Andrew Strak (abstrak@accesswave.ca)
Sun, 23 Jan 2000 18:23:37 -0400
Though, the Mb in the centre of the loin would need some time while exposed
to oxygen to turn into the Oxy form. But -20C is not a very low storage
temperature to preserve the colour of myoglobin in tuna, I presume. I have
heard that something close to -50C is required.
Andrew Strak
----- Original Message -----
From: <Jslkk@aol.com>
To: <abstrak@accesswave.ca>
Cc: <seafood@ucdavis.edu>
Sent: Sunday, January 23, 2000 1:11 PM
Subject: Re: Freezing of tuna loins
> Myoglobin (Mb) is the muscle pigment which changes to give the tissue its
> various colors. It has an iron atom surrounded by a complex of pyrrole
> rings and the Iron atom is capable of losing and gaining electrons and
thus
> causing changes in the color of the tissue. Mb is the storehourse for
oxygen
> in a living muscle.
>
> In muscle right after the fish is harvested, the color would be deep
purplish
> red. As oxygen comes in contact with Mb in the exposed meat surface it is
> absorbed and turns into bright red pigment called oxymyoglobin (OMb). Mb
and
> OMb have the capacity to lose an electron - called oxidation, and turning
to
> a brown pigment called metMb.
>
> OMb is associated with the desirable color for fresh tuna and maintaining
> this color requires that the meat surface be free from any contamination
> which would cause an oxidation reaction resulting in the formation of the
> brown pigment, MetMb. Also in oxygen in the air must be available to
combine
> the Mb forming OMb. The pigments Mb, OMb, Met Mb can all be changed from
one
> to the other under appropriate conditions of storage. However, once the
meat
> is denatured through cooking for example, a brown pigment called denatured
> MetMb is formed which is difficult to reverse.
>
> Meat in the center of the loin is cut off from oxygen but not the
outside
> portion. You say that the meat on the outside returns to its normal
bright
> red color. The other possibility is the inner tissue could have been
> denatured irreversibly. IN either case the result is browning. My
suspicion
> is that the freezing rate is too rapid. Your could try increasing the
rate
> to 4 hours. Studies show that rapid rates freezing does have detrimental
> effects as far as muscle tissue is concerned.
>
> Hope the above explanation helps you in some way in figuring out asolution
to
> your problem.
>
> Laxman Kanduri
>
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