Beef Rainbows

I've often noticed this phenomenon. Occasionally wondered what caused it, and sometimes suspected it must be due to toxic chemicals.

image source: imgur

Turns out, it's totally normal and nothing to worry about. The common name for it is 'beef rainbows,' but the technical term is birefringence. The Texas A&M meat science page offers an explanation:

It is caused by the reflectance of light off of muscle proteins, and it is analogous to the color distribution produced by a prism. Muscle proteins are arranged in strands called myofilaments, which are bound together to form myofibrils. Myofibrils are bound together to form muscle fibers, which form together to form muscle bundles and finally whole muscles. When the myofilaments are cut at the appropriate angle, exposing a cross section of the myofilaments, the reflectance of light off the proteins produces the characteristic appearance associated with iridescence.

The USDA also reassures consumers that it doesn't mean that meat is spoiled:

Iridescent Color of Roast Beef
Sliced cooked beef or lunch meat can have an iridescent color. Meat contains iron, fat, and many other compounds. When light hits a slice of meat, it splits into colors like a rainbow. There are also various pigments in meat compounds which can give it an iridescent or greenish cast when exposed to heat and processing. Iridescent beef isn't spoiled necessarily. Spoiled cooked beef would probably also be slimy or sticky and have an off-odor.

     Posted By: Alex - Fri May 29, 2020
     Category: Food

Thanks, Alex. I always suspected spoliage.
Also, I have heard "bicycle reflector beef."
USDA could take a lesson in clarity from the Aggies. If you are thinking toxic chemicals, how does the line "Meat contains iron, fat, and many other compounds." help you out? Same with the "various pigments in meat compounds".

BTW, a bug man told me you can determine the type of termite by whether the discarded wings show this iridescence or not.
Posted by Virtual in Carnate on 05/29/20 at 01:50 PM
AKA The Arby's Effect
Posted by Stephen Taylor on 05/29/20 at 06:13 PM
Not spoiled necessarily (good quibble from the USDA there), but not fresh, either. I've noticed that when I have a packet of sliced sandwich beef, the older it gets the more I see this effect. The last slices are still fine to eat, but also noticably older than the sandwich from the week before which didn't have the shimmer.
Posted by Richard Bos on 05/30/20 at 03:50 AM
I think the same, Richard, because I don't remember ever seeing this iridescence when the beef is fresh from the store, and for a few days after. Maybe it needs a very slight layer of slime to start showing, a layer we can't feel with our fingers. I always thought once it gets noticeably slimy, it's spoiled. USDA says it would have the odor as well, but they hedge that one with "probably".
Posted by Virtual in Carnate on 05/30/20 at 08:12 PM
It happens immediately after cutting and then again as the beef begins to dry out. Between those times, moisture/juices on the beef scatters/bends the light. Fat content is also a factor because cutting smears it across the surface (rub butter on your glasses and check how clearly you see).
Posted by Phideaux on 05/30/20 at 09:56 PM
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