The Key to Sweeter Sweet Potatoes

Ever roast sweet potatoes just to have them turn out…not that sweet? Yep, me too.

There is a trick to super-duper sweet, sweet potatoes. And no, it’s not adding maple syrup and marshmallows.

It’s all about how you cook them.

Sweet Potatoes’ Magic Enzyme

Sweet potatoes are full of starch. And starch is made from sugar! So the more starch, the sweeter the sweet potatoes, right? Well, not exactly.

Think about a starch-filled normal potato (like a Russet) and compare that to a less starchy one (like Red potatoes). If starch was the reason for sweetness, Russets should be like walking into a candy store. 🍭 But they aren’t. 😢

So why is a sweet potato, sweet then?

Starch has to break down into simple sugars for it to taste sweet. And sweet potatoes have a naturally occurring enzyme that make this happen.

However, here’s the kicker. That enzyme is only active between 135 and 170°F (57 and 77°C).

Freeze Time for Sweeter Sweet Potatoes

So if you want a sweet, sweet potato, you have to cook it until it reaches 135–170°F (57–77°C). And you want to keep sweet potatoes in that range for as long as possible.

That will maximize how sweet the potatoes taste.

In The Food Lab, Kenji Lopez-Alt tested three preparations of roasted sweet potatoes. Here’s what he did to each group before roasting them in a 350°F (175°C) oven for about 30–40 minutes:

1. Group 1: Nothing
2. Group 2: Par-cooked them in a 150°F (65°C) temp-controlled water bath (sous vide) for 1 hour
3. Group 3: Par-cooked them in a 150°F (65°C) temp-controlled water bath overnight

Here’s what he discovered: The par-cooked potatoes tasted sweeter! Hahah, science! 🤓

But that’s not all. They browned better too! All the sugar helps them caramelize faster.

The nice thing is that Kenji didn’t notice a big difference between the ones par-cooked an hour vs overnight. So the extra time isn't worth it. 😎

So you don’t forget it, it’s worth repeating.

Maximize the time your sweet potatoes spend between 135 and 170°F (57 and 77°C).

Did you know that if you find a “yam” in the US, odds are that it is actually not a yam but a sweet potato! True yams are really only found in Africa, South America, and the Pacific Islands.

No Fancy Temperature-Controlled Bathtub? 🛀

Don’t have a sous vide machine? (Until Christmas, I didn’t either. Thanks, sis!)

All good! I’ve got two techniques for you to try that make it easier to max out the time the sweet potatoes are in that magical range.

The first one I learned from Cook’s Illustrated: Start your sweet potatoes in a cold oven.

1. Add oiled, seasoned sweet potato rounds to a sheet pan and then cover them with foil.
2. Place the pan in a cold oven and then turn up the heat to 425°F (220°C)—starting with a cold oven increases the time the potatoes are in that magical 135–170°F (57–77°C) range.
3. About halfway through (~30 minutes), remove the foil—once steam escapes, the potatoes brown better.
4. Then finish roasting them for another 30–40 minutes until browned, flipping part way through.

This technique works well if you’re mashing sweet potatoes too. The difference is leaving the sweet potatoes whole when you throw them in the cold oven. They’ll take longer to cook when whole so leave them covered the entire time. In fact, try wrapping the potatoes in a foil pouch. This keeps more moisture in, which will help cook the potatoes evenly and prevent them from drying out.

The next technique is courtesy of Kenji in The Food Lab: Create a makeshift sous vide using hot water and a heavy pot.

1. Take out a handy dandy frying thermometer and heat up four quarts of water to 175°F (80°C) in a heavy Dutch oven—you want to use a heavy pot and lid that maintains heat well because of step 3.
2. Add a few pounds of cut sweet potatoes into water and put the pot in your oven with the light on—the oven light will create a warm environment that helps maintain the water temperature.
3. Then leave the potatoes a couple of hours in the water bath—this gives the sweet potatoes plenty of time in that 135–170°F (57–77°C) range.
4. Then drain the potatoes and let dry.
5. Finally, roast them in a 400°F (205°C) oven after mixing with oil and seasoning for 30–40 minutes until they are browned—flip halfway through to maximize browning on multiple sides.

Where I learned this: The Food Lab by J. Kenji Lopez-Alt and The Science of Good Cooking by Cooks’ Illustrated.

P.S. Do you know why you can’t get angry at a yam? Because they’re such sweet potatoes! 🥁 I know. I know. I am horrible. Sorry. But I yam what I yam. 😉