Saturday, July 30, 2011

Purple, Blue or Black?

Don't you just love that rich vibrant purple hue?
After the batch of delicious Pumpkin Steamed Buns, I was thinking of what other steamed buns I could make by substituting the pumpkin in the recipe. The first thing that came to my mind was purple sweet potatoes. The buns would look so pretty in a purple hue I imagined, like my Purple Sweet Potato Swiss Roll here.  

So purple sweet potato steamed buns I made, or so i thought!

Look what I've got!

Yup, no kidding, those steamed buns were made from PURPLE sweet potatoes! It's just so strange how it ended up blue black. Haha! At a glance they look like bamboo charcoal mantous!  Would anyone please explain what happened? Is it because of oxidation?
Despite the undesirable and rather unappetizing colour, the steamed buns tasted fine and were actually good eaten warm with lashings of  butter.

Purple Sweet Potato Steamed Buns/Mantou

50gm sugar
150gm mashed purple sweet potato
6 tbs water (more or less depending on the moistness of sweet potato)
1tsp instant dry yeast
150gm pau flour (or plain flour)
1tsp double action baking powder
10gm butter

1. Combine sugar, mashed purple sweet potatoes, water and yeast.
2. Combine pau flour, double action baking powder and butter and mix to form crumbs.
3. Mix (1) and (2) together to form dough.
4. Knead dough until smooth and no longer sticky.
5. Let dough proof in a warm place until double in size. Cover it.
6. After proofing, punch down the dough and knead for 2 mins and roll out  into a rectangle.
7. Slowly roll up like swiss rolls, cut into equal pieces.
8. Cover and let buns proof for 2nd time for another 45 mins.
9. Steam on high heat for 12 mins. Done!


  1. hi there, your mantous look delicious despite it not being purple! actually this is a pretty common problem that can be solved by adding lemon juice. this will help preserve the color. i also use purple sweet potato powder instead of puree so the color will be more vibrant :D

  2. Despite the dark colour of your mantous, I can see they are very soft & yummy. Thanks for sharing this experiment, this is a surprise to see the result turn out different to the original colour.

  3. well, it looks KINDA purple hahaha. If you didn't tell me they were sweet potato, I really would have thought black sesame or charcoal! still look super lovely though!

  4. i wonder if double action and normal baking powder will have different effects on the color cos i've seen purple sweet mantous before but using baking powder instead of double action.

  5. I never know the colour will change so much... Thanks for sharing with us! :)

  6. strange isn't it? i used the same batch of mashed potatoes as for my swiss rolls and my swiss rolls didn't turn out blue black like that! i definitely think it's due to oxidation, like how apples get brown after being left out for a while. you know, i think the cream of tartar prevented my swiss rolls from being oxidised.

    bossacafez: thanks for the tip on lemon juice!

  7. hey, that's not blue black, that's black! hahaha! next time when i have some purple potatoes, i'll try and see. Let you know. I'm also curious :))

  8. Just sharing my 2 cents: The colour is affected by the pH of the environment. It's purple at its natural pH of ~6 but changes towards pink as the environment gets more acidic (e.g. cream of tartar, lemon juice, vinegar). If the environment becomes more alkaline (e.g. baking powder, baking soda), it changes to blue and then to grey. Hope this helps. P.S. I have read that the colour of some purple sweet potatoes (dark skin) are less affected than others (light skin). :)

  9. Hi Anonymous, that's interesting to know :)Thanks for sharing!


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