15 Meat Colour

15.1 Introduction

Meat colour is extremely important when meat is presented for sale.  If it does not look right, nobody will buy it.  The subject divides naturally into two topics.

1. Light scattering.  This determines whether meat is pale or dark.

2. Pigmentation.  This determines whether the meat is purple (when first slaughtered), red (after exposure to air) or brown (after exposure to air for too long).

We will also consider very briefly,

3. Colourimetry.  Methods for measuring meat colour used industrially.

15.2 Light scattering and pH

15.3 Translucency of meat

Light entering meat is scattered by its complex microstructure. Scattering may occur byreflection and refraction, and may involve elastic scattering (like Rayleigh scattering in a blue sky). Meat has a complex microstructure, with curved membranes in various stages of disruption, myofilaments in various orientations and degrees of overlapping, and fluid compartments changing in refractive index.Scattering tends to randomize the original directionality of the light. Light escaping from meat becomes visible to the observer, while the remainder is  lost deep into the meat.

As shown above, high scattering causes a short path length with minimal absorbance while low scattering allows a long path length with maximum absorbance by chromophores.  A chromophore (colour-bearer) is a coloured pigment. 

15.4 Redness

Strong scattering shortens the light path through the meat, which reduces selective absorbance, and the observer sees a minimal myoglobin effect.Conversely, weak scattering allows a long light path and myoglobin becomes fully visible.Metmyoglobin (brown oxidized form of myoglobin) formation starts in subsurface layers of the meat, and whether or not the observer detects its brownness depends on the degree of scattering and the depth of light penetration.

15.5 Sources of scattering

Extreme PSE pork contains denatured sarcoplasmic proteins deposited on myofibrilsDenatured proteins increase scattering and contribute to the paleness, but this cannot be the whole story, because dark-cutting beef scatters much less light than normal beef, yet normal beef does not contain massive precipitates of sarcoplasmic proteins. Protein precipitation is important, but only in explaining extreme paleness, not the ordinary post-mortem development of meat paleness. 

Scattering from precipitated sarcoplasmic proteins, reflectance from the myofibrillar surface, and refraction through myofibrils all contribute to meat paleness, as shown above.

The diagram aboveshows how both scattering from precipitated sarcoplasmic proteins and reflectance from myofibrillar surfaces immediately scatters light back to the meat surface to be perceived as paleness. But it may not be immediately obvious how refraction does the same thing. Light not reflected from the myofibrillar surface must enter the myofibril. If the myofibril has a higher refractive index than the sarcoplasm, the incident ray will be refracted towards the normal, and vice versa when leaving the myofibril. Thus, having traversed a few myofibrils with a high refractive index, the light may be scattered back to the meat surface to be perceived as paleness, as shown in the diagram belowWhen sarcoplasm and myofibrils have similar refractive indices, the incident illumination penetrates deeply into the meat, which then appears dark and strongly pigmented.

15.6 Post mortem changes in scattering

Changes in scattering after slaughter - sometimes a little decrease at first, but always a large increase later (except if the meat is dark-cutting or DDD when it does not increase).

15.7 Derivatives of myoglobin

 Myoglobin derivatives differ in their absorbance and reflectance spectra and the ratio of measurements at two different wavelengths may be used to calculate the relative amounts of myoglobin derivatives. An isobestic point occurs when two or more spectra intersect to give the same value at the same wavelength. An isobestic point for myoglobin, oxymyoglobin and metmyoglobin is at a wavelength of 525 nm. An absorbance peak for metmyoglobin is at 630 nm. Thus, the ratio of measurements at 630 nm (mostly metmyoglobin) to measurements at 525 nm (sum of all three myoglobins) contains information on the amount of metmyoglobin as a fraction of the total myoglobins.

Reflectance spectra of myoglobin (1),  metmyoglobin (2) and oxymyoglobin (3).

 The strong absorbance bands that occur at low visible wavelengths are called Soret absorbance bands. The Soret absorbance bands for myoglobin, oxymyoglobin and metmyoglobin are at 434, 416 and 410 nm, respectively. Myoglobin has an absorbance band at 555 nm that is replaced in oxymyoglobin by a strong absorbance band at 578 nm and a slightly weaker band at 542 nm. In most practical situations, however, the formation of oxymyoglobin in fresh meat is accompanied by a trace of metmyoglobin formation so that these two absorbance bands (at 578 and 542 nm) are approximately equal. 

15.8 Colourimetry

There are three words with special meanings in relation to the human perception of colour.

  1. The general type of colour, such as red, blue or green, is called a hue. If, for the sake of explanation, red paint was mixed in increasing quantities into a pot of dull white paint, the paint would change through pale pink to dark red, yet the hue (red) is unchanged.

  2. The property of colour which changes in this example of paint mixing is the intensity of the colour, and this is called the chroma.

  3. If the paint mixing example had started with bright white paint instead of dull white, the final product would be brighter, and would have a greater luminosity.

There are several different systems for the measurement of colour in these terms, and the choice of which system to use depends largely on what equipment is readily available. In the method recommended by the International Commission on Illumination (CIE), the primary hues, red, green and blue, are added or subtracted from each other to match any colour. By a mathematical manipulation, it is possible to specify both hue and chroma in the CIE system by means of a single pair of chromaticity coordinates called x and y.

Changes in hue follow the contour shown from 380 to 700 nm in the diagram above,  while changes in chroma radiate from the central position of white. Luminosity is specified by a third coordinate relative to the plane of the chromaticity coordinates. To illustrate this dimension the diagram above of the  chromaticity coordinates  is shown pinned to a drawing board, and a pencil is held perpendicular to the board.

Luminosity is measured at a point along the pencil, and this dimension is called percent Y. In meat with a lot of marbling fat, it might be expected measurements made by reflectance spectrophotometry would be biassed by light reflected from marbling fat. However, this source of error appears to be quite small in actual practice.

15.9 Meat colour problems

Further information

Swatland, H.J. 2004. Progress in understanding the paleness of meat with a low pH. South African Journal of Animal Science, 34: 1-7.