Ingredient

Sweet potato

A sweet potato is not a potato and is not closely related to one. It has far more sugar and a different kind of starch, which is why it caramelises readily, why it goes soft rather than fluffy, and why sweet potato chips almost never come out crisp.

Why it behaves differently

An ordinary potato is high in starch and low in sugar, so it dries out into something fluffy and crisps readily. A sweet potato is high in sugar and its starch converts further to sugar as it cooks, so it stays moist, browns very fast and goes soft rather than mealy.

That sugar is why sweet potato caramelises so well — the edges of a roasted chunk go dark and sticky in a way a potato never does — and also why it burns before it crisps. Sweet potato chips are the standing example: by the time the surface is dry enough to be crisp, the sugar has scorched.

The practical consequence is to stop expecting it to do a potato's job. Roast it, mash it, purée it, put it in soups and curries and traybakes. Do not try to make roast potatoes out of it.

Getting the best from it

• Roast at a slightly lower temperature than potatoes — 190–200°C — so the inside cooks before the sugary outside burns.
• Cut into larger pieces than you would potatoes. Small pieces dry out and blacken.
• Spread out on the tray. The usual rule, and it matters more here because they release a lot of moisture.
• Bake whole for the best flavour. An hour at 200°C concentrates the sugars and the flesh goes almost custardy.
• For chips, toss in cornflour before oiling. It gives a coating that can crisp when the potato itself cannot. They will still be softer than potato chips.
• Pair with heat, acid and salt. Their sweetness needs balancing — chilli, lime, feta, yoghurt, smoked paprika, black beans.

The varieties

Orange-fleshed

The common supermarket type. Sweetest, softest, highest in moisture. Best roasted or mashed.

White or pale-fleshed

Drier and starchier, closer to a potato in texture and much less sweet.

Purple-fleshed

Dense, dry and nutty, holds its shape well.

Yam

A completely different plant, drier and starchier. What is sold as "yam" in some countries is a sweet potato; a true yam is not.

Buying and storing

• Firm, with smooth unbroken skin and no soft spots or sprouting.
• Store somewhere cool, dark and dry — but not the fridge. Cold damages sweet potatoes and gives them a hard core and an off taste.
• They keep for weeks at cool room temperature and go off quickly once they start.
• A little sticky dark syrup seeping from a cut end is normal and is a sign of a sweet one.
• Cooked sweet potato freezes well, mashed or roasted.

Why slow roasting makes it sweeter

A sweet potato contains enzymes that convert its starch into maltose, and those enzymes work fastest in a temperature window the potato passes through on its way to being cooked.

Roast one slowly and it spends a long time in that window, so a great deal of starch is converted and the result is markedly sweeter and softer. Microwave one and it shoots past the window in a couple of minutes, so almost no conversion happens and the result is starchier and blander.

This is why a slow-roasted sweet potato tastes like a different vegetable from a quickly cooked one, and why boiling gives the least sweet result of all. If sweetness is what you want, start it in a moderate oven rather than a hot one and give it an hour.

Common questions