colours - lunds tekniska högskola · examples of synthetic colours. summary •our eyes identify...

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.