.c.wekell@noaa.gov)
Date: Mon Aug 14 2006 - 10:28:17 PDT

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    Brine freezing, where brine containing almost saturated salt, uses solutions that are chilled down to 0°F. It is used to rapidly chill and freeze fish (primarily tunas). In some fisheries there is a widespread use of refrigerated seawater (RSW) systems (used in the salmon industry). About 20+ years ago, the Japanese were investigating the use of calcium chloride brines since they could be chilled down to -55°F, where freezing would be expected to be very rapid. Unfortunately, the rapid freezing had a tendency to split or crack the fish. Further, the uptake of calcium ion into the tissue yielded a very bitter tasting tissue.

    Over 20 years ago, in response to FDA concerns over the sodium content of foods and labeling requirements,  we investigated the salt uptake in various fish species, either through brine freezing, refrigerated seawater, and even calcium chloride brines (Wekell et al 1983. Food Tech. September 1983, pg51-58 &63). Basically, we found that sodium continued to be taken in by fish tissue even after freezing. The Tuna Foundation, I believe even before our studies, was somewhat aware of this phenomenon and suggest to tuna boat owners that after freezing was complete to pull off the brine and hold the tuna in cold storage. Nevertheless, some tuna boat captains thought that once the fish were frozen solid, no further uptake of sodium would occur. Unfortunately, leaving batches of fish, taken on widely separated days during a trip, if left in the brine or RSW systems, led to extreme variations in sodium content, literally from fish to fish. This in turn, led to significant labeling issues. See our paper for a discussion of this.

    A salt laden fish tissue would be expected to "increase" in apparent sodium concentration (based on finished, cooked product) due to the cook-out of water. Of course, sodium chloride is not volatile and stays with the cooked meat. Another possible factor for an "apparent" (i.e., by sensory evaluation) increase in saltiness in cooked (fried or otherwise) might be the production of certain flavor enhancers, particularly for salt, e.g., sodium glutamate, 5'-ribonucleotides such as IMP and GMP (see Wekell and Barnett, "Seafood Flavors and Quality" in the Wiley Encyclopedia of Food Science and Technology, Wiley & Sons, 1999, pp2071-2095).

    Oleks Tokarskyy wrote: 

    Hello,
My guess is because cooked tuna in oil looses weight while frying due to
moisture loss, but retains salt.
You can evaluate it by cooking yield.
second, salt might be intentionally added to improve flavor - again, cooking
in brine, and what is brine?

might be wrong :-)

thanks,
Oleksandr Tokarskyy
graduate student
Mississippi State University

----- Original Message ----- 
From: "Richard Chivers" <richard@fishonline.co.uk>
To: "seafood" <seafood@ucdavis.edu>
Sent: Monday, August 14, 2006 7:06 AM
Subject: Sodium content of tuna


  
    I have just received a query regarding the sodium content of tuna.

According to McCance and Widdowson tuna has the following sodium content:

Raw tuna 40-50 milligrams/100g

Cooked tuna in oil 290 milligrams/100g

Cooked tuna in brine 320 milligrams/100g

My contact wants to know why the sodium content increases so much through
cooking.

Thanks in anticipation

Richard Chivers
Seafood Audit International

    --
    John C

    John C. Wekell, Ph.D.

    Research Chemist

    NOAA-Fisheries

    Environmental Conservation Division

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    Seattle, WA 98112

     

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