The optimal way to bake sweet potatoes

March 21, 2014 at 12:25 pm (B plus (3.5 stars, like a lot), Root vegetables)

I was going to be home late on Tuesday, and so I asked Derek to bake some sweet potatoes, so that they’d be ready to eat when I got home. He asked me how and I said I didn’t remember exactly, but that they’re pretty forgiving. When I got home I found that he had rinsed them off, pricked the sweet potatoes with a knife in a few places, and put them on an (unlined) cookie sheet. He had been baking them at 375 for about an hour and they were not even close to being soft. I was surprised, as I feel like sweet potatoes are usually done after an hour in the oven. They were were quite large, but I think that even large sweet potatoes shouldn’t take much longer than an hour to get soft.

After 20 more minutes the potatoes were still hard. I poked the sweet potatoes a few more times and turned the oven up to 400. I also turned the sweet potatoes and added some water to the cookie sheet, to keep the skin from burning before the flesh got soft. After another 30 minutes or so they were finally done, but the cookie sheet was covered in burnt sweet potato juice. What a mess.

Clearly this wasn’t the optimal way to cook sweet potatoes. So what is? I did some quick internet research to try to figure it out.

To prick or not to prick?

Everyone seems to agree that you should pierce the potatoes’ skins with a fork (or sometimes a paring knife) before baking. I’m not sure why. I assume this speeds up the cooking process, but does it also achieve something else? Keep them from exploding? I imagine this is a possibility in the microwave, but has anyone had a sweet potato explod in the oven?

To wrap, to oil, or neither?

After I saw how Derek had baked the sweet potatoes I remembered that I usually wrap mine individually in aluminium foil. I think that keeps some of the moisture in and they steam as well as bake, which causes them to cook faster. It certainly prevents a mess. According to Serious Eats, wrapping them in foil also keeps the outer layers from overheating as they roast. (See the section below on temperature.)

Cook’s Illustrated, however, says that instead of wrapping them individually, you should rub each sweet potato with about half a tablespoon oil, in order to guarantee a soft, caramelized skin. But half a tablespoon of oil? Really? Seems unnecessary. I never rub my sweet potatoes with oil, as I think the tin foil keeps the skin moist enough, and I like to add butter or coconut oil or nut butter to my sweet potatoes. But if you’re not going to wrap them in tin foil then maybe it’s a good idea, as otherwise the skin (which is normally my favorite part of the sweet potato) gets kind of dried out and hard and not so tasty. Alternatively, you can rotate your potatoes while they are baking. The part touching the pan usually gets moist and gooey with sweet potato sugars, so if you rotate them maybe no one side will get too dried out.

To turn or not to turn?

CI says not to turn the potatoes during baking, as turning will keep the bottom skin from caramelizing. But I usually do turn my sweet potatoes once or twice, in order to get more of that nice caramelized bottom skin.

What temperature and for how long?

CI says that at 400-degrees an oiled 2-pound sweet potato should cook in about 40-50 minutes. While many online recipes agree that you should set the oven at 400 F, a few reliable sources say to cook the sweet potatoes at a lower temperature.

Harold McGee, in On Food and Cooking, says that there’s an enzyme that converts starch to sugar which works best when the sweet potato is between 135°F and 170°F. So for maximal sweetness you should keep the sweet potato in that temperature range for as long as possible. Some people, like this the Kitchn author, interpret that to mean that it’s best to bake at 350°F or even lower. I wonder how long that will take? This Serious Eats recipe suggests  roasting sweet potatoes at 300°F (150°C) for a couple of hours. I’ve done this before and they come out great. It’s also pretty fullproof, as unlike at 400 F you can’t really burn your sweet potatoes. The recipe says to bake for 2 hours but I’ve left them for 3 or more hours I think with no noticeable negative effects. I sometimes put the sweet potatoes in the oven in the late afternoon, then take Alma to the park or go shopping. When we come home they’re ready to eat for dinner!

Other tips

It’s a good idea to throw some sweet potatoes in the oven whenever you’re baking something else on a low temperature. It saves energy and gives you a nice healthy, ready-to-eat snack to keep in the fridge.

Unlike for a cake or more finicky recipe, you don’t need to finish pre-heating the oven to put them in. Just put them in whenever they’re ready, even if the oven isn’t up to temperature yet.

If you don’t wrap your sweet potatoes in tin foil, definitely cover your baking sheet with tin foil or parchment paper. Otherwise you’ll have a sticky, gooey, burnt mess on your hands.

Sweet potatoes benefit from the addition of some fat, both for taste and for nutrition. (You need some fat in any sweet-potato-containing meal, in order to absorb the beta carotene.) Butter is traditional but I also really like coconut oil or nut butters like peanut or almond butter. I usually also add salt and cinnamon to my sweet potatoes.

Whatever you do, don’t throw away the skin. It’s full of minerals, and (more importantly) it’s delicious! It’s as if a sweet potato comes with a built-in dessert. Yummm.