Why It Works
- Freezing sweet potatoes prior to roasting causes the water inside the sweet potato to crystallize without breaking the skin, which yields an extraordinarily soft and fluffy texture.
- Roasting the sweet potato from frozen at a lower initial oven temperature gives it plenty of time in the sweet spot for a natural enzymatic conversion of complex starches into sweet simple sugars.
- Finishing the sweet potato at a high temperature allows for a hint of complex caramel flavor to develop.
- Whipped creme fraiche and a sprinkling of granola each contrast the roasted sweet potato's sweetness and softness.
I grew up in Hong Kong, so when winter arrives, I find myself nostalgic for the sweet potato vendors who used to suddenly turn up on every street corner when it got cold. I remember walking through the streets, smelling wafts of wok-roasted sweet potatoes, chestnuts, and quail eggs from blocks away. On one end of these carts there'd be a massive wok set over coals, filled with hot pebbles and sweet potatoes, which the vendors would constantly flip to ensure an even roast. They'd hand over a paper bag with whole, piping hot, roasted sweet potatoes inside, the pillowy and bright flesh protected only by a brittle skin. It’s the perfect snack.
One secret to recreating the concentrated and sweet flavor of those whole roasted sweet potatoes is to use the Northern Chinese practice of freezing them before roasting. As the sweet potato freezes, ice crystals form within the cell, slowly rupturing cell walls, transforming the texture of the flesh. As the sweet potato roasts, the water that leeches out of the cells turns to steam, eventually bursting through the skin so that the sugary juices leak out of the sweet potato as a gooey caramel. At that point, the properly roasted sweet potato is done.
As Tim Chin has written, based on his own tests (in which he found, for the record, that this method produces the very best whole roasted sweet potato), there's probably even more going on with the freeze-roasting process than just a textural transformation. Cooking the sweet potato from frozen also likely allows it to spend more time in a critical temperature zone from 135° to 170°F (57° to 77°C), when an enzyme in the sweet potato called amylase becomes very active, converting the tuber's complex starches into the (very sweet) simple sugar maltose. This is a phenomenon Kenji López-Alt also takes advantage of in his diced roasted sweet potato and mashed sweet potato recipes.
The resulting sweet potato is so good you can eat it out-of-hand, exactly how it's enjoyed as a street snack in Hong Kong. But it can also be dressed up.
Here, I serve the roasted sweet potato with a light but thick whipped cream that's slightly tart due to the inclusion of crème fraîche. For a finishing touch, I sprinkle granola on top. This dessert is really an homage to a sweet potato pie, with the granola acting as a stand-in for the pie crust. Both the whipped cream and the granola provide contrast for the sweet potato: the tanginess provided by the crème fraîche gently complements the potato's sweetness, and the granola adds pops of crunch to offset the silky softness of the flesh.
- 4 sweet potatoes (about 1 pound; 450g each), scrubbed (see note)
- 1 cup (240ml) heavy cream
- 1/4 cup (60g) creme fraiche
- 4 teaspoons (15g) confectioners' sugar
- 1/2 teaspoon vanilla extract
- Pinch ground ginger
- Pinch ground cinnamon
- Pinch kosher salt
- Homemade or store-bought granola, for topping
Place sweet potatoes in freezer until frozen solid, at least 6 hours and up to 3 months.
Line a rimmed baking sheet with aluminum foil. Wrap each frozen sweet potato in aluminum foil and seal tightly so that there are no loose seams. Place wrapped sweet potatoes on prepared 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. (Note that roasting times may vary significantly depending on the size and shape of the sweet potatoes.)
Remove baking sheet from oven and carefully unwrap potatoes; discard aromatics, if using. Increase oven temperature to 450°F (232°C) and return unwrapped sweet potatoes to oven. Roast until sweet potatoes are fragrant and sugars begin to caramelize, about 40 minutes. Let roasted sweet potatoes stand until cool enough to handle.
While sweet potatoes cook, using a stand mixer fitted with the whisk attachment, whip the heavy cream with the crème fraîche, vanilla, ginger, cinnamon, and salt until soft peaks form; alternatively, hand-whisk the ingredients in a large mixing bowl until soft peaks form.
Split open the sweet potatoes and arrange on plates. Spoon the whipped crème fraîche on top and sprinkle granola over. Serve immediately.
Stand mixer (optional), rimmed baking sheet.
This recipe works equally well with dry or moist sweet potato varieties. Dry sweet potatoes such as Okinawan purple or Hannah varieties will be fluffy and less sweet. Moist sweet potatoes such as Jewel, Garnet, or Beauregard varieties will taste juicier and sweeter. My favorite, however, are the purple fleshed varieties that are small, roast quickly, and end up sweeter and fluffier due to their higher moisture content.
It is possible to speed this recipe up by partially freezing the sweet potatoes for at least 45 minutes; you will still get some of the benefit of the freezing process. You can also speed up the cooking time by freezing and then thawing the potatoes prior to roasting. Once thawed, you’ll notice that the sweet potato is entirely soft, almost as if it’s already been cooked; fully thawed sweet potatoes will take about 45 minutes at 450°F (230°C).
Make-Ahead and Storage
Frozen sweet potatoes can remain in the freezer for 10-12 months. However, sweet potatoes frozen for overnight or longer will either need to be thawed for at least 30 min prior to roasting or roasted for longer until the flesh is totally soft.
Fully roasted sweet potatoes can also be frozen after roasted and eaten cold.