It's a chef's trick that has crept into some recipe collections and often appears in lists of cooking tips and hints. It is not a good practice.

The main use of bicarb in the kitchen is as a raising agent for things like cakes and some types of bread. It makes them lighter.

This happens because bicarb reacts with liquid and gives off bubbles of carbon dioxide, which are then trapped by the cake mixture. It's like filling your dough with lots of tiny balloons.

Now, carbon dioxide is also the gas that is used by supermarkets to make your fruit and vegetables look good. Those rosy red apples didn't ripen that way. In fact, they're not even fully ripe. They got their color through being exposed to carbon dioxide while still in storage.

So did much of the other stuff which looks so good on the shelves and tastes so much like the cardboard boxes it comes in, and with about as much juice.

So why not take advantage of this known quality of bicarbonate of soda and use it in cooking to improve the appearance of your green beans?

The answer is simple and direct: the chemical reaction that produces the carbon dioxide also destroys the nutrients in food, and in particular the vitamins. You are trading nutritional content for the sake of appearance. And it is a severe trade; almost total loss.

There is not even any need to do it. Most color loss in vegetables occurs either as a result of overcooking, or through covering during the cooking process.

You can avoid the first by cooking your vegetables until just done ? when they still have a certain amount of ?bite? to them ? and then immediately refreshing in ice-cold water. You reheat them by plunging them into boiling or very hot water for a few seconds.

The second problem mainly affects pulses such as green beans and peas. Cook these uncovered in the minimum amount of water and without adding any salt. Try using a little sugar instead. This will enhance the flavor without any noticeable adverse effect.

Refreshing green beans is also a great way to ensure they remain slightly crisp, particularly if you intend to add some kind of sauce or vinaigrette just before serving them. They will also keep in the fridge, in the iced water, for several days after cooking.

Of course, for the occasional dinner party appearance may be what is uppermost in your mind. Should that prove to be the case, you can always cheat. Just remember what a little bicarb can do.