sweet potatoes

Hello, I’m Andrea Castillo and this is Seasonal, a newsletter that connects you to the Bay Area food system, one fruit and vegetable at a time. Each post highlights the most interesting aspects of one seasonal fruit or vegetable grown and harvested around the Bay Area. Find me on Instagram at @seasonalbayarea or e-mail me at seasonal@substack.com to share your feedback and suggestions, I’d love to hear from you. If you’d like to unsubscribe, click the link at the bottom of this post.

This week we explore sweet potatoes (peak availability September-December).

Sometimes the best way to understand what something is, is to start from what it isn’t. And the best way to nerd out on a sweet potato is by starting here: a sweet potato is not a potato. It is not a potato that happens to be sweet. It is not even closely related to a potato. Not a distant auntie. Not a twice-removed cousin. Potatoes, one of the members of the nightshade family, have more in common with tomatoes, peppers, and eggplants than they do with sweet potatoes.

If a sweet potato has nothing to do with the potato, is it then a yam? In the United States, orange sweet potatoes and yams are often confused, but a sweet potato is also not a yam. You may see “yams” for sale at markets but there is a high chance those “yams” are actually sweet potatoes because real-deal yams are uncommon in American grocery produce sections. Yams are members of the Dioscoreaceae family, can be as large as a human arm, and do not taste sweet like sweet potatoes.

what it is, what it does, what it is, what it isn't

What, then, is a sweet potato? Sweet potato plants are vines that belong to the Convolvulaceae, or morning glory family. There are thousands of varieties of sweet potatoes, and about 100 are grown commercially in the United States. Those varieties are usually divided into two main groups: dry and moist. When cooked, dry sweet potatoes tend to taste mild and fluff up, while moist sweet potatoes are sweeter and denser.

Unlike potatoes and yams, which are the edible stems of those plants, sweet potatoes are the freakishly large edible roots of the sweet potato plant. These engorged roots store loads of nutrients and energy that sustain the plant and nourish us with carbs (starches and sugars), fiber, potassium, and vitamins. For a root, sweet potatoes are quite versatile in the kitchen, and we eat them as chips, fries, wedges, and hasselbacks, and in pies, mashes, and casseroles.

Sweet potatoes may be edible roots, but rooted they are not. Apparently, sweet potatoes aren’t homebodies. Itching to move and expand, this not-a-yam-and-not-a-potato vegetable has managed to uproot itself throughout the course of natural and human history.

what’s the story, morning glory?

Up until the 21st century, everyone thought European colonizers introduced sweet potatoes to the world after taking them away from their home turf in Central and South America. But there was a strange sweet potato remnant in Polynesia that dated back to approximately 1,100 A.D.—400 years before European colonizers even set foot in the Americas—and researchers couldn’t explain how it got there. That thousand-year-old remnant has fueled a debate questioning the trajectory of sweet potatoes that has not been settled for decades. In 2013, researchers examined sweet potato specimens Captain James Cook obtained from New Zealand and the Society Islands from his 1769 voyage. The genes from these sweet potatoes had not yet been muddled by the genes from the sweet potatoes the Spaniards and Portuguese were introducing to the rest of the world and researchers were finally able to obtain genetic evidence that traced Polynesian sweet potatoes back to Ecuador and Peru.

How did those sweet potatoes find their way across the Pacific? One theory is that sweet potatoes, or kumara/cumal in Quechua, originated in Central and South America and that Polynesians sailed across the Pacific and brought back sweet potatoes, or kuumala in Polynesian, with them (notice that linguistic similarity there?). This would’ve been a long, insane journey, but not impossible. Or… maybe sweet potatoes found their way to Polynesian islands from Asia and did not originate in the Americas after all.

Sweet potatoes descended from ancient morning glories. We’ve always thought sweet potatoes originated in the Americas because there’s ample evidence of 4,000 year-old sweet potato remains found at sites in Mexico and Peru, and fossils suggest morning glories took root in North America 35 million years ago. In 2018, however, a group of researchers discovered a 57 million-year-old morning glory fossil in India and questioned that origin story. If sweet potatoes descended from a morning glory ancestor and originated in the Americas, what the heck was the oldest known morning glory fossil doing in India? Could the sweet potato have originated outside the Americas?

There is no strong evidence to demonstrate it did—morning glory fossils are rare and fragile—especially because the fossil found in India is not a sweet potato. Still, it opens up the possibility that Asia could be the birthplace of sweet potatoes and that instead of migrating on ancient Polynesian canoes, these roots migrated naturally, over the course of millions of years from Asia, to Polynesia, and then the Americas. For now, it seems that no one knows the story, morning glory, and the sweet potato origin debate rages on.

wild thing

At first glance, it may seem like a trivial debate, but sweet potatoes are one of the most valuable crops in the world—up there with wheat, rice, corn, and cassava—because of their high yields, relative ease of growing, and good nutritional value. The orange variety is one of the top sources of beta-carotene (which our bodies convert into Vitamin A, just like they do when we eat an orange honeydew melon), the purple sweet potato is also rich in anthocyanins—powerful antioxidants also found in blueberries—and is a major source of subsistence in food-insecure countries.

