Ask Ottolenghi: what’s the secret to keeping cooked green vegetables green?

What’s the best way to cook greens so they stay bright green and don’t turn a dull, browny-green colour? Does it depend on the type of greens, do I have to add salt to the water, should the water be boiling or just hot? Arti, Italy
The conventional wisdom is that we salt the water to prevent discolouration and “fix” the colour, but Heston Blumenthal, after numerous controlled experiments, says it’s more to do with the quality of the water itself. What salt does do, however, is raise the boiling temperature of the water, which is especially helpful if you’re cooking green beans, say, because if the temperature comes down too far, this activates an enzyme that destroys chlorophyll and, hence, the colour. Salting also helps enhance the vegetable’s flavour.
The most practical way to ensure that the water temperature doesn’t fall too much when the vegetables are added is not to overcrowd the pan, so, whenever possible, cook your vegetables in batches (or in a very large pot), which will mean the water comes back to a boil as quickly as possible. Another common theory to maintain a vegetable’s green vibrancy is not to cover the pot, but in fact the lid helps the water return to a boil more quickly, which helps retain that “green-ness”.
One thing everyone agrees on, though, is that vegetables shouldn’t be cooked for too long, though there is also something to be said for the soft tenderness of a well-cooked green vegetable, especially if it’s then dressed with chopped fresh herbs, olive oil, lemon juice and, perhaps, a sprinkle of pomegranate seeds and/or toasted nuts.