Re: Why does Deli Roast Beef have that rainbow-like sheen on it?

The iridescence ("rainbow-like sheen") of the roast beef and ham you have 
noticed is due to the regular muscle fiber structure of the meat combined 
with water droplets to create a "diffraction grating". The reflection of 
light off the water in the regular fiber grating causes separation of 
colors much like happens with a prism.

In the meat industry, it is known that the effect is enhanced when 
phosphates are used to increase the amount of water held in the meat. A 
dry meat surface scatters rather than diffracts or reflects light.

Equivalent poultry products typically are made from white muscle as 
opposed to the red muscle of beef and pork products. White muscle tissue 
has a higher natural pH which changes the conformation of the proteins in 
the muscle fibers, so a much less noticeable rainbow effect occurs. Also, 
these products are typically lower in salt, which affects the swelling of 
the muscle fibers.

You might check "turkey ham" which is made from dark meat and is more 
similar to pork ham in manufacture: You might find the iridescence there.

An interesting experiment would be to examine each type of product under a 
microscope where the cause of the effect should be much more obvious.