LEC15

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

Meat with a low pH is generally more pale than at a high pH.

Paleness related to pH is caused by light scattering.

Myofibrils are a primary cause of pH-related light scattering in meat, but light scattering is also related inversely to sarcomere length.

We do not yet know the relative importance of surface reflectance from myofibrils versus refraction through the depth of myofibrils.

Precipitation of sarcoplasmic proteins is added to myofibrillar scattering when pH is extremely low, or when pH reaches low levels while meat is still hot.

Scattering tends to decrease the length of the light path through meat. This reduces selective absorbance by myoglobin. Thus, the colour of meat is more conspicuous when pH is high.

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

The dominant pigment of meat is myoglobin.

It is dissolved uniformly through the sarcoplasm of myofibres, particularly those having strong aerobic metabolism in the live animal.

After slaughter, myoglobin may move into the intercellular space and may be lost in fluid dripping from the meat surface.

Myoglobin strongly absorbs green light.

Thus, light escaping from meat is dominated by yellow, orange and red, depending on the concentration of myoglobin.
In living muscle, myoglobin without much oxygen has a purple colour. When myoglobin is oxygenated (carrying oxygen but NOT oxidized), it is bright red. Most consumers prefer their meat to be bright red.
After long exposure to oxygen, myoglobin becomes oxidized (a chemical change) and is brown. Gourmet cooks prefer meat with a brown colour because it is likely to have a better taste than bright red meat.

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 myofibrils. Denatured 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.

Scattering originates at boundaries between the sarcoplasm and the myofibrils.
Although the sarcoplasm of living muscle may have a higher refractive index than the A band, and the A band may have a higher RI than the I band, the myofilament lattice shrinks when pH decreases.

Thus, the myofibrillar refractive index increases as pH decreases, eventually exceeding that of the sarcoplasm, and this increases light scattering.

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 below. When 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

Sometimes we can detect an early post mortem decrease in scattering, perhaps reaching a minimum when sarcoplasm and myofibrils have the same refractive index.

But this transient effect might also originate at the boundary between the intercellular fluid and the plasma membrane of the myofibre, because increased osmotic pressure from glycolysis within the myofibre may cause a transient uptake of intercellular fluid.
This uncertainty results from the difficulty of making optical measurements on the muscles of meat animals as they are being slaughtered, dressed and refrigerated. But there is no doubt scattering eventually increases after slaughter, unless a carcass is to remain as a dark-cutter because of minimal glycogen, reduced glycolysis and a high ultimate pH .

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

The familiar colours of meat are the purple colour of myoglobin, the bright red colour of oxymyoglobin, and the brown colour of metmyoglobin.<>
<>Dietary supplements of vitamin E retard the formation of metmyoglobin.
Green pigments caused by the formation of sulphmyoglobin are rarely seen in the normal course of meat handling. Sulphmyoglobin may be formed if bacteria are able to produce hydrogen sulphide which then acts on 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.

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.

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

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

Meat processed with sodium nitrite develops a heat-stable pink colouration from dinitrosylhaemochrome. This is the attractive pink colour of ham.

However, formation of this or similar pigments when they are not wanted can create problems. Meat retaining a pink colour after extensive cooking causes consumers to suspect the meat has not been properly cooked. Nitric oxide and carbon monoxide from oven gases may be involved in forming heat?stable pink derivatives of myoglobin. Similar problems may be associated with high nitrite levels in the feed, treatment with sodium erythorbate, irradiation and freezing.

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.