Err… yeah! What I said above.
Why does deli-cut meat have an iridescent sheen?
On the outside? That seems to change as the meat grows older?
Anyone know?
Ooh, Ooh, I know this one!*
The phenomenon is known as birefringence.
(*check out the first link in my sig),
Oops, actually it may not be birefringence, but your question is explored in depth in the linked thread.
Great question. It made me realise how inattentive we are. It’s one of those things that hadn’t even crossed my mind to think about thinking about. If you see what I mean.
I noticed that more the meat was cured; i.e. the saltier it was, the more the sheen there was. My WAG was that the dissolved salt was refracting light. They’re regular crystals, aren’t they
?
I’ve never noticed any sheen on the organic deli-meats I buy, or the roasts I make myself, and cut into deli-style slices. My guess, it’s a byproduct of processing, or a deliberately added chemical for added shelf-life. Either way, if it’s not there if you make it yourself, you probably don’t want or need it.
I am so happy to see the return of birefringence. It is a beautiful word that I learned from SDMB.
There seem to be two main theories out there (not counting the evil additive theory). The first is that it’s oxidized oils; the second is that it’s a interference pattern resulting from light diffracting off of parallel muscle fibers (like a CD). This cite on pre-roughening reducing iridescence, originally found by Squink (http://www.foodscipress.com/JMFV13.1.htm#Surface%20Roughening%20Of%20Precooked ) indicates to me that the reason we see this in deli meat is probably because of the uniformity of the surface caused by the rotary slicer. The slicer probably creates an exceptionally flat surface, and may also burnish the surface as well, causing good conditions for optical phenomena.
Forgive me this one indulgence:
I always thought the sheen was from where the jockey hit it.
I’m weak and couldn’t resist.
No; this is incorrect - it isn’t a chemical sheen, it is more like the iridescence you see on tropical fish and peacock feathers - I’m pretty sure it is caused by some kind of refractive effect when light enters the semi-translucent (it is particularly noticeable on bacon) muscle fibres - I’ve only ever observed the effect in meat where the meat is sliced at 90[sup]o[/sup] across the muscle fibres, so you’re viewing them end-on.
I would hazard a guess that the effect is more prevalent in deli-cut meat because the cutting equipment is sharper and the cut involves less tearing or other disruption of the structure.
Ok, we indulged you, now indulge me…
I don’t get it 
Presumably alluding to the possibility of the meat being of equine origin, but sold as something else?
Eleusis, He’s saying it’s horse meat. Which we know isn’t true. Horse costs too much 