Muscle iridescence in yellowfin tuna (Thunnus albacares)

Microscope spectrophotometry was used to investigate strong iridescence in yellowfin tuna steaks. Iridescence was restricted to the axes of transversely sectioned myofibers. Sometimes groups of adjacent myofibers all had the same iridescent colors, but colors sometimes differed between adjacent myofibers; thus, myofibers were optically isolated. Iridescent colors were not changed by rotating a polarizer in the illumination pathway, nor by rotating a polarizer in the measuring pathway. But when both pathways contained polarizers, and the polarizers were crossed, both specular reflectance from the meat surface and iridescence from within myofibers were completely extinguished. The reflectance spectra of iridescent colors all showed multiple interference peaks, with a strong dependency on angles of illumination and measurement Thus, iridescence in tuna muscle exhibited the same optical properties as iridescence previously reported in beef: the most likely cause was multilayer interference from A-bands at different depths. If low-order reflective interference from A-bands can cause iridescence, then high-order interference might be a source of pH-related light scattering along myofibers in food myosystems. (C) 2012 Elsevier Ltd. All rights reserved.