Roll Cake with Whipped Cream

December 31st, 2020

I will confess that before looking into how a roll cake was made, I assumed it was done using some sort of complicated spiral mold. In retrospect, I should have realized that—just as with the previous dish I prepared—the name should be interpreted quite literally, and that a flat sponge cake is rolled up with the filling inside. Roll cakes here in Singapore are also occasionally referred to as Swiss Rolls, which I believe is the common name in the US. Growing up, I was familiar with the snack cakes produced by Little Debbie which were a chocolate coated cake with a sort of cream/frosting filling. In Singapore, I generally see them with whipped cream fillings, sometimes with fruit flavoring. In addition to clarifying the artwork, making this roll cake gives me an opportunity to fulfill a pledge I made in my first ever entry: discussing the delightful and fascinating material, whipped cream.

What is whipped cream?

Continuing the theme of descriptively named food, whipped cream is made from a dairy cream that has been vigorously whipped to incorporate air and create a type of foam that can hold its shape. As I’ve said before, this kind of material is a bit amazing; processing a liquid together with a gas produces something that acts like a solid. Previously, I introduced the idea of yield-stress fluid microstructures that can be categorized as either gels or glasses, and that with their jammed air bubbles, foams fall into the latter category. However, outside of model materials, it is overwhelmingly the case that microstructures are quite complicated and may not neatly fall into a single category or subcategory. In my early research into the design of yield-stress fluids, I presented a framework for conceptualizing new and existing material microstructures by juxtaposing model structures. Such a juxtaposition is how I have always conceptualized the microstructure of whipped cream.

The cream precursor to whipped cream is an emulsion of milk fat in water, together with stabilizers that prevent the fat from aggregating. By whipping the cream, air is entrained, but also the stabilizers protecting the fat globules are partially stripped away. These partially stripped solid globules settle at the air-water interface and also begin to agglomerate with each other, stabilizing the entrained air bubbles. Thus, I believe that the microstructure of whipped cream can be conceptualized as a juxtaposition of a jammed foam with a networked particulate gel. The air bubbles in whipped cream are certainly at a high enough volume fraction to push and jam against each other, but the interstitial stabilizing fluid is composed of a network of solid fat particles. If whipped cream is agitated too much, the aggregation of the fat becomes too great, the globules form large grains and can no longer coat the interfaces between bubbles, collapsing the foam.

Making the roll cake

Following the recipe at this link, the initial construction of the sponge cake was fairly standard. One interesting aspect was incorporating a pattern onto the surface of the cake through a sort of lithographic method. Making a paste of butter, sugar, egg, and starch, I ended up with a paste with a yield stress of around 10 or so Pascals. This relatively low yield stress meant that while the paste couldn’t really hold a 3D shape, it could be deposited on the baking pan in a specific pattern. The pattern became embedded in the batter and was transferred to the surface of the cake during baking.

For rolling the cake, it seems as though we make use of a sort of “creep deformation”, rolling it up just out of the oven and stuffing it into a poster tube so that the shape can set as a cylinder. Creep is the term for permanent deformation of a material over time. For the yield-stress fluids that I always discuss and for simple solid materials, the “yield stress” is a single critical stress that indicates when a material will start to undergo permanent deformation. However, for both sorts of materials, permanent deformation can still occur well below this critical stress if applied for a long time (and typically at elevated temperatures). For everyday materials, it is most common to have seen this phenomenon with plastics. The example that always comes to my mind from my youth was sitting on a soccer ball on a hot summer day. Before long, the nice round ball had deformed into an unsightly oval shape. However, it is also possible that what is occurring with the cake is not quite creep, and that there is still just some evolution of the microstructure occurring just after the cake has been pulled out of the oven. To test if this cake truly does undergo creep, I would first allow it to cool down fully and complete any irreversible processes from the baking, then reheat it to the baking temperature and see if at that point I was able to induce permanent deformation by rolling it up.

Regardless of the precise mechanism, I was able to set the cake in the shape of a spiral cylinder. The first time I made this, I was surprised at how flexible the cake was as I could unroll it and slather the inside with a strawberry whipped cream I prepared. The whipped cream could hold “medium-firm” peaks indicating the presence of a yield stress, but inferring a particular yield stress is more difficult for me to do visually with lower density materials like foams. Tasting the whipped cream, I estimate that it had a yield-stress in the range of 10 to 30 Pascals, more than enough for it to effectively coat the inner surface of the cake. For stability, we also include gelatin in the whipped cream which further gels the continuous phase of the foam, complicating the microstructure even more but strengthening the material after refrigeration.

The final cake was quite delicious, and I think looks great with the patterned surface. I recommend this recipe if you’re interested. I hope you all have a safe and happy new year and I look forward to bringing you exciting new things in 2021.