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That “shiny rainbow” is called iridescence, your second question is easiest and the answer is, it has nothing to do with the quality of a meat product. You may be surprised to learn this also happens in specific fish cuts.This question has been asked and answered previously, see below, unfortunately it's all links so I will lay it out.Iridescent Color of Roast BeefUnfortunately the iridescence in meat is often greenish, and being green leads us to believe it's “bad” this is not the case at all.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.A typical photo of what you are referring to.When light hits a slice of meat, it splits into colors like a rainbow. This is something called a "diffraction grating," essentially what happens when light waves bend or spread around a surface and create a pattern. It's the same thing that happens to make rainbows on the surface of a DVD.Different cuts of meat such as ham, beef and even fish are not the only meats known to have rainbows. However, when cooked beef is sharply sliced against the grain of the muscle fiber, this, coupled with the moisture in the beef, creates an excellent surface for producing rainbows."In my opinion," Dr. Thomas Powell, Executive Director of the American Meat Science Association, told me, "The reason it shows up in roast beef is because the cuts of meat that are used in most roast beef are more prone to iridescence, particularly in the round," hence the reason why the USDA singles out roast beef as being especially colorful.[1]Now for the super detailed scientific answer combining chemistry, physics and meat science.Iridescence is an optical phenomenon in which the light is diffracted from a photonic structure at a specific wavelength or a rainbow-like colour. The iridescent colours are not caused by pigments, but by the interference of light with the morphology of the structure. These photonic structures comprise of periodically ordered materials (or air gaps) with varying refractive index. The interference of light with the structure and its scatter causes a photonic effect in which only a dominant wavelength or multiple wavelengths are scattered back. Photonic structures including surface gratings, multilayers and thin films have been observed in nature on the surface tissues or cuticle of a variety of species including birds, insects (think butterfly wings), marine molluscs, and plants.On to the meat end and understanding musculatureMuscular tissues are formed by elongated cells in a regular arrangement showing a well-ordered structure. When viewed at specific angles, such photonic structures may produce iridescence. The orientation and spacing of the ordered structures are different from muscle to muscle and type of species; the iridescence, if present, is also likely to be different. For instance, different types of muscle tissues could produce different colours and intensities. It has been found that fish tissue slices produce colours that vary depending on the species and environmental conditions. Muscle fibres in mammals are supported by structures formed by myofilament proteins grouped in fibrils that form cell fibres; when cross-sectioned at a certain angle, the diffracted light may produce iridescence depending on the structure. The arrangement of the cells in the whole muscle may have a different order and/or orientation depending on the muscle tissue type, however, it is usually homogenous. In fact, certain cuts of pork and beef exhibit iridescence, while others do not. In the case of beef, iridescence is most frequently found in semitendinosus muscle; it has a homogenous size distribution of cells and cylindrical fibrils that form a regular grid in transverse sections. Several studies have investigated the association of iridescence with physiological factors such as pH, phosphate concentration, maturity, cooking temperature, fat thickness, surface roughness, shear force, animal type and sex type. Along with these factors, iridescence in muscle tissue can be directly associated with water content. Since water is the main composition of the cell, changes in water content can cause evident changes in structure or optical properties. Thus, the water content can be related to other factors like pH or ionic strength. Therefore, characterisation of the structure and optical properties of the materials is crucial for understanding iridescence in natural structures. Previous studies have shown that iridescence in muscle tissue appears at a wavelength range seen as green, orange or red colours. In recent investigations of cooked beef iliocostalis and yellow fin tuna (Thunnus albacares), multilayer interference has been suggested as an explanation for the diffracted colours with strong angular dependence. It is shown that the intensity of iridescence is dependent on the water content of the tissue and the diffraction angle.The gratings present on the surface of the muscle tissue display colour diffraction at different angles of view (or illumination), suggesting that the structure is a well-ordered surface grating rather than a multilayer structure. Tilting the viewing angle caused a change in colour and intensity observed due to both the diffraction grating and the background colour, which also causes a back scattering.Figure 1. Muscle tissue of a pork loin sample displays iridescence when viewed at a specific angle. (a) The loin muscles sliced transversely to the long axes of muscle fibres. (b) Section planes of muscle tissue prepared for observing fibres from different angles. (c) A schematic of the periodicity of muscle fibres and fibrils. (d) The light interference with a diffraction grating comprising of a constant refractive index and periodicity. (e) A pyramidal cut of the muscle tissue showing the angular dependence of iridescence colours, and the prevalence of the iridescence along the sample. (f) Microscopic images of the surface of the muscle tissue illuminated at the angle θi to produce the maximum diffraction.ConclusionsIridescence is a structure dependent light phenomenon that is influenced by the optical properties of the muscle tissues, such as refractive index, that are directly related to the physiological factors such as water content. The surface of the muscle tissue consists of quasi regularly oriented and spaced bundles of fibres, which comprise of well-ordered myofibrils. When the fibres are sliced, fibrils protrude from the surface of the muscle tissue, which creates a periodic array which acts like a diffraction grating. Upon illumination, light waves scattered by the well-ordered protrusions and microfibrils constructively interfere and diffract light at specific wavelengths. We quantified the diffraction from pork loin muscle tissues with real-time spectrophotometry during drying. Drying the surface of an iridescent muscle tissue sample, thus, removing water from the constituent fibres caused the intensity of the diffraction to vanish while the diffracted wavelength remained stable. This phenomenon is a result of a change in refractive index and/or surface structure due to the decrease in water content. In contrary to the analogous evidence provided for the muscle tissues of beef iliocostalis, and yellow fin tuna (Thunnus albacares), our data indicates the presence of a blazed surface grating in pork (Sus scrofa domesticus). 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