Green vegetables and chlorophyll

Why

The green colour of vegetables comes from chlorophyll, which has a magnesium atom at its centre; in acidic conditions hydrogen pushes it out and the bright green pigment turns olive brown. The vegetable releases acid itself as heat damages its cells, so the longer it cooks, the more olive it gets; lemon juice or vinegar added to the water has the same effect. In tests a lid made no difference to the colour; what matters most is cooking time. Baking soda makes the water alkaline and keeps the colour, but it quickly softens vegetables to mush and destroys some vitamins, mainly vitamin C and thiamin, so it isn't worth it. Blanching means cooking briefly in plenty of boiling water until the vegetables turn bright green and are tender but firm, then plunging them straight into ice water to stop the cooking.

How to do it

• Cut the vegetables into evenly sized pieces and drop them into a large pot of rapidly boiling water.
• Cook for just a few minutes, until bright green and tender but firm; taste as you go.
• Drain them at once and plunge them into ice water; once cool, drain and pat dry.
• Add lemon juice, vinegar and sharp dressings only just before serving.
• Don't add baking soda to the water and don't worry about the lid; watch the time above all.

If it goes wrong

Olive colour can't be reversed; blend overcooked vegetables into a soup or purée where the colour matters less. In dishes where greens braise for a long time in a sauce, such as saag or Greek green beans, an olive colour is normal and belongs to them. Next time cook the vegetables for less time and cool them straight away.