Due to its structure, this molecule absorbs light and allow only a mixture of wavelengths so that it
appears green. Like most pigments, chlorophyll is very sensitive to heat and extreme pH.
· Effect on cooking green vegetables :
Initially on cooking the green colour brightens. This is because the air is the spaces in the cells
escapes, and this allows the green pigment chlorophyll to be seen more clearly.
However, continuing to cook causes the vegetables to start to lose their green colour. Heat readily
causes the central magnesium ion to be pushed out, and other ions if present, will replace the
chlorophyll centre, which may cause a change in colour.
If beans are cooked in very acidic cooking water (for example vinegared water), the numerous
hydrogen ions present will replace the magnesium at the centre of the chlorophyll molecule, changing
its overall structure and causing it to change slightly which wavelengths of light it can reflect and which
it can absorb. Instead of absorbing all wavelengths except green, the pigment now reflects a mixture
of wavelengths, giving an undesired horrible brown colour. Additionally, the yellow and orange
pigments also present in green plant tissue are no longer masked by the chlorophyll. These new
pigments formed thus reduce the perceived “greenness” of the vegetables.
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So to preserve the desired green colour, green beans should never be cooked in acidic cooking water.
Equally if cooked vegetables are to be dressed with an acidic vinagrette (which would promote H+
displacement), they should be dressed just before serving. Green vegetables turn drab if they stand in
a tart dressing for more than a few minutes. Raw salad should be dressed at the very last minute as
well, but this is not only to prevent magnesium displacement. The oil in the vinagrette penetrates
cracks in the waxy cuticle that cover vegetable leaves much more than water, and prevents refraction
of the bright green light, making them appear to “loose their colour”.
However, green beans lose their appetising colour even when cooked in water to which acid has not