Extreme heatwaves from climate change, however, threaten the future of sweet potatoes and people’s access to an important source of nutrition. Taming wild sweet potatoes, i.e. domesticating them, has reduced the diversity in the sweet potato gene pool. With sweet potato crops, long-lost wild cousins may have genes that are particularly adept at enabling sweet potatoes to thrive in spite of dramatic temperature changes. If sweet potatoes did in fact originate in Asia, those older, wild Asian varieties could help researchers find biodiverse genes and incorporate them into future sweet potato varieties. With more genetically diverse sweet potatoes in hand that blend a bit of the new with a bit of the old, farmers in future could grow sweet potato crops that are better equipped to thrive in harsher climates.

hasta la raíz

What’s odd about sweet potatoes is that humans have grown them for about 8,000 years, but they never thrived in North America until relatively recently. Europeans uprooted sweet potatoes from Central and South America, brought them back to Europe, and then sailed across the Atlantic again to establish them as a crop in Virginia in 1648 (or as early as 1610). In fact, indigenous peoples in North America did not eat sweet potatoes until European colonizers arrived. According to ethnobotanical research, two southeastern North American tribes, Cherokee and Seminole, are the only ones that adopted sweet potatoes into their traditional diet—a new culinary staple for people who had been living and eating in North America for 14,000 years.

Sweet potatoes also became part of a new eating tradition for the people in North America who were brutally uprooted from their kingdoms and villages throughout West Africa and enslaved for centuries in Portuguese, Spanish, French, and British colonies in the present-day southeastern United States. With many of their culinary traditions no longer accessible, enslaved West African people and subsequent generations born into slavery sought to retain parts of their identity, culture, and knowledge through food. Sweet potatoes—by no means a perfect substitution—had to make do instead of yams, a staple starch in the West African diet.

Foragers unearthed wild yams throughout West Africa as far back as 25,000 BCE. About 5,000 years ago, they tamed the wild yam, domesticated it, and turned it into a key crop. Not only did yams feed the yam belt population (stretching from present-day Ghana to Nigeria) for millennia, they also became important social, religious, and cultural items. For example, the yam sits at the center stage of the New Yam Festival, a thanksgiving ritual rooted in the traditions of the 9th century Kingdom of Nri, the ancestral clan of the Igbo in Nigeria, one of the main ethnic groups enslaved by the British during the trans-Atlantic slave trade.

Yams were often the only food available on slave ships, but for those who survived the horrendous journey, those yams were the last they ever had. Yams never took root in North America as a major crop, perhaps because of how difficult they are to grow. However, these new shores had an unfamiliar food: sweet potatoes. As culinary historian and James Beard award winner Michael Twitty puts it, “when Africans encounter sweet potatoes as enslaved people they utilized them in ways similar to tropical yams.” A common way enslaved people prepared sweet potatoes was likely inspired by the traditional way of roasting yams: they placed them directly in hot coals, wrapped in cabbage or other types of leaves.

The roasted sweet potato texture offered some semblance to the longed-for yams. But the resulting flavor wasn’t quite right and took some getting used to. The fatal flavor flaw that downgraded sweet potatoes for yam fans was their distinct sweetness. And the thing about sweet potatoes is that the longer they’re cooked at lower temperatures, the sweeter they will taste.

sugar coat it

Why are sweet potatoes sweet? The sweetness comes down to the ability of sweet potatoes to transform their starch into sugars as they cook. This transformation is only possible because of an enzyme that sweet potatoes have but yams and potatoes lack: amylase. 

Approximately half of a sweet potato is starch, which functions like a silo: starch is where plants store their supply of sugar. The starchy root dispenses the energy that enables the plant to grow and survive harsh climate conditions like droughts or cold winters. For humans, starch is also a source of energy. Amylase and other enzymes in our saliva and pancreas convert starches into glucose, the simplest type of sugar our cells like to munch on. Even though starches store sugar, they do not taste sweet when we eat them because they have not been converted into sugar yet—that’s the reason starchy potatoes and yams do not taste sweet. Cooked sweet potatoes, on the other hand, taste sweet because some of their starches have already transformed into sugars before we even take a bite. 

it’s getting hot in here

Just as we have enzymes in our digestive systems that break down starches into sugars, the in-house sweet potato amylase enzymes convert starch into maltose, a type of syrupy sugar. But sweet potato amylases are inactive by default. Temperatures between 135F and 170F create the perfect environment for sweet potato amylases to wake up and divide and conquer. Literally. Higher temperatures weaken bonds that bind starch together and amylase takes advantage of this starchy Achilles heel to break apart starch molecules and create maltose. With enough time in the 155F-ish temperature sweet spot, amylase can break down as much as 75% of starches into maltose, resulting in sweet potatoes that are saturated with a light, molasses-like syrup.

