Why do my green vegetables go grey or olive coloured?

There is a short window, usually the first few minutes, in which green vegetables are actually brighter than raw — air trapped between the cells escapes and the chlorophyll shows through more clearly. After that the colour only goes one way, and how fast depends entirely on heat and acid.

Step by step

1. Understand the mechanism, because it dictates everything else.Chlorophyll is a ring with a magnesium atom at its centre, and that structure is what makes it green. Acid replaces the magnesium with hydrogen and the molecule becomes pheophytin, which is olive brown. Every green vegetable releases organic acids as its cells break down, so the clock starts as soon as it is hot.
2. Cook fast and hot, in plenty of water.Boiling water, a large pan, and a small quantity of vegetable at a time so the water barely drops off the boil. A big volume of water dilutes the acids the vegetable releases. Small pan, lots of vegetables, water off the boil for two minutes: that is the grey combination.
3. Leave the lid off.This is the single most useful habit. The volatile acids released by the vegetable escape as steam from an open pan; a lid condenses them straight back into the water. Lid off for green vegetables, and it barely changes the cooking time.
4. Stop early, while they still look bright.Broccoli and green beans: three to five minutes. Peas: two or three from frozen. Spinach: under a minute. Test one piece rather than watching the clock, and remember that carryover heat continues the cooking after draining.
5. Refresh in cold water if you are not eating immediately.Draining into iced water stops the cooking and locks the colour. Essential if the vegetable will sit, be reheated, or go into a salad. Drain thoroughly afterwards and reheat briefly in butter or hot water.
6. Keep acid away until the last moment.Lemon juice, vinegar, a vinaigrette or a tomato-based sauce will turn green vegetables drab within minutes. Dress them at the table, not in the pan.
7. Salt the water, and skip the bicarbonate of soda.Salted water seasons and slightly speeds cooking. Bicarbonate of soda does keep the colour brilliantly green, because it neutralises the acids — but it also destroys vitamin C rapidly and turns the vegetable slimy and soft. It is a restaurant trick that is not worth it at home.

Tips

• Steaming keeps colour well too, provided it is quick and the lid is only on for a short time. It is the long lidded cook that does the damage.
• Stir-frying and roasting avoid the problem almost entirely, because there is no water for acids to concentrate in and the cooking is short.
• Blanching and refreshing in advance is the standard way to have bright green vegetables ready for a big meal without last-minute panic.
• A vegetable that has gone olive is perfectly safe and tastes only slightly duller. The colour is lost, not the food.

Common mistakes

• Putting the lid on to make it boil faster — It traps exactly the acids you want to lose. Lid on to reach the boil, lid off once the vegetables go in.
• Cooking a large quantity in a small pan — The temperature crashes, the cook takes twice as long and the acids concentrate. Cook in batches.
• Adding lemon to the cooking water — It turns the vegetables drab immediately. Squeeze it over the served dish instead.

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