Why do green vegetables go dull when cooked?

Because heat and acid destroy chlorophyll, converting the vivid green pigment into a drab olive one — and the change is chemically specific and largely preventable.

What happens. Chlorophyll contains a magnesium atom at its centre, which is what gives it the bright green colour. When vegetables are heated, cell walls break down and acids trapped inside the plant cells are released. Those acids displace the magnesium and replace it with hydrogen, forming pheophytin — which is olive-brown.

The reaction is not instant. Briefly cooked vegetables actually look greener than raw, because heat drives out the tiny air pockets between cells that scatter light and dull the colour. Beyond a few minutes, the pigment change takes over and the colour degrades.

What makes it worse:

Long cooking — the single biggest factor.

Acid. Adding lemon or vinegar to green vegetables during cooking accelerates the change dramatically. Dress them after cooking, not during — a salad of green beans dressed while hot and left to sit goes dull for exactly this reason.

Covering the pan. Volatile plant acids released during cooking condense on the lid and drip back in, concentrating acidity. Cook green vegetables uncovered.

Overcrowding, which lengthens cooking time.

What preserves the colour:

Cook briefly in plenty of rapidly boiling, well-salted water, so the temperature does not drop when vegetables go in and cooking time stays short.

Uncovered, letting acids escape.

Refresh in iced water immediately — this stops cooking instantly and is the standard professional method. Draining and leaving vegetables to cool slowly means they continue cooking in their own heat.

Adding bicarbonate of soda does work, by keeping conditions alkaline — and it is a bad idea, because it destroys vitamin C and turns the texture slimy.

Why some vegetables behave differently. Red and purple vegetables contain anthocyanins, which are pH-sensitive in the opposite direction: red cabbage turns blue-purple in alkaline conditions and stays vividly red with added acid, which is why recipes include vinegar or apple.

Carotenoids in carrots and peppers are far more stable and survive cooking well.