Purple Yam Swiss Roll

Recipe: Purple Yam Swiss Roll紫薯瑞士卷

Yield: 1 roll (8-inch)

Ingredients

Tools needed 1 baking pan 11.5-inch x 8-inch parchment paper / baking sheet hand whisk electric mixer Batter 4 egg yolks (large eggs) 1½ TBSP castor sugar 3 TBSP vegetable oil 3 TBSP fresh milk 120g mashed purple yam 30g cake flour (non self-raising)

Egg Whites Ingredients 4 egg whites (large eggs) 3 TBSP castor sugar 1 tsp. distilled white vinegar or lemon juice Filling Swiss meringue buttercream 1 tsp. blueberry jam *optional

Related post: Stages of Stiffness In Beating Egg Whites
Measurements: tsp. = teaspoon | TBSP = tablespoon

Preparation
1. Peel purple yam. Cut yam into small cubes. Arrange yam in a plate and steam over hight heat for 30 minutes, or until fork tender.	2. Mash yam with a fork. Leave to cool.	3. Line a 11.5-inch x 8-inch baking pan with parchement paper or baking sheet. Measure the ingredients. Set aside. Preheat oven to 350oF / 180oC.
4. Prepare 2 large mixing bowls. Bring eggs out from refrigerator and separate the yolks and whites into the prepared bowls right away.	5. Remove any chalazae (the white stuff attached to the yolks) from yolks.	6. Add sugar to the yolks.
7. Whisk the yolks with a hand whisk.	8. Whisk until yolks turn creamy and color turns light and pale.	9. Add oil to the yolk mixture. Whisk to combine. Mixture should be glossy and creamy.
10. Add mashed yam into the yolk mixture. Whisk to combine until incorporated.	11. Stir in milk.	12. Sift flour to the yolk mixture. Mix to combine until incorporated.
13. Set aside the batter.	14. Let's move to the egg whites part. Whisk whites until foamy, about 10 seconds at highest speed.	15. Add half of the sugar to the foamy whites.
16. Continue to whisk at highest speed for about 1 minute, or until color of the foamy whites mixture turns white. Add the remaining sugar and vinegar / lemon juice.	17. Continue to beat whites mixture at medium speed for abour 1 plus minutes, or until you get soft peaks when beaters are lifted. Beat at low speed for another 20 to 30 seconds.	18. Using a spatula, put one third of the whites into the batter in step 13.
19. Gently cut and fold whites in "O" circular motions (from 1 to 2) as indicated in the photo. Start with cut (move spatula downward to the bottom of the batter), then begin to fold (gradually move spatula upward to the top) as you move half way to each "O". Repeat to cut and fold while turning the mixing bowl in either clock or clockwise direction until mixture is incorporated.	20. Add the remaining whites to the batter. Continue to cut and fold gently until mixture is incorporated.	21. Pour batter into a lined baking pan. Gently leveling batter by using spatula. Bake cake at 350oF / 180oC preheated oven for 12 to 15 minutes.
22. Doneness Test: Insert a wooden toothpick in the center of the cake.	23. Doneness Test: Cake is done when toothpick comes out clean without any batter sticking on it. Crumbs are fine. Turn cake upside down onto a wire rack and leave cake to cool completely, about 30 minutes.	24. Remove parchment paper from the cake.
25. Combine Swiss meringue buttercream with blueberry jam using a spatula.	26. Spread buttercream over cake, leaving 0.5 to 1-inch border around the edges, leveling with a spatula. Cut a few scores vertically on one of the short sides, beware not to cut through the cake.	27. Cut a piece of baking sheet slightly larger than the size of the cake (with additional 3 to 4-inch on each side). Place cake on the baking sheet. Start rolling up the cake from the scored short side. Hold each side of the baking sheet with your hands, keep rolling the baking sheet gently to roll up the cake. Roll as tight as you can and be gentle not to crack the cake.
28. Twist both end of the baking sheet. You might secure the wrap by using cellophane tape. Keep swiss roll in the refrigerator for about 1 hour.	29. Take out swiss roll from the refrigerator. Unroll it.	30. Cut swiss roll into 0.75-inch thick slices and serve.

Notes

Beating egg whites mixture at low speed for the final stage is to break down the large bubbles into tiny little fine foams. So that texture of the cake will turn out fine and smooth. When mixing beaten egg whites into batter, always mix one third of the lightweight portions (whites) into the heavy one (batter). Cut and fold until incorporated. Then only add the remaining lightweight portions into the heavy portion. Swiss roll can be stored in the refrigerator for up to a week.