Castella cake is a light, fluffy cake popular in East Asia, and there are two main methods to make it. Today, we’re taking a closer look at the Taiwanese version (古早味蛋糕, gǔzǎowèi dàngāo). A typical recipe starts with cake flour and cornstarch in hot oil, then adds milk, egg yolks, and an egg white meringue. In this post, we’ll focus on the ingredient that’s less common in cakes: the hot oil. We’ll bake two cakes, one with hot oil and one with oil at room temperature, and we’ll discuss the effects of the hot oil on the flour and cornstarch, the batter, and the final baked cake.
Experiment Overview
Goal: To taste and see differences in texture and appearance between castella cakes made with hot oil and with room-temperature oil
Recipe: Adapted from castella cake by @lings_creation
Method: Prepare two cake batters, one with hot oil and one with cool oil. Bake, cool, and compare.
Results: Between the two types of cake, we noticed differences in
– Batter thickness
– Rise of the cake
– Cake texture
Conclusions: Adding milk to hot oil and starch gelatinizes the starch, which thickens the batter and creates a moist, fluffy cake with a tall rise.
Test Method
Ingredients and Equipment
- 90g Pillsbury Softasilk cake flour
- 15g Argo corn starch
- 92g Carlini vegetable oil, divided
- 118g Friendly Farms whole milk, divided
- 8 Goldhen large egg yolks, divided
- 1/4 tsp McCormick cream of tartar
- 8 Goldhen large egg whites
- 80g Baker’s Corner granulated sugar
- 2 8×4-inch aluminum loaf pans lined with parchment
- 8×8-inch pan for water bath
Making the cakes
- Preheat oven to 300°F and bring a pot of water to boil.
- Sift the cake flour and cornstarch (105g). Whisk to combine and divide into two portions (52g each). Whisk 46g vegetable oil into one portion.
- Heat the remaining 46g vegetable oil to 140°F. Add it to the second portion of the flour mixture and whisk well to combine.
- Whisk 59g milk into each batter. Then add 4 egg yolks to each batter and combine well.
- In a separate bowl with clean beaters, add the cream of tartar to the egg whites and whip until foamy. Gradually add the sugar while whipping the egg whites to stiff peaks.
- Stir 1/8 of the egg whites into each batter to lighten. Then fold the remaining egg whites into each batter in batches, dividing the meringue evenly between the two (160g each).
- Pour each batter into a prepared loaf pan. Tap the tin firmly on the counter a few times and draw a zigzag through the batter with a chopstick to remove large air bubbles. Place the two cakes in the 8×8-inch pan, put the pan in the oven, and pour boiling water into the pan until it reaches halfway up the side of the cakes. Bake for 60 minutes until lightly brown on top. Remove from oven, cool, and taste.
Note: My cool oil cake tilted a little bit in the water bath, so the top is slanted
Results
To better understand some of these results, I suggest reading through the last few posts about starch, especially the introduction and the one specifically about starch in flour. At the end of the flour post, I discuss the effects of cooking flour before baking it.
Batter thickness
When we first combined the oil with the flour and cornstarch, the hot mixture was thinner than the cool one. This makes sense since heat gives the oil molecules more energy to move freely and flow past each other. But as soon as we added the milk, the consistencies changed. The mixture with hot oil thickened because the starch absorbed the water in the milk and swelled, and the whisk left lines in the batter. The cool starch, on the other hand, could not absorb much water so the mixture remained thin. From this point forward, the hot oil batter remained thicker than the cool oil batter, though the difference became less noticeable as we added other ingredients.
Both this recipe and the tangzhong bread from last time use heat to pregelatinize starches in the flour. But unlike the tangzhong bread, where we cooked a mixture of flour and water, for this cake, we first combined the flour and starch with fat to create a roux. This is a common technique for sauces and other liquids thickened with starch. The fat separates individual starch particles so that they do not clump when they absorb water and swell. Once the starch is evenly dispersed, we can add the water or milk. Each individual starch granule now has plenty of room to swell and thicken the liquid without clumping.
Rise of the cake
As a consequence of the difference in batter thickness, the hot oil cake rose higher than the cool oil cake. (This was more obvious as the cakes were baking.) As you might recall from our discussion about leavening, the thinner the batter, the more quickly gas rises through it. Think of bubbles in water and honey. In a thin liquid like water, bubbles quickly float to the top and pop, but in a thicker liquid like honey, bubbles move more slowly.
The majority of the gas in this recipe comes from the air we beat into the egg whites and then fold into the cake batter. When we bake the batter, the gases expand and lift the cake. Since the hot oil batter is thicker than the cool oil one, more bubbles stay in the cake as it cooks. Furthermore, the pregelatinized starches set at a lower temperature, which means that the hot oil cake solidifies more quickly than the cool oil cake and the air bubbles have even less time to escape. Ultimately, these factors give us a cake with greater volume and height.
Texture
Both cakes were incredibly light and fluffy. Even with both of them in front of us, two taste testers and I found it difficult to discern which cake was which. With effort, however, we determined that the hot oil cake was softer and moister, and the cool oil cake was lighter and bouncier. Many of these differences can be attributed to moisture. Like in the tangzhong bread from the last post, the pregelatinized starches in the hot oil cake retain more water throughout the baking process because the molecules remain trapped in the swollen starch granules. As the cakes stale over time, this difference becomes even more salient. The pregelatinized starch holds water in the crumb of the cake while the water in the cool oil cake migrates to the crust.
Conclusions
Taiwanese Castella cakes use hot oil to pregelatinize the starches in the flour and starch. The resulting batter is thicker and better at retaining gas, which helps the cake rise higher. The pregelatinized starches also hold moisture throughout the baking and staling processes, making the cake moist, light, and spongy. As with all cakes, the manipulation of starch changes thickness and texture.
References
Bajaj, R; Singh, N.; Kaur, A. Effect of native and gelatinized starches from various sources on sponge cake making characteristics of wheat flour. Journal of Food Science and Technology, 56(2), 1046–1055, 2019.
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Hesso, N.; Loisel, C.; Chevallier, S.; Le-Bail, A. Impact of Pregelatinized Starches on the Texture and Staling of Conventional and Degassed Pound Cake. Food and Bioprocess Technology, 7, 2923–2930, 2014.
Seyhun, N., Sumnu, G., Sahin, S. Effects of Different Starch Types on Retardation of Staling of Microwave-baked Cakes. Food and Bioproducts Processing, 83(C1), 1–5, 2005.