Carbon dioxide is all around us and inside us. It's in the air we breathe in, and there is even more in the air we breathe out. Even so, we don't normally think about this gas as something we use in the kitchen - although most people know that it gives the pop and sparkle to champagne and the fizz to a glass of cola.
Carbon dioxide is the gas produced by yeasts when they grow and consume sugar for energy. It has played a part in cooking ever since man first baked a loaf of bread, let grape juice turn to wine or brewed a pint of beer. These days we can make carbon dioxide on an industrial scale. We can even buy cylinders of it at the local supermarket to make sparkling drinks and soda water at home. So let's look at some of the things this gas can do.
In Heston's kitchen he and his chefs have looked at how carbon dioxide can be used to make fizzy fruit - you can do this yourself if you have a fizzy drink machine.
One of the things that humans seem to like about carbon dioxide is that it gives a pleasant tingle on the tongue and palate. Why carbon dioxide gives this tingle is something that has only been understood quite recently. The sensation arises when carbon dioxide triggers off the nerves in the mouth that detect pain. Although no harm is being done to the skin, the brain is fooled into thinking that it is being pricked with tiny needles.
Most people actually like this sensation, although many still wrongly believe that it is caused by the bubbles of carbon dioxide bursting in the mouth. If you don't believe that pain sensors are involved, then try this little test:
Of course, when you drink a fizzy drink, you will feel the bubbles bursting as well, unless you happen to be in a pressure chamber. So, if a diver is put in a decompression chamber after working in deep water and he drinks a (normally) fizzy drink, he will find that there are no bubbles, because the pressure of the chamber stops them bubbling out of the drink, just as it stops nitrogen dissolved in the diver's blood from causing bubbles and giving him the bends.
But the surprise to the diver is that the drink still tastes fizzy and that he still gets the tingle when the carbon dioxide activates the pain sensors in his mouth - even though there are no bubbles.
Carbon dioxide has lots of other uses that go way beyond making sparkling drinks - making bread, for example. When we add yeast, water (and sometimes sugar) to flour, the yeast is activated and starts to grow and release carbon dioxide. This gas gets trapped inside a film of gluten (this is the protein present in wheat flour and some other flours), which is quite elastic and stretchy.
As more bubbles of carbon dioxide form, the bread starts to rise - it can easily double in size within an hour. Bakers usually knead the bread at this stage to distribute the bubbles more evenly and to release some of the excess gas. Then they allow more carbon dioxide to form during the process known as 'proving'. It is the carbon dioxide gas bubbles, trapped by the gluten, that give bread its light and fluffy texture when it's baked.
We also take advantage of carbon dioxide when we make cakes without using yeast. Making carbon dioxide is quite easy and there are several ingredients in the kitchen which we can use to do this. One of the most important is sodium bicarbonate, also called bicarbonate of soda. In the presence of an acid this chemical immediately releases carbon dioxide. See for yourself in this experiment:
Sodium bicarbonate is used in baking powder and in self-raising flour. During baking, chemical reactions caused by the natural acidity of the ingredients, and speeded up by the high temperatures during baking (which makes the bubbles expand), take place. The result is that carbon dioxide is released, making cakes and pancakes lighter and fluffier by creating bubbles within the batter as it cooks. Try it for yourself:
Carbon dioxide also has another very useful property - it acts as a preserving agent and at high enough levels actually stops the growth of some micro-organisms, especially yeasts. This means that fizzy drinks are less likely to go off once they have been opened than is the case for still drinks. Some food manufacturers take advantage of this when they package their products. If you ever find bread - especially sliced bread - sold in puffed-up plastic packaging, then it is very likely that the air around the bread is high in carbon dioxide to reduce the risk of moulds forming on the bread.
So as well as being useful to Heston in creating new food ideas, this gas which we all breathe in and out is also a food preservative, a texture modifier and one that gives us a strangely pleasant sensation of pain.
Carbon dioxide has lots of other uses that go way beyond making sparkling drinks - making bread, for example. When we add yeast, water (and sometimes sugar) to flour, the yeast is activated and starts to grow and release carbon dioxide. This gas gets trapped inside a film of gluten (this is the protein present in wheat flour and some other flours), which is quite elastic and stretchy.
