Colour of milk and milk products in CIE L*a*b* space

Since the colour of objects is a feature originating in human mind, the perception and interpretation of colour remains subjective. An illuminated object reflects light energy, either partially or completely. A reflected electromagnetic wave reaching an observer is captured by the eye, and, after passing the signal to the brain, an impression of colour is generated. An analogous phenomenon is observed during sunbathing. People cannot see solar radiation, but can feel it warming them up – thus an impression of warmth is created in their minds (also subjectively). Since our bodies differ, we experience impressions in different ways. The reception of colours depends on the eye structure, educational background, natural environment and mental aptitude (27, 42). A mood or illness can also affect colours created in the brain. Due to disturbances in the eye or brain structure, a certain percentage of society is unable to generate the whole range of colours. Thus, as an instrument of colour evaluation, man is not free from drawbacks. An objective colour description is indispensable, particularly wherever slight differences in colour are significant, as in textile, paint and foodstuff industries or in dentistry (22). Nowadays, spectrophotometers are most frequently used for a quick evaluation of product colours. They convert energy reflected from an object into numbers defining the colour. The space for the expression of product colour is nowadays usually the CIE LAB colour space. In this space, the colour is expressed by means of three coordinates (constituents): L*, a*, b*. For better visualisation of the results, constituent L* can be presented as an independent vertical line with the following values: 0 – black, medium values – greys, and 100 – white. Values a* and b* can be presented as a point in the ab coordinate system, that is, on a colourful plane where colours fluently pass from red (+a*) through yellow (+b*) to green (–a*) and blue (–b*), just like the colours of the white light spectrum after passing through a prism. By connecting a point with coordinates a*b* with the centre of the coordinate system, a segment line is obtained, whose length is defined as chrome (c). The higher the value of c, the more intense (stronger and clearer) the colour of the object, and vice versa: the lower the c (closer to the centre of the system), the paler grey the colour is. The angle created by the axis +a* and the line segment c is called the hue (i.e. colour), and denoted by the letter h. The parameters obtained while measuring colour depend on spectrophotometer settings: the source of light (31), the angle of light incidence and observation (measurement geometry), the observation angle of the observer (2° or 10°), the diameter of the measurement area, and, for thin layers of samples, the parameters depend on the surface colour (3, 45). The Colour of milk and milk products in CIE L*a*b* space