A science-based approach of getting the best out of your sweet potatoes. [Photographs: J. Kenji Lopez-Alt]
Have you ever wondered why sweet potatoes are so darn insecure? They come to the table hidden under a blanket of marshmallows or a sticky sweet layer of sugary syrup. You eat them and taste cinnamon and nutmeg, the natural toffeelike aroma of the sweet potato buried underneath layers of spices. I often feel like sweet potatoes are that friend who you just want to sit down to tell them they don't need all that makeup or fussy fashion to be at their best.
Poke around and you'll find streusel-topped casseroles that read more like dessert. You'll find recipes that call for a whopping full cup of sugar. You'll bump into recipes that hide behind a mask of cream and butter. What you won't find much of is a real testing-based approach to making mashed sweet potatoes in a way that brings out the best in them with minimal extra ingredients and a bit of good technique.
It's time to say goodbye to the days of hiding sweet potatoes behind a mask of sugar and bolted-on marshmallows. What we have here is a technique for making mashed sweet potatoes that are so sweet, rich, and packed with sweet potato flavor, they need only the simplest of embellishments to shine.
The Science of Sweet
I've talked through some of this science in the past in this article about the best roasted sweet potatoes, but it's worth a recap, as it all applies here as well.
As all of you highly educated and well-groomed Serious Eaters probably already know, the thing that we call a yam in the U.S. is not in fact a yam. A true yam is a gigantic, starchy, sticky root from a large grass-like 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 U.S.
Up here, the things we call yams are in fact 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 less sweet and starchier 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 commonly available variety in the U.S. They have a higher water content and sugar content than dry sweet potatoes, and cook up creamy and rich rather than fluffy.
The latter is the one we're interested in roasting today.
Sweet potatoes, like a few other starchy tubers and gourds, have a secret hero super-power locked within them. By unlocking that superpower, you can actually make a sweet potato taste sweeter without adding any extra sugar to it.
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 that 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 and 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.
As Harold McGee says in On Food And Cooking:
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 at around 135°F, and it stops when the rising heat denatures it, at around 170°F.
You see where we're going here? By holding a sweet potato in that, ahem, sweet spot between 135° and 170°F for an extended period of time, you can actually induce it to naturally convert its starches into sugars.
To test this, I started with a half dozen sweet potatoes. Each one I cut into quarters. I left one of those quarters carefully wrapped in the refrigerator. The others I encased in vacuum-sealed bags and held in a water bath set to 145°F for one hour, 6 hours, and 24 hours respectively*. After that, I roasted them with a bit of olive oil in a hot oven until tender.
*For the record, most vegetables will never soften at 145°F as pectin, the main structural compound in a vegetable's cell wall doesn't break down until above 180°F or so. The sous-vide sweet potato quarters were still as hard feeling as their refrigerated counterparts.
There's a direct correlation between how sweet the potatoes taste, how well they brown, and how long they've been held at 145°F. Though the sous-vide potatoes actually seemed to lose a little pigment (the flesh was not as bright orange as the refrigerated potatoes), the flavor difference was immediately apparent. A little more testing revealed that the most prominent effects occur within the first couple of hours. Any extra time spent in the bath is incremental.
If you've got yourself a good sous-vide rig, then the process here is simple: throw your whole sweet potatoes directly into a 145°F water bath, let them rest for a few hours, then roast and mash them. But I wanted a method that would take advantage of this phenomenon without the need for any fancy equipment. I turned to the oven instead.
Slow Roast for the Most Maltose
An oven is not as precisely controlled as a sous-vide water bath, but I figured that with a low enough temperature I should be able to give the potatoes a good long stay in that 135 to 170°F sweet spot. Roasting potatoes 300°F for a couple of hours was certainly an improvement over a more standard hour in a 375°F oven, but it still wasn't ideal—the exteriors of the potatoes just get too hot too fast.
Much better was to wrap the potatoes in a tightly sealed foil pouch. This traps in any escaping moisture, which in turn prevents the outer layers of the potatoes from overheating as they roast. You end up with more evenly cooked and, more importantly, sweeter sweet potatoes that are incredibly easy to peel. By placing the potatoes directly in a cool oven and allowing them to heat as the oven preheats, you can also enhance their enzymatic sweetening.
The foil pouch method also allows you to add aromatics as you're roasting. A few sprigs of thyme is a classic pairing.
If you're hell-bent on the absolute best sweet potatoes and don't have a sous-vide circulator, you can easily use the beer cooler method. Just fill a cooler with water at around 170°F, add your sweet potatoes whole, cover the lid, and let them sit for a couple hours before packing them in foil pouches and roasting for the recipe.
Brown is Better
With perfectly sweet, tender, and easy-to-peel sweet potatoes, all that was left was a bit of fine-tuning. Thanksgiving is not the time to be shy with the butter, and there was no doubt that these potatoes would be getting some. I'd brought an early test batch of these potatoes over to a friend's place for an early Thanksgiving meal (we do a LOT of early Thanksgiving's 'round here) and he commented on the sweet potatoes asking if they contained any brown butter.
Nope, but by gum, that's a fantastic idea! The potatoes themselves get a rich, almost toffee-like complexity to them due to the slow-roasting. Adding butter that's been cooked down until its milk solids just begin to turn brown and nutty accentuated that flavor even more.
Some folks like to add cinnamon, nutmeg, or other spices to their sweet potatoes. My theory is that they're just overcompensating for sweet potatoes cooked in a way that doesn't bring out their best flavor. We don't have that problem here. The only other additions I made before whipping them up was some salt, a little fresh thyme to complement the sprigs that I'd placed in their foil pouch, and a small dash of maple syrup to accentuate that caramel flavor even more.
What you end up is rich and complex, with a natural built-in sweetness that is far more complex and satisfying than anything that extra sugar or a marshmallow topping could ever get you. These are sweet potatoes for true sweet potato lovers.
Sweet potatoes? It's your day to shine. Go out there and show 'em how it's done.