Cooking science

Chlorophyll degradation

A green vegetable dropped into boiling water goes brighter for the first minute and then, if you leave it, turns a dull olive-brown. Two different things are happening, and knowing which is which tells you exactly when to stop.

The brightening, then the dulling

Raw plant tissue holds small pockets of gas between the cells. Those pockets scatter light and dull the appearance of the chlorophyll underneath. Brief heat drives the gas out, the cells collapse slightly against each other, and the green suddenly reads much more vividly. That flash of brilliant colour in the first minute is the signal that the vegetable is ready.

The dulling is a different, slower and permanent change. Chlorophyll holds a magnesium atom at its centre, and that is what makes it green. Acid displaces the magnesium and replaces it with hydrogen, producing a compound that is olive-brown. Once that swap has happened, nothing brings the colour back.

The acid comes from the vegetable itself — released as the cells break down — plus anything acidic you added. So the longer it cooks, the more acid accumulates around it and the further the colour goes.

What keeps the colour

• Cook briefly. By far the largest factor. Two to three minutes for most green vegetables.
• Cook uncovered. A lid condenses the volatile acids and returns them to the pot. Uncovered, they escape as steam.
• Use plenty of water. A large volume dilutes the acids released and recovers its temperature faster when the vegetables go in.
• Refresh in iced water. Stops the cooking dead. Without it, residual heat keeps the process running.
• Add acid at the very end, or at the table. Lemon, vinegar and dressings all turn green vegetables olive within minutes.
• Salt the water well. It seasons, it slightly firms the cell walls, and it helps the vegetable hold its colour.

The things that seem to help and do not

Bicarbonate of soda

It genuinely keeps vegetables vividly green, by neutralising the acid. It also breaks down the pectin so fast that the texture collapses, and it degrades some nutrients. A bad trade, and an old restaurant trick worth not copying.

A copper pan

Copper ions replace the magnesium and hold the green. Also a traditional trick, and not something to encourage.

Leaving the lid on

Traps the acid. Actively harmful for green vegetables.

Cooking longer "to be safe"

The commonest cause of the problem.

What genuinely works

Short cooking, plenty of water, no lid, iced water afterwards, acid at the end. Nothing exotic.

Where it shows up

• Broccoli, green beans, peas, asparagus, mangetout — the classic cases.
• Spinach and greens, which go from vivid to drab in under a minute.
• Pesto and herb sauces, which dull as they sit. Blanching the basil briefly first keeps them green for far longer.
• Green vegetables dressed in advance — a green bean salad dressed an hour before serving will be olive by the time it reaches the table.
• Reheated greens, which take a second dose of the same process.
• Brussels sprouts and cabbage, which additionally develop strong sulphur smells the longer they cook.

Common questions