Name: Purple Yam Mooncakes
Prep time: 60 min
Cook time: 50 min
Total time: 110 min
Ingredients: 2 cups cake flour ((270g)); ½ cup golden syrup ((120g)); 1/3 cup vegetable oil ((80ml)); 1½ tsp alkaline water ((7.5ml)); 1 egg yolk (mixed with 1 tbsp water for eggwash); 2 tbsp mooncake glaze; 1 lb purple yam ((peeled and cubed) (450g)); ¼ cup vegetable oil ((60ml)); ¼ cup sugar ((55g)); ½ lb orange sweet potatoes ((peeled and cubed) (225g)); 2 tbsp vegetable oil; 2 tbsp sugar; ¼ cup kuachi ((melon seeds) (30g))

Recipe ▼

Vegetarian Purple Yam Mooncakes filled with homemade purple yam and sweet potato paste center to simulate salted egg yolk. Melon seeds provide texture.

I know it sounds crazy but I am always on the lookout for purple yam or purple sweet potatoes. It seems these purple tubers are no where to be found in my neck of the woods. That is, until the Sunday before this recently past Mid Autumn Festival. On our way home from the Minnesota Zoo, we stopped by an Asian market, and lo and behold, there they were!

Purple Yam / Sweet Potatoes

I was ecstatic! (just kidding) My waiting and hoping that someone would sell them are over, at least for now. The price was more than double that of regular sweet potatoes but I wasn’t going to pass it up. I bought two pounds, enough for making these Purple Yam Mooncakes and also Bubur Cha Cha, which I’ll be sharing soon.

Purple and Orange Sweet Potato Paste

The color of these purple yam is not as deep as those found in Malaysia. When cooked and pureed, it is more like a lilac or a pale purple color. Not a brilliant purple but quite pretty, especially when paired with orange sweet potatoes. It did deepen slightly after baking. The flesh is mildly sweet and very smooth, perfect for my purpose of making it into a paste. 🙂

Mooncake / Mid Autumn Festival

I made these Purple Yam Mooncakes last Wednesday, knowing that both my sons will be home from college on Friday. Since the purple yam paste was quite moist, the crust soften in a day, just in time for the Mid Autumn Festival on Thursday, September 15th. Between the four of us, we had our fill of these tasty treats for tea time over the weekend.

Purple Sweet Potato Paste Turning Green

You may or may not see some green tinges along the inside of the crust. If you do, do not be too alarmed. I did see it in the first cake I cut into and was duly concerned. Only one day and my mooncakes have gone bad? I almost threw it away but then I have 5 more untouched, beautiful mooncakes. What should I do?

I remember that purple beans turn into green beans when cooked and so I promptly search the internet for answers. I found them here and here. A few other food bloggers also experienced this when they baked purple sweet potatoes in cakes and bread with baking soda (an alkaline). Apparently, the anthocyanins in these tubers are pH sensitive, appearing blue or purple in the presence of acidity. In this case, it reacted with the alkaline water used in the dough, losing its acidity and turning green in the process. Thankfully, it is just food science!

Pídàn Mooncakes Instead of Purple Yam Mooncakes?

One way to get around this is to reverse the position of the purple yam paste and the sweet potato paste. If you do that, you will have a purple center instead of an orange center. Imagine a pídàn (皮蛋) or century egg yolk in your mooncake…yikes! Haha! Whatever you decide, I hope you give these Purple Yam Mooncakes a try. They are quite delicious and you can adjust the sweetness to your liking. That said, they are best eaten the day after baking and consumed within 3 days. Make them in small batches and eat them while they are fresh and moist.