Short answer
Cooking pushes chlorophyll to lose the magnesium atom at the centre of its ring. Hydrogen from the acids that cells release when heated takes its place, turning bright green chlorophyll into olive-brown pheophytin. How far this goes depends on acidity, time and temperature: short cooking in plenty of water, uncovered, keeps more green, while long simmering, a lid and acidic ingredients speed the change.
Chlorophyll is a large flat ring (a porphyrin) holding a single magnesium ion at its centre, with a long fatty tail (phytol) that anchors it in the chloroplast membrane. The magnesium is what tunes the ring's absorption to give the bright, slightly bluish green of raw beans or spinach. In acid, and faster when warm, two hydrogen ions displace the magnesium. The product, pheophytin, absorbs differently and looks olive to khaki-brown. Nothing is burnt and nothing is lost from the vegetable; one metal atom has been exchanged, and the colour shifts with it.
Plant cells keep organic acids in their vacuoles, away from the chloroplasts. Heat breaks those compartments, so as soon as a vegetable is cooked its own acids reach its chlorophyll. That is why the first minute of blanching often makes green vegetables look brighter — air between the cells is driven out and the surface looks more transparent — and a few minutes later they start to dull. A lid traps volatile acids in the steam and returns them to the pan, which is the chemical reason behind the cook's rule to boil greens uncovered. Adding lemon juice, vinegar or tomato before serving rather than during cooking keeps more of the green.
Pickled green olives, gherkins and long-braised greens are olive-coloured for the same reason: a long time in acid converts most of the chlorophyll to pheophytin.
In alkaline conditions the magnesium stays put, and the enzyme chlorophyllase can cut off the phytol tail to make chlorophyllide, which is still green because the coloured ring is unchanged. This is the chemistry behind the old habit of adding a pinch of bicarbonate to cooking water. It works for colour, but alkaline water also breaks down the pectin that holds cell walls together, so the vegetables soften quickly and some vitamins degrade faster. Most cooks now prefer speed instead: small pieces, a large volume of fast-boiling water or a quick steam, then plunging into cold water to stop the reaction.
If the magnesium is replaced not by hydrogen but by copper or zinc, the ring holds on to the new metal far more tightly and the colour becomes a vivid, stable green. Copper complexes of chlorophyll and chlorophyllin are used as a food colour (E141 in the EU). The same exchange can happen by accident: table olives processed in contact with copper salts can develop bluish-green patches, and olive producers and regulators test for copper chlorophyll derivatives because a green that looks too bright can indicate added colour where it is not permitted.
Autumn leaves and ripening fruit also lose green, but not by the cooking route. Living plants dismantle chlorophyll enzymatically, first removing the magnesium and phytol and then opening the ring into colourless breakdown products, which unmasks the yellow and orange carotenoids that were present all along. This is why a banana turns yellow as it ripens rather than olive: the plant is breaking the pigment down completely, not converting it into another coloured form.
Why: The lid returns volatile acids released by the beans, speeding pheophytin formation.
Fix: Cook uncovered in fast-boiling water for the shortest workable time, then cool in cold water.
Why: Lemon juice or vinegar in contact with warm vegetables converts chlorophyll quickly.
Fix: Dress just before serving, or cool the vegetables first.
Why: Bicarbonate in the water keeps chlorophyll green but breaks down pectin in cell walls.
Fix: Drop the alkali and shorten the cooking instead.
Each statement is labelled by kind — established fact, a standard’s requirement, observed market data, a convention, or Colourwise’s own interpretation or analysis — with the strength of the evidence behind it.
FactStrong evidence
Pheophytinisation is the replacement of chlorophyll's central magnesium ion by two hydrogen ions; it is favoured by acidic and mildly heated conditions and turns the bright green colour olive-brown.
Source: The Fate of Chlorophylls in Alkali-Treated Green Table Olives: A Review
FactStrong evidence
Chlorophyllase removes chlorophyll's phytol tail without changing the coloured ring, so the resulting chlorophyllides remain green.
Source: The Fate of Chlorophylls in Alkali-Treated Green Table Olives: A Review
FactStrong evidence
Copper can replace the hydrogen in pheophytin to form copper–chlorophyll complexes; such complexes are identical to the E141 colourant and are associated with bluish-green staining and colour adulteration in table olives.
Source: The Fate of Chlorophylls in Alkali-Treated Green Table Olives: A Review; Food colours (topic page)
FactModerate evidence
In senescing plant tissue chlorophyll is broken down enzymatically into colourless products rather than converted to pheophytin by acid.
Colourwise interpretationLimited evidence
Cooking greens briefly, uncovered and in plenty of water, then cooling them quickly, preserves more green than long covered cooking because it limits both the time and the acid concentration the chlorophyll is exposed to.
Based on: Follows from the acid- and heat-driven pheophytin reaction described in the cited review; not a measured comparison of cooking methods.
Caveat: Vegetables differ in acidity and structure; the size of the effect varies.
Reviewed 29 September 2026. Colourwise summarises its sources in its own words and does not reproduce standards text or proprietary colour data. Spotted an error? Tell us.