Heston Blumenthal ... how to cook the perfect potato. Although it looks like a solid knobbly lump, about three-quarters of a potato is water. The rest is known as ''dry matter'', principally starch. It's the dry matter that gives a potato its flavour, and managing the dry matter is central to cooking good mash, chips and roast potatoes. Growing conditions have such an influence on dry matter that the amount in a single variety can vary virtually from field to field, depending on the temperature, rainfall and drainage. It's not an exact science. You can cook a particular variety perfectly one day, then find it responds badly the next. In general, floury potatoes, such as anya, king edward and maris piper, have more dry matter, while the waxy charlotte and marfona have less. You might therefore think it's a simple case of going for a potato with high dry matter, after all, maximising flavour is what it's all about. But there's more to it. All starch granules are made up of a proportion of two molecules - amylose and amylopectin - which respond to heat and cooking in different ways. Amylose tends to form a strong, tightly knitted gel; amylopectin coheres more loosely.

I'm looking forward to the day when bags of potatoes come with information about dry-matter percentages and amylose-amylopectin ratios printed on them - it will really help cooks find the perfect potato. For chips or roast potatoes, you're looking for a variety that's high in dry matter, with a reasonable proportion of amylose to help keep the shape. For mash, too much amylose is best avoided as it can lead to a gummy consistency. When cooking roast potatoes (and chips), I want an interior that's soft and fluffy, with a crisp, almost glass-like, crust. Although you're working towards these aims simultaneously, it's helpful to think of them as separate tasks, each requiring a specific process. For that soft interior, it's best to use a type of potato with a high dry-matter content and simmer carefully until it's almost falling apart. Be brave about this because the outer crumbliness is the key to a good final crust, as well as a soft inside. For the crust to be properly crunchy, the fat needs to pool and collect in little pockets that harden and crisp up. If you've simmered the potato until it's only just holding together, lots of cracks and fissures will have developed in the surface - perfect places for the fat to collect. (Fat will penetrate the potato only where there is a crack.) To give the fat as many places as possible to catch and harden, it's best to use large potatoes cut into quarters or, better still, eighths, so there are plenty of pointy edges where fat will collect. First rinse the cut potatoes in water for five minutes or so: this washes off surface starch, which would make the cooked potato less fluffy and crisp. For roast potatoes, the pieces are then cooked in unsalted water - salt can occasionally make the crust a little chewy, so it's safer not to add any. Potatoes with high dry matter fall apart more easily than those with a lower dry-matter content, so you'll need to keep an eye on them while simmering. Appearances can be deceptive: a potato will often look as though it's firm right up to the moment where it falls apart, so you need to periodically remove one and check it.

When they're ready, the potatoes need to be lifted very gently out of the water with a slotted spoon into a colander (don't just tip them, which will almost certainly turn them to mush). Leave them to steam, which will ensure moisture evaporates (it might otherwise make the crust soft or soggy) and the potatoes firm up and are not too fragile to work with. Once cool, the potatoes are ready for the roasting tray, and all that careful preparation should guarantee a great roast potato. Make sure you hold on to any bits of potato that have broken off and put them in the roasting tray too. They will turn into lovely little nuggets of crispiness. Since, for me, chips have to have a fluffy interior similar to a roast potato's, the first stages of cooking them are much the same: rinsing off surface starch, simmering the cut pieces until they're very soft but just holding together, with lots of little cracks in the surface. This requires careful and regular checks with a slotted spoon. By the time the chips are ready, they'll be fairly fragile and have to be handled delicately, although they'll become slightly more robust once they have cooled. As far as crisp chips go, moisture is the enemy. When I discovered this, I went to extraordinary lengths to try to combat the problem - oven-drying chips or even individually pin-pricking them to get rid of excess moisture. The most practical solution for the home cook, however, is a concerted drying-out process, drawing on the air-drying abilities of the freezer, both before and after the first session in the deep-fryer. Double frying is the other key to great chips. The purpose of the first session in the fryer, at a gentler temperature, is to make any starch left in the surface cells dissolve and combine to create a rigid outer layer that can withstand the higher temperature of the final frying, which will colour the chips golden. Skipping this can undo all the trouble you've taken to dry out the potatoes and drive off moisture.

A single frying at a high temperature leads to a thin crust that can easily be rendered soggy by whatever moisture remains in the chip's interior. Although this first, low-temperature frying might seem a time-consuming process, once the chips have had it, they can be stored in the fridge for up to three days before their second, high-temperature frying. As for mashed potato, you can, of course, simply boil and mash some spuds. It might be a bit lumpy, but it will work well enough to go with a couple of bangers. However, there are times when a dish calls for something more refined, and no less than pommes purees will do. Creating a perfect, velvety texture requires a little more work, but it's worth it. The end result will be a real taste of luxury. The success of pommes purees is largely dependent on managing the starch. Cooking has to break down the cell walls of potato without damaging the starch granules. If they're overcooked, the granules leak starch, turning the mash into a sticky, wallpaper-paste-like mass. The key to preventing this is an initial 30-minute simmer at precisely 72C, which alters a potato's structure so it responds well to the subsequent boiling and mashing. Cooking the potatoes at this temperature starts a process called ''gelatinisation'', in which the starch granules absorb water and swell to become a gel. (The process starts at 45C and stops at about 75C, hence the need for precision.) It also strengthens the potato's cell walls, so they're less likely to disintegrate.

After this first simmer, it's essential to cool the potatoes completely because this causes the starch molecules to firm up (an activity known as ''starch retrogradation''). At this stage, the potatoes will look uncooked but don't worry, they're now perfectly prepared for the final stage of cooking, which is simply softening by boiling in water as normal. Using this two-stage cooking technique prepares potatoes perfectly for mashing. Nonetheless, it's vital that this final stage is done carefully so as not to undo all your work. Mashing is an invasive process and if it's done too aggressively, there's a risk that, instead of neatly separating the potato, the swollen starch granules will burst, leaking starch into the mash. Using a food processor to mash will just produce a gummy pulp, and even a masher is likely to cause some damage. For the smoothest pommes purees, it's better to use a food mill or potato ricer, which grinds the potato gently, rather than blitzing or squashing it, so the starch granules remain intact. RECIPE: Heston Blumenthal's roast potatoes Loading

Extract from Heston Blumenthal at Home. * Most of these are not available in Australia. Floury Australian potatoes include: king edward, sebago and dutch cream. nicola and kipfler are waxy potatoes. Purple congo has a dry texture. Nashdale Fruit Company sells pink fir, charlotte (waxy) and king edward potatoes at Glenbrook, Castle Hill and Penrith farmers' markets.