You do not need salt in the water in order to keep your vegetables green. There, I have said it - I have committed the cardinal sin of questioning perhaps the single most unquestioned act in the kitchen. People have been stoned for less.
There are key moments in many people's lives; moments that were turning points or milestones. I can honestly say that trying to cook green beans in the old Fat Duck kitchen was one. Traditionally, we have always been told to plunge green vegetables into a large quantity of boiling, heavily salted water, so that the quantity of cold beans added was not enough to bring the water off the boil. The old Fat Duck kitchen was fuelled with gas from a domestic pipe. This meant that we could not, in fact, bring a particularly large quantity of water to the boil. Because of this, we had to cook our green beans in batches of about eight at a time. Any more than this, and the water would come off the boil, leaving the beans more or less to stew and develop a murky brown colour.
I am talking about properly cooked green beans here - the ones with that marvellous fresh, grassy taste and that are easily chewed, not those shiny, green and virtually raw things that are so undercooked they squeak when you crunch into them.
I became increasingly obsessed with beans, and set about researching the mechanics of cooking green vegetables. The thing that was stuck in my mind was that recipes regularly told you that salting the water helps "fix" the colour, but without offering any further explanation as to why this should happen.
Salting the water certainly doesn't season the beans, as they always require seasoning afterwards, anyway. The only reason that I could come up with was that salt raised the boiling temperature of the water.
I had read in Harold McGee's brilliant book, On Food And Cooking, The Science And Lore Of The Kitchen, that there is an enzyme that can destroy chlorophyll and hence the colour. Apparently, this enzyme is activated in warm to hot water and destroyed by evaporation. This seemed to explain the reason why salt is added to the water - it raises the boiling temperature and helps to maintain the water at a boil when the beans are added, thus preventing the enzyme from doing its damage. Hooray, I thought - I've cracked it.
Purely out of interest, I decided to measure the difference in boiling temperatures between water with and without salt. (In case you were wondering, we salted at the same level that we usually do for this sort of thing: 40g per litre.) To my amazement and disappointment, there was little noticeable difference between the two. So I decided to try and cook a green vegetable in unsalted water. I allowed a head of broccoli to turn yellowy brown on the tips of the florets, and then cooked it in unsalted water from the tap at home. I purposely overcooked the broccoli, and do you know what? Even though it was old and yellow in parts, the green parts stayed vividly green. It was at this moment that I realised I had to find a scientist who was interested in cooking.
After a good deal of sleuthing, I got hold of Peter Barham, lecturer in physics at Bristol University and occasional contributor to Weekend's food pages. Peter has since become a very good friend, a keen proponent of molecular gastronomy and author of the terrific The Science Of Cooking, which was published earlier this year.
When I told him that I had reluctantly come to the conclusion that salt was not necessary to keep the green colour in vegetables, his response sparked a fruitful friendship. It transpired that, although I was correct in my conclusion, some of the basis of my findings were a little misplaced. The most important factor is the quality of the water itself, and of the amount of calcium it contains. Calcium is an enemy of green vegetables - a high level of calcium will be more likely to cause the beans to discolour. It just so happened that the water at home had considerably less calcium than the water at the restaurant.
So, as long as your water is not high in calcium (not more than 20mg per litre), and comes back to the boil almost immediately after the beans are added, unsalted water will cook those beans, and other green vegetables, perfectly (buy low-calcium mineral water, if all else fails). So McGee's destructive enzyme may not be so bad after all.
Incidentally, another old wives' tale is that you must never put the lid on the pan when cooking green vegetables, as they will discolour. Nonsense. Putting the lid back on the pot helps to bring the water rapidly back to the boil and doesn't harm the beans in any way.
Cooking French beans and other green vegetables
1 If cooking green beans, top and tail them, then soak in cold water for a couple of hours before using. Most beans have been picked long before they reach the shelves, a couple of months in some cases, and soaking helps to rehydrate them.
