What is the difference between our everyday white sweet potato and the purple sweet potato? Anthocyanins, a vibrant plant pigment that is also a powerful antioxidant.
These are naturally occurring water-soluble pigments that give many of the blue, purple, pink and red fruits and vegetables their colour. These include blueberries, blackberries, red grapes, purple cabbage, purple corn, purple carrots, eggplant and purple sweet potatoes. Butterfly blue pea flower (shankhpushpi or aparajita) is also rich in anthocyanins, which give it the vibrant blue colour.
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Anthocyanins are a unique group of pigments, adding a bit of magic to food with their changing colours. My son often says, “There’s no magic, there’s only science.” There is an interesting bit of high school chemistry behind this magic. This family of pigments turns to a bright pink or red in the acidic state (for example, when lemon juice or vinegar is added to it) and turns blue in the basic state (for example, in the presence of baking soda). This happens due to a shift in their molecular structure, which absorbs a different light spectrum and appears in a different colour to the human eye.
As anthocyanins are water-soluble (except for the ones in the purple eggplant), you should always cook purple sweet potatoes in a bowl kept in a pressure cooker and not directly in water, to ensure the pigments don’t leach out in the water. Another thing to note about purple sweet potatoes is that these are denser and drier than the regular white variety, so give them an extra five minutes in the pressure cooker.
Knowledge of these basic chemical reactions helps us deal with pigmented produce the right way. Purple cabbage, when cooked in water, turns into an unappetising shade of bluish grey. The best way to cook purple cabbage and retain its vibrant colour is to cook it in slightly acidic water, adding lime juice, vinegar or citric acid crystals. Pickled purple cabbage in a salted vinegar solution also retains its brilliant colour.
For me, finding something new in the produce aisle is one of the best ways to jumpstart my creativity in the kitchen. When I recently saw purple sweet potatoes at the local supermarket, I was the human version of the starry-eyed emoji. The craze for trying out exotic produce is not to neglect our own backyard vegetables, but a way to escape the mundane nature of everyday cooking. I loaded my basket with enough to cook different dishes and to let some sprout so I can grow them in my kitchen garden.
I was curious about the origin of purple sweet potatoes. It turns out there is more than one variety. The one from the US is called Stokes Purple (trademark), with a dark purple flesh and a light purple skin. There are two varieties from Hawaii, one with a cream-coloured skin (Okinawan) and another with a purple skin (Molokai). In India, purple sweet potatoes are being grown by Mati farms in Odisha and Krishi Cress in Delhi. I have been buying my stock from Namdhari’s in Bengaluru. Deep purple is my favourite new colour!
Purple Sweet Potato Stir-Fry
2 medium-sized purple sweet potatoes
1 tbsp coconut oil
A pinch of asafoetida
Half tsp mustard seeds
Half tsp cumin seeds
2 dried red chillies
1 sprig curry leaves
1 tsp salt
2 tbsp grated coconut
Scrub and wash the sweet potatoes. Place them in a bowl and keep the bowl in a pressure cooker with around half a cup of water in the cooker. Cook for two-three whistles. Turn off the flame and let the cooker cool. Open it once the pressure drops. Remove the cooked sweet potato and peel off the skin. Dice the peeled sweet potato.
In a pan, heat the oil. Fry the asafoetida for a few seconds, followed by the mustard seeds, cumin seeds, red chillies and curry leaves. Once the mustard seeds pop, add the diced cooked sweet potatoes and salt. Combine well. Cover and cook for two minutes on a low flame. Remove to a serving bowl and garnish with grated coconut.
This can be used on toast or served along with rice and dal.
Purple Sweet Potato Kheer
1 litre full cream milk
Half cup sugar
1 medium-sized purple sweet potato
3 green cardamom pods
Dried rose petals for garnish
In a heavy bottomed pan, keep the milk to boil. Once it comes to a boil, reduce the flame and simmer for 25-30 minutes. Give the milk a stir every few minutes to ensure it does not burn at the bottom. This can also be done in an electric pressure cooker (Multipot or Instant pot). Use the porridge function and keep the milk to cook for 30 minutes.
Meanwhile, scrub and wash the sweet potato well. Place it in a bowl and keep the bowl in a pressure cooker with around half a cup of water in the cooker. Cook for two whistles. Turn off the flame and let the cooker cool. Open it once the pressure drops. Remove the cooked sweet potato and peel off the skin. Mash the sweet potato until smooth, using a fork or in a small mixer.
Peel the cardamom pods and crush the seeds until finely ground. Once the milk has simmered for half an hour and turned somewhat thicker, add the sugar and simmer until the sugar has melted.
To this add the sweet potato purée and the green cardamom powder. Stir to combine and simmer for another five minutes, until the sweet potato mixture is well blended into the milk, giving it a lilac colour.
This kheer tastes best chilled. Chill the prepared kheer for two hours. Serve in small bowls and garnish with rose petals.
Double Tested is a fortnightly column on vegetarian cooking, highlighting a single ingredient prepared two ways. Nandita Iyer is the author of the newly released book This Handmade Life—7 Skills To Enhance And Transform Your Everyday Life. @saffrontrail on Twitter and Instagram.