Colours
Björn BergenståhlSeptember 2008
Colour and wavelength
Green650RedBlue600OrangeViolet550YellowRed500GreenOrange450BlueYellow400Violet
Complementarycolour
Wavelength(nm)
Colour
Colour circle
Colour
• Cone (tappar) and rod (stavar) cells• Three types of cone cells.• 445 (blue), 535 (green-yellow) and
570(yellow-orange) nm• The rods has their peek sensitivity around
505 nm
The three cone cells
Colour circle and the cone cells
X
Y
Z
CIE Diagram
• x=1+Y+Z• y=Y/X+Y+Z
Chemical origin of colour
• Energy levels in the electron shells• Pair of electrons• Opposite or parallel spin• Parallel spin represents an excited state
Chemical origin of colour
• Firm electron shells (σ)-high energiesneeded for excited state.
• Softer electron shells (π)- lower energiesneeded.
• Aromatic rings -UV adsorption• Delocalised double bonds- visible
Conjugated double bonds- carotenoids.
The plant cell
• The plant cell• The chemistry
differs between theplasma and theliquid of thevacuoles.
Chlorophyll
• The electrons ofthe double bondsare fullydelocalised.
Absorption spectra of chloropylls• Chlorophyll is
green• Pheophytin
(chlorophyllwithout Mg) isbrown.
• Pheophytin isformed as aresult ofovercooking
Carotenoids
• Type of terpenoid Yellow to red colours β-ring act as pro-vitamin A
Caroten and lycopene
• Lycopene is the red colour of tomatoes.
Carotenoids
• Carotenoids areantioxidants
• β-ionone may beformed as a result ofperoxide reaction
• Strong smell of hay
Anthocyanins and Flavonoids
• The flavannucleous.
• The flavonoid orflavanoid structure
• Kept in thevacuole of the cell
Anthocyanin structures
• The flavylium cation
Anthocyanines
• pH dependent• Antioxidative capacity
Betalaines
• Beetroot (Betavulgaris)
• Less pH dependent• Fairly stable
Flavonoids
Polymerisation and oxidation products give reddishto brownish products
• Theaflavin• Reddishcolour of tea.
Oxidation of flavonoids
• During processing of tea.• During damage of cell tissue.• Role of phenolase.• Formation of melanins• Important antifungal process.• Heat treatment inhibits the phenolase
activity.
Examples of synthetic colours.
Summary
• Our eyes identify the colours through threecones.
• The colours are caused by “soft” delocalisedelectrons.
• A few important groups of natural colours.
Summary
• Chlorophyll• Carotenoides• Anthocyanins• Betalaines• Flavonoids-melanins
Summary
• A few synthetic colours• Some clasical cooking components-caramel
or sugar brown• Some more chemical• Restrictive policy.
The white colour
• Depend on particle size• Particle size relative to the wavelength of
light.
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