As a home cook, you can use the relationship between temperature, time, and amylase to your advantage. If what you’re after is a sweet potato that isn’t too sweet, then don’t give amylase much of a chance by cooking your sweet potatoes at a high temperature for a short period of time. Boiling them seems to be a good way to reduce the starch-sugar conversion and, though I haven’t made it yet, boiling would work well in a recipe like Cookie and Kate’s savory mashed sweet potatoes.

how sweet it is

If you want a sweeter result, then give that amylase the time and temperature it needs to get to work. Planning and patience will pay off. Flora & Vino’s tandoori chickpea stuffed purple sweet potato, for example, calls for roasting sweet potatoes for two hours at 350F. But the syrupy goodness you get from this method makes it worthwhile: sugars overflow from the sweet potato and pool and caramelize in the foil (full disclosure: I’m into sweet and savory dishes and enjoy maltosey sweet potatoes).

You can also submerge your sweet potatoes in a warm water bath, as Kenji Lopez-Alt suggests, and then roast them to get more of the sweetness and flavor out. I was curious to test this so I roasted four different types of sweet potatoes: Stokes Purple, Golden, Diane, and Beauregard. Two-thirds of the sweet potatoes soaked in warm water for three hours before roasting, one third went straight to the oven without the soak. The warm water bath completely transformed and improved the overall flavor and texture of the Diane, Golden, and Stokes Purple sweet potato, which went from dry and dull to moist and sweet.

The Stokes Purple transformation was so drastic with the warm water bath—it tasted like rose petals!—that I decided to make a bourbon spiced, purple sweet potato pie from scratch. For my first homemade pie, it wasn’t bad but I made the mistake of peeling the sweet potato after the soak, but before roasting it. The stuffing was a bit dry as a result, but next time I’ll keep the peel on as I roast it and add more spices and bourbon. (I crowd-sourced several recipes and combined them into one, if you want the recipe send me an e-mail and I’ll share it with you).

I was also intrigued by steaming. A Zhejiang A & F University and USDA study showed that steaming can yield more maltose than baking for certain varieties of sweet potato, like the Chinese purple Zimei variety. I wanted to replicate part of their experiment as I made Carla Lalli Music’s steamed sweet potatoes with tahini butter with a Red Diane sweet potato. Using a meat thermometer, I kept track of the internal sweet potato temperature during the 45 minutes or so it took to steam. After 20 minutes, the internal temperature reached 135F, kickstarting the amylase. But the amylase party didn’t last long. In just 10 minutes, the internal temperature had already reached 170F and it was time for the amylase to call it a day. Fifteen minutes later, the sweet potato was fully cooked and fluffy and tasted delicious with the tahini butter (and a squeeze of makrut lime).

However, with such a short window for amylase to do its thing, the Red Diane tasted mild, as I expected it would. In contrast, the amylase enzymes in a particularly high-starch sweet potato like the Zimei go nuts when steamed, making the purple Zimei an outlier variety in how sweet potatoes usually respond to different cooking methods. With dozens of sweet potato varieties grown in California, the variety you bring home will have an impact on how mild or sweet your sweet potato turns out.

By the way, those sweet potatoes you bring home should hang out in a cool, dry place, away from onions. Earlier this year I stored them in a mesh bag next to a couple of onions and in just a couple of weeks, they rotted and left a pool of liquid underneath the bag. Lesson learned. Keep them isolated, unwashed (harvested roots don’t do well with the added moisture), and out of the fridge—cold temperatures alter their flavor and harden their flesh. They should last for about a month.

what’s on the inside

Sweet potatoes are generous roots: they have nourished us for thousands of years, demand little from farmers who grow them, and make dishes pop with their bright orange or purple colors. Beyond the farm and kitchen, sweet potatoes bring us together and provide a sense of place. Throughout their round-the-world journey, sweet potatoes, camotes, or batatas provided and connected. Polynesian and Andean civilizations may have been separated by a vast ocean, but one way or another, were connected by this humble morning glory root. African enslaved people endured terrible treatment and conditions, and when so much was lost, sweet potatoes offered a humble hand in claiming a new sense of place and identity. And due to favorable sweet potato growing conditions in the southeastern United States, sweet potatoes have grown there for four centuries and are now a connective fiber in the identity of many in the American South.

Nerding out on sweet potatoes encouraged me to think about my own diverse set of roots in a different way. Roots aren’t tied to a place. We may grow those roots in the different towns or cities where we learned to walk, went to school, or fell in love. But instead of staying anchored in the soil, those roots live within us. Leaving a place isn’t always easy. Sometimes we have no choice but to uproot, sometimes uprooting is a choice. Either way, we leave a lot behind, but we never leave behind our roots. We carry those roots with us, replant ourselves, and start anew.

Thank you for nerding out with me. If you’ve already subscribed, thank you! If this email was forwarded to you and you'd like to sign up for the newsletter, click the button below:

Andrea Castillo is a food nerd who always wants to know the what’s, how’s, when’s, and why’s of the food she eats.