Lemon Battery Cake

Circuit completePhrosting and Physics

When the last day of my super awesome coworker Ana was approaching (in early December), someone commented, “We should get her a cake.” I perked up. Did someone just say cake? I like cake. I like making cake. Both of these things may be somewhat understated–it’s possible I’m slightly obsessed. Since I work in science, we decided that the cake needed to be science themed. Having grown up with an engineer for a father whose birthday cake creations were constructed more than baked (think piano, waterfall, full bowling set, etc.), I eagerly accepted the challenge. I searched the internet for inspiration, and let me tell you, there are some crazy and awesome cakes out there. Many of them involve dinosaurs and/or explosions, which we all know are two of the best parts of science. But in the end, I went with a lemon battery: at-home science, something we encounter every year in kids’ science fair projects, and really interesting chemically. Also lemons are delicious.

Because the cake is a wee bit complicated (two kinds of cake, four kinds of toppings, and of course I made it on a weeknight when I had dinner plans, when else was I going to make it?), this is a special deal two-for-one get it while it lasts DOUBLE science post. Because we have to talk about the science of the cake itself and the science that the cake depicts. There just isn’t any other way.

Without further ado: frosting. Basic frosting really lives up to its name: butter, powdered sugar, milk, and vanilla until it looks right and tastes good. The problem with this kind of frosting, I’ve found, is that with sugar as the primary solidifying agent, it is way too sweet by the time you get to the right consistency. Cream cheese is an excellent solution, but the cooked frosting from my “Baked” cookbook (by Matt Lewis and Renato Poliafito) took a different approach: boil milk, cream, sugar, and flour together until thick, then add butter and flavoring and whip it for creaminess. I was skeptical, but in the end it blew my mind. The flour and the heat thicken the frosting without the addition of so much sugar that it’s the only thing you taste, the butter and cream make it rich and, well, creamy, and it’s an excellent base for any sort of flavoring you want to add (one of the downsides of cream cheese frosting is that it always tastes like cream cheese frosting, while this guy became both lemon and cinnamon without complaint). Usually boiling milk is practically a cardinal sin, so how does it fly here?

Turns out it’s thanks to the fat in the cream. We talked about curdling milk when adding lemon juice to make impromptu buttermilk, and heat curdling is essentially the same deal: the casein proteins lose their aversion to each other and decide to be friends, clumping together in the lumps you seen in acid-treated or boiled milk. Cream solves the problem by bringing fat into the picture. Because milk and cream are both water-based and fat and water repel each other (think about oil and vinegar salad dressings), the fat tends to stick together in spheres we call globules. In low-fat milks, both the number and size of the globules are small, but in cream there are more of them and they’re much bigger. The fat globules attach to the casein proteins, making it much more difficult to stick to each other and form those lumps. Think of the casein proteins as being covered in double-sided tape. When they’re heated, they run into each other and stick because of the tape. When you throw in the fat globules, though, they also stick to the casein–but they’re not sticky themselves. The fat uses up most of the available tape area so that it’s much harder for the casein proteins to stick to each other and form lumps. To hit the magic number of 25% fat (by weight) to avoid the curdling, you’ll need anything with whipping cream in the title (light cream just won’t cut it).

So that’s the science of the cake, now let’s get into the science depicted by the cake. The whole point is to use a lemon to light a lightbulb (or power a clock, or whatever floats your boat), and to do that you need electricity. Basically we stick two wires into a lemon and hook the other ends to a lightbulb, and it lights up as soon as you get electrons flowing through the wire. The tricky part is getting the electrons going. Like water, electrons take the path of least resistance, so they aren’t going to just run through the wire because we want them to. So we hook up one wire to a piece of copper and the other to a piece of zinc. The two pieces of metal go into the lemon, making the completed circuit as follows: lemon to copper to wire to lightbulb to wire to zinc and back to lemon.

The acid solution converts neutral zinc atoms (Zn), separating them into positively charged Zn(2+) ions and two electrons, which are negatively charged. The zinc ions disperse into solution while the freed-up electrons stay on the metal. But the metal is connected to wires leading to a different part of the lemon, where the acidity of the lemon juice means that there are lots of free positive H(+) ions. And free electrons love positive ions… (For more detail, check out Jerry Goodisman’s article “Observations on Lemon Cells” in Vol. 78 of the Journal of Chemical Education.)

So we’ve got zinc becoming Zn(2+) and leaving electrons behind, and those electrons flow through the wire (and the lightbulb!) to the lonely positive hydrogen ions, giving us bubbling hydrogen gas and a lit lightbulb. Hooray!

Three cakesLemon Battery Cake

I realize that occasions are few in which people really need the whole lemon battery cake package, but here it is in all its glory–both cakes and all four fillings/frostings. I suppose you could also make a round lemon cake, but come on, that would just be weird. All of these wonderful recipes come from “Baked: New frontiers in baking” by Matt Lewis and Renato Poliafito.

Yield: One three-layer lemon cake and one one-layer chocolate cake

Lemon cake with lemon curd filling and lemon buttercream

The Cake

The Cake

2 1/2 c. cake flour

3/4 c. all-purpose flour

1 T. baking powder

1 t. baking soda

3/4 t. salt

1/2 c. unsalted butter, softened

1/2 c. vegetable shortening

1 3/4 c. sugar

1 T. vanilla extract

Zest of one lemon

1 egg

1  1/2 c. ice cold water

3 egg whites

1/4 c. cream of tartar

Mix the flours, baking powder, baking soda, and salt together. