Call it pommes purée call it mashed potato - or just plain mash if you prefer. Call it whatever you like, but there's little doubt that it's one of Britain's favourite comfort foods. Nothing very scientific about that, you might think. But have you ever had mashed potatoes that were watery, lumpy or, perhaps worst of all, so gluey that they had the consistency of wallpaper paste? The fact is, that in order to make perfect mashed potatoes, it pays to know a little science beforehand.
Broadly speaking, potatoes fall into two different types: those with cells that tend to separate and that break down easily when cooked, and those with cells that are more firmly held together and stay relatively firm when cooked. The first group are referred to as 'floury' and the second as 'waxy'. In Britain, floury varieties include King Edward or Maris Piper while waxy types include Charlotte, Nadine or Pink Fir Apple; most new potatoes are also waxy.
There are two general schools of thought about what makes perfect mashed potato: the French school and the Anglo-American school. The former uses waxy varieties of potato to make a mash that is thick, smooth and silky textured. The latter uses floury varieties of potato to make a mash that is light and fluffy, and thick enough to stand a sausage or two in.
Whichever school of thought you subscribe to, however, everyone agrees that mash that is sticky and gluey, reminiscent of wallpaper paste, is definitely off the menu.
So, how do you attain mash nirvana (fluffy or smooth) while avoiding the wallpaper-paste pitfalls? Knowing something about the structure of potatoes and how they behave when cooked is the secret to good mashed potato of either type.
Potatoes, like many other root vegetables, lay down reserves of starch in their cells to allow them to survive the winter months. The starch is tightly wrapped up as starch granules inside a thin protein coat. There are hundreds of starch granules inside each cell of the potato.
Floury potatoes contain, on average, more starch granules per cell than waxy potatoes do, which gives them a different density. In fact, there's an easy way to find out whether your potatoes are floury or waxy in this quick experiment:
The cell walls of potatoes are largely made of insoluble cellulose molecules, which are held together by a sort of 'glue' (called hemi-cellulose), made from other molecules. Within these cell walls are the starch granules. It is not normally possible to easily see the starch granules, but there is a simple way of making them visible using iodine solution which can be bought at the chemists. Try this experiment to find out how:
During cooking the hemi-cellulose breaks down, the cell walls get weaker, the cells start to separate and to break open. At the same time the starch granules absorb water and expand and many of these granules are also released from the cells. Floury and waxy potatoes behave differently because in the former the cells separate more easily and they burst, releasing the larger number of starch granules.
Deciding to use floury or waxy potatoes is only the first step to making the perfect mash. When you're cooking them, it's essential prevent the potatoes from absorbing water during cooking, which would make the mash too wet. Two ways of preventing this from happening is either to cook the potatoes in their skins and then peel them or, alternatively, to first peel the potatoes but then to steam them.
Some cooks, including Heston, prefer to cook the potatoes twice before mashing them. The potatoes are first cooked at a temperature below the boiling point for several minutes. They are then allowed to cool before giving them a final cooking just before they are mashed. This technique gives firmer potatoes - and the reason for this is quite subtle.
Pectin molecules
Potato cells contain pectin molecules; during the first cooking, and before the cells get too hot, enzymes are activated which change the nature of the pectin, allowing it to react with calcium that is naturally present within the cells of the potato. The combination acts as an insoluble 'glue' between the potato cells and makes them hold together more during the second cooking stage, with the result that the potato is more firm.
Try this final experiment to see the difference between mash made from floury and waxy potato varieties:
To sample the difference in texture between floury and waxy potatoes, try making a classic dish of Gratin dauphinoise. Simply follow a recipe, such as Antony Worrall Thompson's Gratin dauphinoise but divide the ingredients between two small gratin dishes (this needs to be quite precise). In one use a floury variety of potato and in the other use a waxy variety. Cook according to instructions, although you will want to reduce the cooking time by about 20-30 minutes as they are smaller dishes. Serve and pay attention to the difference in texture and how much liquid has been absorbed.
There are two general schools of thought about what makes perfect mashed potato: the French school and the Anglo-American school. The former uses waxy varieties of potato to make a mash that is thick, smooth and silky textured. The latter uses floury varieties of potato to make a mash that is light and fluffy, and thick enough to stand a sausage or two in. 
