Lemon Layer Cake with Fluffy Icing | Or: Everything You Learned in High School Chemistry is Actually Important

Today it snowed. Actually, it’s been snowing since Thursday night and the driveway has now been plowed twice, thanks to my tenacious husband. I, on the other hand, decided to bake. The chemistry of baking is incredibly interesting to me. I love that a combination of fat, sugar, and eggs – tempered and heated to 170° – creates custard. What exactly does that? What causes the difference in how pastries bake up when liquid is added to dry ingredients, versus when dry ingredients are added to wet ingredients?

Publications from Cook’s Illustrated have filled some of the void in my search for why a certain process must proceed according to certain predetermined steps, but I still have quite a few questions. Today, I decided to tackle Lemon Layer Cake with Fluffy Icing, compliments of The Cook’s Illustrated Cookbook. This cookbook is a bible. I just love it.

This particular recipe includes a white cake, lemon curd for the filling, and a fluffy white icing made with egg whites – kind of like a shiny meringue, like marshmallow. Sounds easy enough. The recipe was fairly involved and, with the three separate components, took me several hours to complete.

I started with the lemon curd. This was a fairly basic curd recipe, but included an additional 1 tsp of gelatin to tighten up the texture after it cooled and make it more spreadable as a filling.

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While the lemon curd was setting up in the refrigerator, I baked the cakes. This cake recipe was a little different than the traditional white cake recipe in two ways. First, it omits the egg yolks from the egg component, going with six egg whites instead. Not that unusual, since egg yolk tends to lead to a more rich and heavy batter. The second difference was to combine the dry ingredients, and then beat in the softened butter to the dry ingredients before adding the wet ingredients.

This approach reminded me of a pie crust I make where the butter is cut into a portion of the flour, followed by the addition of the rest of the flour. The reason for this is the reaction of water and flour to create gluten strands. In a pie dough recipe, you are looking for the perfect combination of tender flakiness and sturdiness, and this depends on the amount of flour that reacts with the water to create gluten. By blending the flour and butter to a portion of the total flour first, the butter creates a protective, hydrophobic (won’t absorb water) coating around the flour and reducing the gluten-producing reaction of flour and water. Later in the pie dough recipe, the remaining flour is added along with water, effectively controlling the amount of gluten produced and keeping the pie dough from becoming to tough.

Back to the cake! Since this cake is supposed to be light and tender, the replacement of egg yolk with egg whites makes it light and fluffy, but the addition of softened butter to the dry ingredients also helps to keep it light by reducing the amount of naked flour which could create too much gluten and make the cake tough. This is especially important, because the batter is beat pretty vigorously in the stand mixer to make it light and fluffy.

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Even with all of that information, I ran into a problem. My cakes fell.

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It’s not totally visible from this photo, but there was an obvious bowl in the top, which made things difficult when I started stacking and filling the layers. I did some reading into “why cakes fall,” and determined that I may have contributed to this fiasco. Cakes can fall for a number of chemistry related reasons including altitude adjustments for high elevation, old baking powder, low temperature, too much or too little sugar (sugar amount determines the temperature at which the batter sets), over or under beating the batter which incorporates too much or too little air, as well as opening the oven half way through the baking and excessive jarring of the batter in the pan before it sets. Oops. Let me explain. I had positioned the pans on separate racks – one above the other. When I checked (half way through) I noticed that the top cake was nicely browned while the bottom cake was still pasty white. I decided to switch the pans from top to bottom which probably burst some of the bubbles before the cake had set, causing them to fall into oblivion.

While the cakes were cooling, I whipped up the icing. This recipe is almost like a meringue with whipped egg whites. It included corn syrup to make it shiny, sugar, a splash of water, and egg whites. The concoction was heated to 160° F over a double boiler…

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and then whipped into oblivion in the stand mixer.

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Now for ASSEMBLY:

Since this was a four layer cake made from two cakes, each one needed to be horizontally divided. First of all, I didn’t have a long, serrated bread knife, so that was difficult but manageable. There’s something to be said for the right tools for the job. Second, and not so manageable, was the giant bowl in the middle of each cake, leading to a cake that would ultimately sink in the middle, rather than mound, and for some pretty thin layers in the middle. I started by cutting each of the two layers in half to create four layers. Then I began layering them, separated by the lemon curd.

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Since some of the layers were less than sturdy (being approximately four grains of flour thick in the middle where they had fallen) I used a flat baking sheet to transfer and slide them onto the stack.

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Assembly proceeded mostly without a hitch (this was actually spectacular since I’ve never actually assembled a layer cake) but it was a little off center and, frankly, looked like it had too much to drink. I frosted it with the icing and everything looked wonderful.

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…for about two minutes. Then my icing started oozing and melting all over. It is really devastating to show my cake failures all over the internet, but it inspires me to learn what went wrong. Namely acid. After some reading, I discovered that egg whites are an alkaline pH and the proteins in them cause them to whip up so nicely. When egg whites are beaten, the protein coils unravel and begin to form bonds and networks. These networks form over the surfaces of air bubbles and keep them from popping. This is excellent, while it lasts. Over beating egg whites causes an overbonding of proteins and begins to create protein coagulation, and there is a very fine line between perfectly beaten and overbeaten. Overbeating causes coagulation which keeps the proteins from holding their network form and – thus – they can’t maintain the surface integrity over the air bubbles, so it begins to slump and deflate. The fix? Acid. Cream of tartar is highly recommended since it is the only solid acid, and it is flavorless. Vinegar and lemon juice work as well, but they have distinct flavors, and sometimes adding additional liquid to a meringue or icing is undesirable. The point of adding an acid is to broaden the margin in which the egg whites are “perfectly beaten” to “over beaten.” An acid increases the number of free hydrogen molecules, allowing more hydrogen bonds to be created. Hydrogen bonds are incredibly strong and help the egg whites to hold their stiff-peaked form.

Here is the lumpy, slouching form of my deflated icing and uneven cake

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As you can see from the next photo, the layers were a little even. I tried adding additional lemon curd in the middle of the cake to even out the slumping middle, but I think it just became a gooey  mess

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On a side note, I seriously thought I just lost my entire post and I almost had an aneurysm. Moving on.

Overall, the cake was delicious. I didn’t personally think the icing fit with flavor the the cake. It was almost cloyingly sweet and tasted like marshmallow. According to my husband, it tasted exactly like Fluff, which got high marks from him. I am considering adding lemon juice as an acid just to cut the sweetness a touch when I remake the recipe.

Overall, I learned quite a bit about the problems I encountered, and it was exciting to use my prior knowledge when I asked myself “why?” I hope y’all learned something today! I know I did.

This is actually the first time I’ve encountered any of these problems, so tell me: who has had problems with their cakes falling? did you discover how to fix it? Did your icing run all over the kitchen after you worked so hard on it? Please tell me, I’m dying to know that I’m not the only inept cook.

XOXO

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