Summary Food Properties and Function. The summary contains information on all parts of the course. Also some background knowledge for the tutorial is included in the summary. The summary contains (parts of) lecture slides for better understanding of the course material.
A conjugated system is responsible for the colour of food products:
- Alternating single and double bonds
- Responsible for absorption of light
- At least -7 conjugated double bonds needed
A colorant absorbs certain wavelengths and reflects others. The reflected colour is observed. The shorter the
wavelength, the more energy light contains (blue > red)
Function of colour in food:
- Gastronomic function (appetizing, attractive)
- Indicator function (smell, ripeness, freshness, nutritional value, processing)
! Sometimes the indicator function is misleading you (e.g. green tomato variety)
Colour additive – single substance, chemically made, extracted from natural source, E-number
Colouring food – mixture of substances, edible raw material, might be concentrated, no E-number
Regulation (EC) No 1333/2008 on Food Additives: “‘Colours’ are substances which add or restore colour in a
food, and include natural constituents of foods and natural resources which are normally not consumed as
foods as such and normally not used as characteristic ingredients of food.”
Food additive ay be included if it serves one of the following purposes:
(a) Restoring the original appearance of food of which the colour has been affected by processing, storage,
packaging and distribution, whereby visual acceptability may have been impaired
(b) Making food more visually appealing
(c) Giving colour to food otherwise colourless
Guidance note on the classification of food concentrates with colouring properties: “As for food safety
aspects, generally, the use of colouring foods could be regarded as safe if the levels of exposure would not
exceed those resulting from normal use of the colouring foods source materials in the human diet.
Furthermore, the extraction process should not lead to the concentration of contaminants such as naturally
occurring toxicants, or to the generation of reaction products or residues of a nature and in amounts as to be
of toxicological concern (EC, 2013).”
Safety concerns of food colorants:
1. Lack of uniformity (the same food colour can be legal in one country but illegal in another)
2. Hyperactivity? (synthetic colours and their link to hyperactive behaviour in children)
3. E-number free trend (trend of using ‘colouring foods’ instead of colour additives: these substances are
not covered by the Regulation on food additives in the EU)
Azo-colorants – 6 synthetic colour additives which share a similar structural
element. In some studies, these azo-colorants have been linked to increase the
hyperactive behaviour in children.
Colour is defined by three attributes:
- Hue (shade, tone) – dominant wavelength
- Saturation (chroma) – intensity of dominant wavelength (‘amount’ of colour)
, - Lightness – average light intensity over the whole spectrum
! For the human eye it’s difficult to distinguish between saturation and lightness
Subjective colour measurement – visual (with the eye) comparison of two colours, by using colour cards,
coloured discs and glass colour filters
Objective colour measurement – instrumental measuring of colour and shininess, by using a
spectrophotometer, tristimulus colour meters and shininess meters
! In subjective colour measurement, the circumstances during the measurement have to be taken into
account: e.g. properties of the background, angle of vision, type of lightning and training of the observer.
Reflection curve – curve of the light that you see. It shows how the surface influences – changes – the light
coming from a light source:
White: all light reflected back of all wavelengths
Grey: none of the wavelengths reflected back more than
another
Black: all wavelengths absorbed
A usable system of colour specification will have to consider the 3 following aspects:
- The light source
- The properties of the food product
- The response of the eye to colour
Daylight: distribution of energy over wavelengths in the range of 380-780 nm
CIE Standard Light Sources (illuminants) – different light sources, all with their own emission spectrum
- A – Tungsten lamp with a colour (2854 K)
- B – Resembles direct sunlight (4780 K)
- C – Resembles afternoon daylight (6770 K)
- D – Theoretical light source, resembles daylight including UV-light (6500 K)
The measured reflection curve depends on the light source used. Correction for the light source gives the
spectral energy distribution of the reflected light:
Food products can be transparent, translucent (semi-transparent) and opaque.
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,Materials can have a different effect on light, which influence colour measurement:
1. Mirror reflection – light is reflected specular
2. Diffuse reflection – light is reflected into different directions (opaque)
3. Transmission – light travels through the material (transparent)
4. Diffuse transmission – light diffuses into several directions within the material (translucent)
5. Refraction – light hits the boundary surface of materials with various refractive indices, changing the
direction of the light
6. Absorption – light is taken up by the material and is lost, because energy is absorbed
In the human eye, the cones and rods are situated on the retina:
3 types of cones (SML-cones):
- Short wavelengths (blue)
Medium wavelengths (green/yellow)
- Long wavelengths (red)
Monochromatic colour – colour of only one wavelength
Any colour of light can be made by mixing RGB (red, green, blue) in different relative amounts. These
additive colour mixtures are always lighter than any of the individual components. If the equal amount of light
of the three primary colours is added, the sensation of white light is produced.
For some colours, ‘negative’ amounts are needed. This means that the colour to be measured is mixed with
one of the primary colours.
Colour specification systems:
1. CIE XYZ colour specification:
- Use imaginary colours XYZ instead of RGB
- Advantage: no negative amounts needed
2. CIE xyY colour specification:
- XYZ doesn’t take lightness into account
- Y is a function for lightness
3. CIE Lab colour specification:
- Differences in XYZ or xyY are not always different for human perception
- Lab differences are also different for the human eye
CIE XYZ colour specification system – RGB (real colours) are translated into xyz (imaginary colours). The
primary colours have been adjusted in such a way that negative amounts are avoided. These xyz (imaginary
colours) are subsequently recalculated into XYZ (tristimulus values).
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, Xλ Yλ Zλ are the values for one specific wavelength, so for monochromatic light. To get a colour specification system
for polychromatic light, there should be integration over the entire visible spectral area to get the 3 total stimuli on
the colour sensitivities of the eye.
Disadvantage XYZ: with the found trichromatic coefficients X, Y and Z, a visual impression of the colour can
still not be obtained
2. CIE xyY colour specification system – a more advanced system that takes into account the chromaticity of a
surface, after which the colour can be represented in a colour triangle or colour space and compared. XYZ
(tristimulus values) are translated into xyz (chromaticity coefficients). The chromaticity coefficients x and y
are good indication for hue and saturation. The tristimulus value Y is a measure of lightness.
The dominant wavelength in the CIE xyY colour space can be found by drawing a line starting in the middle of
the triangle, going through the colour to the end of the triangle. In this case, the dominant wavelength of the
colour C is around 510 nm.
The degree of saturation can in the CIE xyY colour space can be found by the formula below. The closer to the
white point W, the less saturated the colour.
Threshold ellipses – show areas where different colour differences are not distinguished by the eye. In the
violet region, small colour differences can easily be distinguished by the eye, while the eye is least sensitive for
changes in the green region of the diagram.
Disadvantage xyZ: the distance between 2 points in the CIE colour triangle is no measure for the difference
between 2 colours that is perceived by the eye. Geometrically equal colour distances are thus not judged
visually as equal in the CIE colour triangle.
3. CIE Lab colour specification system – colour spaces are constructed according to the theory of opposite
colours for seeing colours. With this the Lab values give a visual picture of the colour directly. The XYZ
(tristimulus values) are recalculated into L*a*b* values. L* is the lightness axis, a* is the red-green axis and b*
is the yellow-blue axis of a colour space.
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