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

Ingredients:
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

Method:
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!


Cake