Second episode of sponge cakes and staff! :) Let’s see how we can avoid the most common mistakes when preparing whipped batters.
1. The choice of the flour
The preparation of sponge cakes & co requires the use of weak flours, namely flour with a low protein content. The % of proteins is always indicated in the nutritional labels, therefore, can be easily checked. Flours with a protein content of 9-10% are usually the best. Often, some potato starch is added to the flour to further weaken it. Why strong flour can be so deleterious for these types of preparations?
Wheat flours contain two proteins: gliadin and glutenin. During kneading, those two molecules associate with each other to form the gluten net. I will get to this topic more specifically in other posts.
Briefly, the gluten net increases the elasticity of the dough. This characteristic is highly desirable in complex preparations as panettone and brioche, but it must be avoided when dealing with sponge cakes and shortbread. If we prepare a sponge cake (or another whipped batter) with a strong flour, the viscosity of the batter will be so high that the sponge cake will detach form the pan during baking. This phenomenon strongly limits the rising of a sponge cake during baking.
2. Whipping the batter
Whipping the batter with an electrical whisker or a counter top mixer is crucial to incorporate air. We commonly find two ways to proceed:
a) whipping the eggs with all the other ingredients (sugar, flour...)
b) whipping the eggs with sugar only and add the flour just after
Option b) has the advantage to limit the formation of the gluten net. The mechanical action determines, as discussed above, the association of gliadin and glutenin.
When do we have to stop whipping? Until we obtain a stable foam. In Italian, we say that the whipped batter is ready when it “writes”. To verify this, take a bit of batter and draw some random curls on the whipped mass. If the curls take a little while to disappear, it means that the foam is stable* and won’t collapse right away.
An unstable foam makes the cake rising, first, during baking and collapsing right after.
3. Adding shortenings
A few whipped batters, e.g. Angel Food Cake or Sacher Torte, require the addition of butter or margarine. The shortening makes the cake softer and richer. However, it must be added when the mass has been already whipped, because the molecules of fats compete with air for the hydrophobic residues of egg proteins. If the shortening is added before whipping, it will be hard to get a stable foam and the cake will be less airy.
4. Adding baking powder
“Heavy” batters (e.g. the one used for Angel Food Cake) require some baking powder. As those batters are commonly less airy than the one used for sponge cake, the addition of baking powder gives a little help with the rising :)
It must occur in a pre-heated, not excessively hot oven (160-180 ˚C). This avoids that the top of the cake will be crunchy, while the core will remain liquid. During this step, the air bubbles expand, which determines the cake rising. If you did select the right flour, the viscosity won’t be too high and will allow the correct expansion of the batter.
That's all folks! The nerdy paragraph of whipped batters comes to an end. Next post, on Thursday (as usual): a recipe for all chocolate lovers! :)
*A foam can be relatively stable, but not stable forever...don’t wait too long to bake it!
Leonardo di Carlo, Tradizione in Evoluzione, 102-103, Chirotti Editore
D. Elgeti et al., Foam stabilization during processing of starch-based dough systems, Innov. Food Sci. Emerg. Technol., 2017, 39, 267-274