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Okay, so sweet potatoes are sweet, but they're not that sweet, right? I mean, sure, you could add maple syrup or honey, and marshmallows on top, but I wouldn't wish one of those monstrous casseroles on my worst enemies, let alone my own family (though, come to think of it, there's some pretty significant overlap between those two groups).
Much better than those casseroles are really well-roasted sweet potatoes. At their best, they're creamy, flavorful, and sweet, with a slightly crisp, caramelized crust. Too often, though, they end up mealy, starchy, and bland. How can the same vegetable produce such different results? How does one get a sweet potato to really live up to its name? We'll talk about that in a second, but first, a few words on sweet potatoes.
I Yam What I Yam
So, as all of you highly educated and well-groomed Serious Eaters probably already know, the thing that we call a yam in the US is not, in fact, a yam. A true yam is a gigantic, starchy, sticky root from a large, grasslike plant native to Africa. These days, they're mostly found in Africa, South America, and the Pacific Islands. Very rarely do they make their way to the US.
Up here, the things we call "yams" are actually a type of sweet potato, a different plant entirely. Sweet potatoes come in a few different varieties, but can basically be broken down into two groups that behave differently when cooked.
- Dry sweet potatoes, like the white-fleshed American sweet potato or Okinawan purple potatoes, are starchier and less sweet than moist sweet potatoes. They turn fluffy when cooked, and in many recipes can act as a good substitute for normal potatoes, albeit with a unique flavor all their own.
- Moist sweet potatoes, like Garnet or Ruby yams, are the more widely available variety in the US. They have a higher water content and sugar content than dry sweet potatoes, and they cook up creamy and rich, rather than fluffy.
The latter is the one we're interested in roasting today.
Here's the deal: Starch is made from sugar. More precisely, starch is a polysaccharide, which means that it's a large molecule consisting of many smaller sugar molecules (in this case, glucose). The thing about sugar is, unless it's broken down to relatively simple forms, it doesn't taste sweet to us. Our tongue simply doesn't recognize it.
It helps to imagine sugar molecules as a bunch of cartoon kids. When they're all standing in a row, it's easy for us to identify them as individual kids. But stack them up on each other and throw a trench coat on 'em, and they're effectively hidden.
Now, sweet potatoes contain plenty of starch molecules. The goal when roasting them is to try to break down as many of the starch molecules as possible into sweet-tasting maltose, a sugar consisting of two glucose molecules. Pull off the trench coat and give that little stack of kids a push, if you will. We do this with the help of enzymes.
Moist sweet potato varieties sweeten during cooking thanks to the action of an enzyme that attacks starch and breaks it down. The enzyme starts to make maltose when the tightly packed starch granules absorb moisture and expand, beginning around 135°F, and it stops when the rising heat denatures it, at around 170°F.
So, essentially, the longer a sweet potato spends in that zone between 135 and 170°F (57 and 77°C), the sweeter it becomes. To test this, I cooked three batches of potatoes. The first I popped directly into a 350°F (180°C) oven and baked until tender. The second I par-cooked in a temperature-controlled water bath at 150°F (66°C) for one hour before baking. The last I par-cooked in the same water bath overnight before baking.
Harold's right. You can immediately see that the par-cooked potatoes browned better, indicating a higher sugar content that allowed them to caramelize faster. This was also reflected in the flavor: The par-cooked potatoes were significantly sweeter and more flavorful than the plain roasted potatoes, which were starchy and bland. Interestingly, the hour-long par-cooked potatoes were nearly as flavorful as the overnight potatoes, which means that, to make the most of this effect, you've really only got to cook them for an hour at 150°F.
If you've got a sous vide device, the path to better sweet potatoes is an obvious one. Just bag your potatoes; cook 'em as long as you'd like at 150°F (any higher, and I've found they soften too much before roasting); then pop 'em in the oven the next day while your turkey is resting.
But what about the rest of us?
There are a couple of options. You could always go the beer-cooler sous vide route. It's cheap and effective, and it will easily hold the proper temperature for the requisite hour. Just put your potatoes in a zipper-lock bag with the air squeezed out, then place them in a cooler filled with water at 150°F. Close the top, wait an hour, and you'll be good to roast.
The good thing about sweet potatoes is that they're less finicky than, say, a steak, which means that you don't have to worry about getting the temperature exactly right. In fact, as long as your water's above 135 and below 170°F, it'll have a positive effect on their flavor.
Here's the easiest way to do it: Bring three quarts of water to a boil in a large pot. Add one quart of room-temperature water. This should bring your water down to around 175°F (79°C). Add a few pounds of sliced or diced potatoes to that water, and it'll come down to well within the requisite range. Pop a lid on the pot, keep it in a warm part of your kitchen, leave it there for a couple hours, then simply roast at your leisure. Your mouth will thank you, if the rest of your family doesn't as well.