Pigments and Flavour Compounds

Plant pigments and colours

Plant pigments are classified by chemical structure, and each class has a characteristic colour range. Knowing the class lets you predict the colour of a raw or processed product, so the pairing of pigment and source is a common examination target.

• Carotenoids — isoprenoid polyenes that give yellow to orange-red colours. Carotenes such as \beta-carotene are hydrocarbons, whereas xanthophylls contain oxygen. Carotenoids are responsible for the yellow-orange colour of peppers (Capsicum) and of many fruits and roots.
• Betalains — nitrogen-containing pigments that give red to violet colours. They occur mainly in the order Caryophyllales, for example in Cactaceae such as cactus pear and in Amaranthaceae such as beetroot. Betanin is the characteristic red-violet betalain. Betalains and anthocyanins do not normally occur together in the same plant.
• Lycopene — an acyclic red carotenoid of tomato and watermelon. It is not the pigment of beetroot, whose red colour comes from betalains.
• Flavonoids — anthocyanins give red, purple or blue colours depending on pH, while flavonols are colourless to pale yellow and do not produce an orange colour.

Provitamin A activity depends on molecular structure. Vitamin A is formed when a carotenoid is cleaved centrally at the 15,15′ double bond to give retinal, and this requires at least one unsubstituted \beta-ionone ring. Thus \beta-carotene, with two such rings, yields two retinal molecules, while \beta-cryptoxanthin, with one ring, is converted to retinol. \alpha-Carotene, with one \beta-ring and one \varepsilon-ring, has lower activity. Lycopene is acyclic and has no ionone ring, so it cannot form retinal and has no provitamin A activity.

Food flavour compounds

Food flavour is produced by a small number of potent odour-active molecules, and each food has its own characteristic compound. For examination purposes, it is more important to link each food with its signature compound than to memorise general families. Formation pathways also differ: Maillard and caramelization reactions give roasted, caramel and cocoa notes, lipid oxidation and fermentation give buttery and fatty notes, and enzymatic release from precursors gives pungent sulfur notes.

• Diacetyl — a vicinal diketone that gives the buttery aroma of butter and some fermented dairy products.
• Eugenol — a phenylpropanoid that is the principal aromatic constituent of clove oil and gives the clove flavour.
• Limonene — a monoterpene that is the main constituent of citrus peel oils and gives the orange flavour.
• Citral — a mixture of geranial and neral that gives the characteristic lemon aroma.
• Diallyl disulfide — the pungent sulfur compound of garlic, formed when alliinase acts on alliin and the resulting allicin decomposes.
• 5-Methyl-2-phenyl-2-hexenal — an aroma-active unsaturated aldehyde formed during roasting of fermented cocoa beans, and a key odorant of chocolate.

Do not confuse these with other well-known odorants. Cyclotene is associated with maple and fenugreek, furaneol with strawberry, and maltol with caramel and bread-like notes. None of these defines the flavour of chocolate.

Caramelization flavour compounds

Caramelization is the non-enzymatic browning of sugars under heat. It proceeds without the requirement for an amino compound, which distinguishes it from the Maillard reaction. Heating causes dehydration, fragmentation and polymerisation of sugars, producing furanones, pyranones and acetyl-substituted hydroxyfurans that carry the characteristic caramel aroma.

• Maltol — 3-hydroxy-2-methylpyran-4-one, a pyranone formed in caramelization with a sweet, caramel-like odour.
• Furanones — for example 4-hydroxy-5-methylfuran-3(2H)-one, generated from sugar degradation and a known caramel-type flavour compound.
• Hydroxyfurans — for example 3-hydroxy-2-acetylfuran, formed during the heating of sugars and associated with caramel-type aroma.