Greening of Steaks
Michael Gray
dropped off as i changed focus in my career, I have a quick question
to ask regarding the subject line
I have noticed occasionally that fresh cut steaks aged minimum 21 days
prior to cutting, that when they are cyrovaced for shipping, tend to
develop a "greenish" tinge. Now once the vac pac is opened and the
steak is exposed to air, the green tint goes away and the steak
appears as normal.
can anyone explain why this happens, what is the cause of it? somebody
said oxidation..but ..well I don't know..
cheers
Michael
carlos hermida
evolved in a cyber chat between no more than 5 gurus considering themselves
the best in their class to argue in front of an international audience who
admires how smart they are. Questions like yours probably never will be
answered because it is not under the enlightment view of the new
establishment.
I will try to help you out of the forum.
Carlos Hermida DVM
Norlyn Tipton
The issue has to do with lack of Oxygen and the steaks being slightly
oxidized and/or it can be an issue of poor lighting. Make sure the
product is being viewed under adequate lighting and the light is not
from yellowed light bulbs as they tend to cast a brownish green haze
to beef (especially beef packaged in clear films). If the lighting is
adequate, the color issue of brown vacuum packaged steaks "blooming"
back to red (upon debagging) has to do with a lack of Oxygen.
Myoglobin is the main color related protein in beef. It is similar to
Hemoglobin in that it has an Iron center that attaches Oxygen (among
other elements/molecules) and causes a color change. In low Oxygen
environments and an un-oxidized state, Myoglobin is in the Oxygen free
(Deoxymyoglobin) form and is purplish red (frozen raw vacuum packaged
beef). In an Oxygen rich environment and un-oxidized state Myoglobin
is in the Oxygen bound (Oxymyoglobin) form and is "Cherry Red" (This
is the color of normal retail beef in a PVC over-wrapped Styrofoam
tray). In an Oxygen rich environment and oxidized state Myoglobin is
in the Oxygen bound (Metmyoglobin) form and is brownish and sometimes
has a greenish tinge to it. Beef can readily switch between these
three states depending on Oxygen levels and oxidative state, but it
will ultimately end up as Metmyoglobin as oxidation increases. This
oxidation process in meat is severely reduced by freezing and reduced
significantly by temperatures below 36F.
When aged steaks are processed they end up in a state of being
partially oxidized and as Oxygen is removed in a vacuum package the
steaks will sometimes turn brown. It just depend on how oxidized the
steaks are. Sometimes they will be a dark "Cherry Red" in the vacuum
package which indicates a lower oxidation state. In other beef (e.g.
stew meats) there will sometimes be a mottling effect (part dark
"Cherry Red" and part brownish) due to different oxidation states of
different pieces of the beef. This can even occur on one piece of
beef. It all depends on the amount of oxidation and the presence of
Oxygen.
The brown color itself is not an indication of the product being
spoiled. Brownish colored steaks will "bloom" back to a "Cherry Red"
color when there is enough un-oxidized Myoglobin to bind Oxygen. The
amount of time, at a constant temperature, a steak remains "Cherry
Red" also indicates the original oxidized state. Brown steak color is
just one of many indications of the freshness state of the steaks, and
it is not a definitive keep or toss issue. It is just an indication
that the end of the shelf life is approaching.
As with all perishable meat products, higher non cooking temperatures
accelerate spoilage and oxidation. At some point of longer age
periods or higher temperatures vacuum packaged steaks will stay brown
even when exposed to Oxygen. This is the point where the steaks are
reaching the end of their shelf life. At this state, it is best to
use sight and smell to determine usability. If a steak has a putrid
smell or is beginning to smell putrid and slime is forming on the
steak it is best to discard it. The general micro load at this point
is about 1 million CFU/cm2. That is not a hard and fast number,
though and steaks with higher or lower numbers can be spoiled
depending on sight and smell characteristics.
Norlyn Tipton, Ph.D.