Post-mortem changes in pork using parallel needles for both light scattering and low-frequency electrical properties

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Parallel 16-gauge hypodermic needles 3 mm apart were used to measure the optical and electrical properties of 60 samples of sternocephalicus muscle from 15 pigs in the first few hours after slaughter (mean sampling time 150 ± 84 min post mortem). Altogether, 1992 sets of measurements were made automatically under computer control at 27.3 ± 4.4 °C. Samples were illuminated at 633 nm by light from a helium-neon laser passing through an optical fiber in one hypodermic needle. Scattered light was collected by an optical fiber in the other needle and measured with a photomultiplier. The myofibers themselves appeared to act as optical fibers, channelling light more readily along, rather than across their length. This effect was confirmed ( p ≤ 0.01) from 450 to 700 nm with white light from a xenon arc. Laser scattering decreased post mortem ( r = −0.18, p ≤ 0.001). The electrical properties of meat between the needles were measured at a low frequency (20 Hz), concurrently with optical measurements. Capacitance across the longitudinal axes of myofibers was lower than when measured with needles coaxial with the myofibers (4.23 ± 1.20 μF versus 4.44 ± 1.38 μF, respectively, p ≤ 0.01). The lowest ( p ≤ 0.001) capacitance was along the longitudinal axes of myofibers (4.04 ± 1.21 μF). Resistance across fibers did not differ significantly from coaxial resistance (1823 ± 253 Ω versus 1834 ± 302 Ω, respectively). The highest ( p ≤ 0.001) resistance was along the fibers (1962 ± 278 Ω). Capacitance increased ( r = 0.35, p ≤ 0.001) post mortem, while resistance first increased ( r = 0.19, p ≤ 0.005) before decreasing overall ( r = −0.30, p ≤ 0.001). This transient change in resistance was unlikely to have been caused by the effect of temperature on electrolyte conductance. In measurements made with needle electrodes coaxial with the longitudinal axes of myofibers, scattering was correlated with capacitance and resistance ( r = 0.31 and r = −0.14, respectively, p ≤ 0.001) with little effect from time of measurement. In this orientation, simultaneous measurement of light scattering and low-frequency electrical capacitance might enhance the early post-mortem prediction of pork quality.