The rate of a chemical reaction increases exponentially with the absolute
temperature (Svante Arrhenius). The mathematical form of the empirical
Arrhenius rate law given above is Rate = A exp(-B/T). The constants A and B
in the Arrhenius rate law are empirical and differ from one reaction to
another (please note that many biochemical reactions pertinent to changes
taking place in the flesh are enzymatic in nature). However, the empirical
values of B are found to be similar for many chemical reactions. One result
of this similarity is the useful generalization that for many reactions
which occur near room temperature, a temperature increase of 10oC
approximately doubles the rate of the reaction. Apart from the temperature
factor, in frozen fish we are also dealing with various unfreezeable water
levels, different for different temperatures, and lots of enzymatic
reactions take place in this particular phase and at much higher
concentrations.
A general rule is the colder the better. From my personal experience, there
are no practical benefits from going much lower than -30C for Gadid species
(Cod and similar). But here we face issues of protein crosslinkage and
textural changes very often related to formaldehyde formation. However, Tuna
for sushimi may require even -50C in order to effectively prevent myoglobin
oxidation. Strangely enough, fish oils oxidation appears sometimes promoted
by lower storage temperatures within the range -10C-15C.
Fast freezing to low temperatures results in small ice crystal formation and
limited cellular damage. But with the increasing temperature of frozen
storage the process of recrystalization should set-up therefore for a longer
term storage at higher temperatures any cryo freezing may be of a
questionable benefit.
Andrew Strak
-----Original Message-----
From: Kurt Jacobsen [mailto:kurt_jacobsen@hnfoods.com]
Sent: Wednesday, August 08, 2001 10:49 AM
To: seafood@ucdavis.edu
Subject:
Dear Members:
When Is It Cold Enough ?
I'm interested in learning more about freezing and cold storage temperatures
for oily types of fish such as mackerels.
I understand the basic concepts behind freezing product rapidly as it
relates to the ice crystal formation and cell structure of the meat, and
that colder is better for both freezing and cold storage temperatures.
My questions are:
A) At what temperature as you go colder, (-30C, -40C, -50C, -60C....?), is
there no longer a benefit to product quality, (texture, color, flavor, etc
after thawing), or is there such a temperature point? (ie: is there a
breakeven point or law of diminishing returns at which going any colder
would not add any improvement to the quality of the product, and therefore,
investing more dollars in refrigeration equipment to go colder for initial
freezing and/or cold storage would not be necessary).
B) If you blast freeze these products at say -40C, but most cold storages
maintain a temperature of -20C, what impact does the temperature differences
have on the quality of the product?
Regards,
Kurt Jacobsen
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