Sweet potatoes are largely made up of starch, but they aren't sweet when raw. Cooking sweet potatoes releases an enzyme that breaks the starch down into a type of sugar called maltose, which is about 33% as sweet as table sugar. This enzyme is active between 135°F and 170°F, so if you steam your sweet potatoes or roast them in small chunks, they'll come out less sweet than if you were to choose low and slow baking as the cooking method. They'll still come out sweet, just not quite as much. This is why sweet potatoes are great in both sweet and savory recipes. Their natural sweetness can be played up by adding sugar to make a dessert or played down if you're including them in a main dish or as a savory side dish.