There’s no denying it: this tart is purple. It’s not lavender or rose, it’s purple. The color didn’t come from a little dropper bottle or baggie of magic powder, it came from purple potatoes. To be specific, Okinawan sweet potatoes.
After seeing the little Chinese grannies at HT Market buying gai lan (good choice) and garlic chives (even better choice), I knew that they couldn’t be wrong about the “Hawaiian sweet potatoes”, or imo (in Japanese). Those ladies know what they are doing! Sometimes I work up the courage to ask them how to cook something (“stir-fry with a little oyster sauce”). Sometimes I just put the strange looking fruit/vegetable into my basket and go about my business.
If you haven’t started buying the some of the stuff that the little Chinese grannies pick up at your nearest product stand or ethnic market, you’d better get on it. Okinawan sweet potatoes may be harder to find than regular sweet potatoes, but are worth seeking out. The bright purple flesh isn’t all just for show either, there’s some serious nutrition behind this root vegetable. The violet pigment seen in Okinawan sweet potatoes (as well as berries, eggplant, flowers and anything else purple found in nature) comes from anthocyanins, which are powerful antioxidants that not only provide protection from free-radical damage and offer protection against harmful UV rays, they also attract hungry animals and pollinators. Scientists are studying anthocyanins to fully understand the effects they have on our bodies.
Food science geek alert: the color of anthocyanins is very pH sensitive. In acidic environments, they appear more red and in basic environments they appear more blue. That’s why one usually adds apple cider vinegar to red cabbage for a lovely rosy hue—blue food can be off-putting sometimes. Try adding vinegar and baking soda to glasses of grape juice and you’ll see what I’m talking about!
Suitable for roasting, mashing, frying, baking, soups and more, (hmm…kinda like a sweet potato?) Okinawan sweet potatoes lend themselves and their beautiful purple hue to countless preparations. Like dessert. Like a sweet potato tart with a smooth filling and crumbly chocolate shortbread crust made with coconut oil. If you haven’t sensed a vegetables-in-dessert theme on this blog yet, then keep reading.
Nutrition Focus: Okinawan Sweet Potatoes
- Good source of dietary fiber, shown to help with satiety, reduce blood cholesterol and control blood sugar levels.
- Low on the glycemic index, meaning glucose (blood sugar) will be released more slowly and steadily than foods high on the glycemic index. This is beneficial to everyone, but especially for diabetics and others whole need to watch their blood glucose.
- High in anthocyanins (antioxidants), which may help protect against cancer.
- Vitamin A to support cellular function and boost immune system.
- Vitamin C to protect cells from free-radical damage and improve iron absorption.
Okinawan Purple Sweet Potato Tart
The crust is inspired by Dorie Greenspan’s chocolate shortbread tart dough in Baking: From My Home To Yours, which is a true pleasure to drool over. My favorite brand of coconut milk is Chaokoh, which can be found at your nearest Asian supermarket or online.
For the dough:
¾ cup (90 grams) whole wheat pastry flour
½ cup (65 grams) all purpose flour
¼ cup (25 grams) cocoa powder
¼ cup (30 grams) powdered sugar
¼ teaspoon (1.5 grams) salt
9 tablespoons (130 grams) coconut oil, frozen and cut into small pieces
1 large (17 grams) egg yolk
For the filling:
4 medium (850 grams) Okinawan sweet potatoes
3 large (150 grams) whole eggs
3/4 cup (150 grams) granulated sugar
¼ teaspoon (1.5 grams) salt
1 teaspoon (4 grams) vanilla extract
½ cup (120 grams) coconut milk
Optional, for serving:
sliced mango, berries, kiwi, papaya or other seasonal fruit
Make tart dough. Pulse together the dry ingredients for the dough until combined. Scatter the coconut oil over the dry ingredients and process in 1-second pulses until the oil is in bits the size of peas. Break open the yolk in a dish and pour about half of it over the dry ingredients, then pulse a few times to combine. Add the remaining yolk and continue to pulse until the dough forms large clumps.
Bake the tart dough. Gently pat the dough into a 9-inch fluted tart pan. Freeze the tart shell for 30 minutes or wrap 3 times in plastic wrap and freeze for up to 2 months. Center a rack in the oven and preheat to 375°F. Prick the dough with a fork, cover with foil and bake for 25 minutes. Reduce the heat to 350° F and leave the oven on. Remove the foil and transfer the crust to a rack to cool.
Make the filling. Peel the sweet potatoes and cut into chunks. Place in a steam basket over boiling water and cook until tender, about 12 minutes. While the potatoes are hot, pass through a ricer or a food mill into a large bowl. Add remaining filling ingredients and whisk together until smooth, then pour into the par-baked tart shell.
Finish the tart. Bake the tart for 45 minutes, until the filling is set and a toothpick inserted in the center comes out clean. Remove the tart to a rack to cool completely. To serve, cut into wedges and garnish with a dusting of powdered sugar and fresh fruit.