Why It Works
- Browning butter packs in tons of flavor, without dulling the sweet potatoes.
- Slow-roasting the sweet potatoes activates endogenous enzymes that bring out their natural sweetness.
- A touch of maple syrup, butter, and a bit of chopped thyme are the only embellishments these naturally sweet sweet potatoes need.
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 only cinnamon and nutmeg, the natural 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 recipes for streusel-topped casseroles that look more like dessert. You'll find recipes that call for a whopping full cup of sugar. You'll bump into dishes 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 good-bye to the days of hiding sweet potatoes behind 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
Sweet potatoes, like a few other starchy tubers and gourds, have a secret hero superpower locked within them. By unlocking that superpower, you can actually make a sweet potato taste sweeter without adding any extra sugar to it. 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.
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 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 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 (57 and 77°C) 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. I cut each one into quarters and left one of those quarters carefully wrapped in the refrigerator. The other three I encased in vacuum-sealed bags and held in a water bath set to 145°F (63°C) for one hour, six hours, and 24 hours, respectively.* After that, I roasted them with a bit of olive oil in a hot oven until they were 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 (82°C) 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 in 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 the potatoes spend 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. 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, put the lid on, and let them sit for a couple of hours before packing them in foil pouches and roasting for the recipe. 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 sweet spot. Roasting potatoes at 300°F (150°C) for a couple of hours was certainly an improvement over the more standard hour in a 375°F (190°C) oven, but it still wasn't ideal—the exteriors of the potatoes just got too hot too fast.
Much better was to wrap the potatoes in a tightly sealed foil pouch. This traps in any escaping moisture, which 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 makes for a classic pairing.
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 over to a friend's place for an early Thanksgiving meal (we do a lot of early Thanksgivings '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 due to the slow roasting. Adding butter that's been cooked down until its milk solids just begin to turn brown and nutty accentuates 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 were some salt, a little fresh thyme to complement the sprigs that I'd placed in their foil pouches, and a small dash of maple syrup to accentuate that caramel flavor even more.
What you end up with 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.
- 4 pounds (1.8kg) moist sweet potatoes, such as ruby or garnet yams (about 4 large potatoes; see note)
- 8 sprigs thyme, divided
- 6 tablespoons butter (3 ounces; 85g)
- 1/4 cup (60ml) maple syrup
- Kosher salt
Adjust oven rack to center position. Place 2 large sheets of heavy-duty aluminum foil on a work surface. Working with 1 sheet at a time, place half of potatoes in center along with 3 sprigs of thyme. Fold up foil and crimp edges to seal tightly. Repeat with other half of potatoes. Transfer pouches to a rimmed baking sheet and place in oven. Set oven to 300°F (150°C). Roast until a thin skewer inserted into potatoes meets no resistance, about 2 hours. Remove potatoes from oven and set aside until cool enough to handle.
Meanwhile, melt butter in a medium saucepan over medium-low heat, swirling gently, until particles are pale golden brown and smell nutty. Immediately transfer to a large bowl or the bowl of a stand mixer fitted with a whisk attachment. Add maple syrup. Pick leaves off of remaining 2 thyme sprigs, roughly chop, and add to bowl.
Peel sweet potatoes and discard skins. Add flesh to bowl with butter and syrup. Beat with whisk attachment or a handheld mixer until smooth and fluffy, about 2 minutes. Season to taste with salt. Serve. See note for make-ahead suggestions.
Sweet potatoes can be made even sweeter by pretreating them in a water bath to activate their enzymes. To do this with a sous vide–style circulator, set your circulator for 150°F (66°C). Place the sweet potatoes in the water bath and let circulate for 2 to 4 hours before proceeding with the recipe. To do this with a beer cooler, fill your cooler with water at 170 to 175°F (77 to 79°C). Add the potatoes, cover, and let rest for 2 to 4 hours before proceeding.
To Make Ahead: Sweet potatoes can be refrigerated for up to 5 days. To store and reheat, transfer the sweet potatoes to a heavy-duty zipper-lock bag with the air pressed out. Reheat by completely submerging the zipper-lock bag in a pot full of hot (not boiling) water, removing the bag occasionally to squeeze contents around, until fully reheated, about 30 minutes. The best way to do this is with a sous vide–style circulator set at 150°F.