Jslkk@aol.com
Sun, 23 Jan 2000 12:11:19 EST
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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