|MadSci Network: Chemistry|
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.
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