Why is a roasted sweet potato so much sweeter than a raw one?

A sweet potato contains a substance that snips starch into small pieces and turns it into maltose. In a raw potato, the starch grains are packed tightly, so they are hard to cut. Once heat makes the starch soak up water and loosen, it finally becomes easy to break apart.

This works best when the inside of the sweet potato is at roughly 60 to 75°C. If it gets much hotter, the substance loses its strength and stops. So the longer the inside stays in that range, the more maltose forms and the sweeter it gets. Rice turning sweet when you chew it for a long time works the same way: a similar substance in saliva turns starch into maltose.

There is a reason street vendors roast sweet potatoes for a long time in a drum. Cooking slowly over low heat keeps the inside in the temperature range where sweetness forms. If you cook one in a microwave in a few minutes instead, the inside heats up quickly and rushes past that range, so the same potato tastes less sweet. Steamed sweet potatoes taste less sweet than roasted ones partly because more moisture stays in and they cook at a different pace.

A roasted sweet potato also loses water, which concentrates its sugar, and the skin side browns and adds a nutty, caramel-like aroma. The sticky, honey-like juice that oozes out of the cracks in the skin is that concentrated sugar.

A common misconception is that sweet potatoes, like many vegetables, last longer in the fridge. Sweet potatoes are a warm-climate crop, so if they are kept below 10°C for a long time, the inside is easily damaged, hard cores can form, and they rot quickly. It is better to wrap them in newspaper and keep them somewhere cool and airy at about 12 to 15°C.

The saying that sweet potatoes stored for a few weeks taste sweeter than freshly dug ones is mostly true. During storage, part of the starch slowly turns into sugar. If you want them sweeter at home, choose to roast them for a long time at a low temperature in an oven or air fryer.