Comparison of D65/10° and A/10° illuminant/observer systems for colour measurement of raw pork

In pork colour measurements, the value of each parameter depends on the type of illuminant used. The spectra of D65 and A illuminants show great differences, with illuminant A having greater emission in the red part of the visible spectrum. Therefore, its application in the colour measurements of the pork longissimus muscle should result in larger changes in redness (Δa*) and hue angle (Δh°) and higher correlation between Δa* and Δh° and pH48 and WHC. The purpose of this study was to compare the suitability of the illuminant/observer systems D65/10° and A/10° for colour measurements of pork longissimus muscle, using the CIELAB and CIELCh systems.

The study involved 168 samples of longissimus lumborum muscle taken from 168 carcasses (mean weight 90.2 ±6.0 kg) of pigs slaughtered on an industrial processing line. The moisture content, crude protein, intramuscular fat content, WHC, and pH48 were determined. Colour measurements using CIELAB and CIELCh scales were carried out with both D65/10° and A/10° illuminant/observer systems and reflectance measurements. The chromatic absorbance value at 525 nm (A525p) and the relative amounts of MbO2, MetMb, and Mb were calculated according to methods proposed by Krzywicki (1979) and AMSA (2012). Meat samples were illuminated and differences in the values of colour parameters (ΔL*, Δa*, Δb*, ΔC*, Δh°), chromatic absorbance at 525 nm (ΔA525p), and in the relative amounts of chemical forms of myoglobin (ΔMbO2, ΔMetMb, ΔMb) were determined. In addition, hue difference (ΔH) and total difference (ΔE) were calculated.

The values of correlation coefficients between moisture content (especially crude protein and intramuscular fat, and colour parameters) were low and often statistically insignificant. Higher and mostly significant values of correlation coefficients were found between colour parameters and WHC and pH48. The A/10° system resulted in higher values of correlation coefficients than D65/10° between (I) WHC, pH48 and (II) h°, Δa* and Δh°. At the same time, in the A/10° system the combined effect of the relative amounts of myoglobin chemical forms on the variation of h° values and the combined effect of changes (Δ) in their amounts on the variation of Δh° and ΔH and ΔE were greater than in D65/10°, with the greatest effect of these changes (Δ) in the amount of MetMb.

Replacement of the illuminant/observer D65/10° system with the A/10° system in colour measurements of raw pork longissimus muscle changed the proportion of pigments and the relative number of chemical forms of myoglobin in influencing the values of colour parameters, primarily h°. Therefore, using the A/10° system for colour measurements allows us to better capture the differences (Δ) in redness Δa* and especially in hue angle (Δh°), as well as hue difference (ΔH) and total difference (ΔE), with an increase in the relative amount of MetMb becoming the main determinant of these differences (Δ). At the same time, measurements using the A/10° system increased the correlation coefficients between WHC and pH48 and changes in redness (Δa*) and hue angle (Δh°). Therefore, the A/10° system compared to the D65/10° system may be more useful for measuring the colour stability of raw pork, especially the determination of Δh° and ΔH and ΔE. >.