Near-infrared birefringence and transmittance of pork in relation to pH, sarcomere length, cold-shortening, and causes of paleness

Abstract

Pork neck muscles, longitudinally restrained during rigor development, were compared with contralateral muscles that were excised and cold-shortened. Restraint had no effect on pH or Japanese pork color scores (JPCS), but restrained muscles had longer (2.52 ± 0.63 μm) sarcomere length (SL) than unrestrained muscles (1.83 ± 0.67 μm, P < 0.005). Transmittance ( T) measurements made on 1 mm slices of pork with unpolarized visible light were strongly affected by pH ( R = 1). But by using near-infrared (NIR) light at 800 nm to reduce scattering, birefringence related both to pH and SL was detected. With some orientations of muscle fibers relative to polarized light, T increased as SL increased from 1.2 to 1.5 μm, peaked at length 1.5 μm, then decreased as length increased to 3.5 μm. Thus, birefringence was proportional to the overlap of thick and thin filaments, until alignment was disrupted by severe shortening. Situations were found where T of polarized light was correlated with SL, R = 0.98 ( P < 0.01). These results support the hypothesis that refraction of light passing through myofibrils may be involved in pH-related paleness in meat.