3. • Color, flavor and texture are primary
attributes that determine food quality
• Color significantly impacts food choices
• Color impacts perception of taste
3
4. Importance and source of colour in foods
• Although considered of no nutritional importance,
colour is a prime factor in the appeal of food.
• Dietary significance now is being confirmed.
• Colours in foods are from:
1. pigments naturally present
2. natural constituents acquired during processing
3. deliberately added to enhance or replace lost
colour – additives, artificial (synthetic)
5. • Energy content of matter is quantized.
• Internal energy of molecules does not vary in a
continuous manner – does so in a series of
discrete steps.
• The molecule is normally in the lowest energy
state.
• Such a molecule can absorb energy if it is equal to
the energy difference between the discrete steps.
Molecular basis of colour
5
6. • Energy in the electromagnetic spectrum can be
regarded as a series of photons of set energies.
• The energy is inversely proportional to the wavelength
– so a photon of a particular wavelength represents a
particular energy.
• So for a particular wavelength of radiation to be
absorbed it must be equal to an allowed energy
transition (difference between discrete steps).
• For visible wavelengths these are electronic transitions
Molecular basis of colour
6
7. Molecular basis of colour
• Certain materials exhibit colour as they absorb
particular wavelengths of light from the visible
spectrum – 380-780nm (photons of energy).
• Different colours are due to the region of the visible
spectrum and to the degree of absorption.
• The colours seen are complementary to those
absorbed. The wavelengths (colour) absorbed is not
transmitted from the food, but the other wavelengths
(colours) are and so are seen.
7
8. Range of electromagnetic radiation
X-
rays
-
rays
UV
region
Visible
region
Infra-
red
Micro
waves
Radio
waves
10-3 10-1 10
400 800
105
103
107 109 nm
Increasing wavelength
Increasing frequency
Increasing energy
9. Relation of observed colour to absorbed
colour
nm absorbed
300-435
435-450
450-490
490-500
500-560
560-580
580-595
595-650
650-780
Colour absorbed
Violet
Blue
Greenish blue
Blueish green
Green
Yellowish green
Yellow
Orange
Red
Complementary hue
Yellowish green
Yellow
Orange
Red
Purple
Violet
Blue
Greenish blue
Blueish green
10. • Less energy is required for transitions with molecules
containing delocalised electrons.
• This means longer wavelengths (less energy) are
required for such molecules.
• Conjugated systems have delocalised electrons –
increase in conjugation leads to absorption of longer
.
• Presence of other groups in molecule can have an
effect on the of absorption.
• Metal complexes coloured.
Molecular basis of colour
10
11. Molecular basis of colour
• Ethane C C bonds * abs.<180nm
• Ethylene C C bonds * abs. 190nm
• Benzene cyclic * abs. 256nm
• Naphthalene * abs. 290nm
• Anthracene
* abs.360nm
11
12. • Inclusion of saturated groups (methoxy,
hydroxyl) may result in absorption of longer
wavelengths – a bathochromic shift by
auxochromic (electron donating) groups.
• Opposite effect with some groups –
hypsochromic shift.
• In visible wavelength shorter is blue end
and longer is red end of spectrum.
Molecular basis of colour
12
13. • Consider benzene and tryosine:
H2N CH C
CH2
OH
O
OH
H2N CH C
CH2
OH
O
H2N CH C
CH2
OH
O
HN
tyrosine phenylalanine tryptophan
Benzene absorbs at 256nm
275nm 280nm
265nm
13
