Absorbance of light by mitochondria in pork muscle fibres differing in myoglobin content

A scanning microscope photometer was used to map the transmittance (T) of white light along short lengths (⩽1 mm) of muscle fibres in transverse sections of pork psoas minor (n=8, Duroc×Yorkshire, hot carcass weight 86.1±5.2 kg). Red fibres (high myoglobin) were compared with white fibres (low myoglobin; n=16 pairs; 60.1±28.1 h post-mortem). Raster scans across cut ends of fibres were eroded in concentric zones to find radial gradients of T. All red fibres had positive radial gradients (mean T mm(-1)=5.389±3.680) from outer to inner zones, showing T was higher (P<0.01) in the axis relative to the periphery. White fibres had either positive or negative gradients (mean T mm(-1)=0.394±2.546). Gradients in red fibres were higher (P<0.001) than in white fibres. Radial gradients matched the visible distribution of mitochondria. It was concluded mitochondria dominate over myofibrils in giving red fibres lower T than white fibres, adding to other effects from myoglobin and pH. Whether or not mitochondrial effects are visible in pork displayed for retail sale remains to be determined.