2 Bring a pan ( not an aluminium one) of unsalted, low-calcium water to the boil.
3 Drain the soaked beans and drop into the boiling water. It is important to make sure that you do not put too many in at a time, as the water must return to the boil as soon as possible.
4 Immediately put the lid on.
5 Cook until done. The time will vary depending on the vegetable. In the case of green beans, they should have a resistance when eaten, but no crunch.
6 Lift the beans out of the water and, if you aren't going to serve them immediately, plunge into a bowl of iced water. This is not essential to keep the colour, but if you are going to leave them out to cool at room temperature, the residual heat will continue cooking them. Alternatively, you may want to allow for this and cook them for a shorter time initially.
7 Reheat in a mixture of butter and water, about one third butter to two water, seasoned with salt and pepper. Drain and serve.
All recipes serve four.
Haricots verts à la crème
Just add butter, cream and mustard - a classic French approach to cooking with green beans.
200g French beans
125g unsalted butter
50ml whipping or double cream
Salt and pepper
1 tsp grain mustard
1 tbsp chopped parsley
Cook the beans as above, but stop the cooking a couple of minutes before you would normally do so.
Peel and very finely slice the shallots, and crush the garlic. Then, in a small frying pan, sweat these off in half of the butter over a low heat, until the shallots are translucent and the garlic has lost its aggressive aroma - this should take about 10 minutes.
Add the cold beans to the mixture in the pan, then add 50g more butter cut into cubes, the cream and the water. Season very generously with salt and pepper, and cook over a high heat for three to four minutes, until the liquid has turned sauce-like in consistency. Stir in a heaped teaspoon of grain mustard, the remaining butter, if needed, and season again, if necessary. Heat through only briefly, sprinkle over a heaped tablespoon of chopped parsley and serve at once.
One note of caution, however: make sure that you add the mustard only at the very last minute of cooking, because if the beans are allowed to cook for too long with the mustard, they will turn an unappetising yellowy brown colour.
Salad of haricots
As well as being an essential part of salade Niçoise, these beans make a quite delicious salad in their own right.
200g French beans
4 shallots, finely chopped
A handful mixed fresh herbs - parsley, chervil, tarragon, thyme
150g white mushrooms, very finely sliced
For the vinaigrette
12.5ml sherry vinegar
37.5ml white-wine vinegar (preferably chardonnay)
200ml extra-virgin olive oil
6 small sprigs fresh thyme (preferably lemon thyme)
clove garlic, crushed shallot
First, prepare the vinaigrette. Cut the half shallot in two through the root, so that each quarter holds together, then mix all the ingredients together and leave to infuse in a sealed container for 24 hours. Use as much or as little as you need, when you need it.
Cook the beans as above. To serve, simply mix the beans with the shallots, herbs and mushrooms in a bowl, pour in a little vinaigrette (with mustard added, if you wish), season, toss well and serve.
Runner beans with cucumber
The addition of cucumber to the runner beans gives them a wonderful freshness. In any case, I am a big fan of cucumber, and think that it is woefully underused in cooking. This is best served as a side dish or garnish.
250g runner beans cucumber
50g unsalted butter
Salt and pepper
Thinly slice the runner beans on the diagonal, so that you end up with strips of roughly 2-3cm long and a couple of millimetres wide, then cook the beans as directed above.
Meanwhile, peel the cucumber and halve it lengthways. Next, run a teaspoon all along the length of the inside of the cucumber, thereby removing the seeds with ease. How you cut it up from here is more or less up to you - all you have to be aware of is that you're after very thin pieces (though, for appearances' sake, you could cut it on the diagonal, like the beans)
Blanch the cucumber for two minutes in the same water that you used to cook the beans, then drain and refresh in cold water. Put the butter and 150ml water in a pan large enough to hold the beans and cucumber to a depth of no more than a couple of centimetres. Add the blanched beans and cucumber, bring the water to the boil, and simmer for a few minutes until the vegetables are nicely glazed. Season generously and serve at once.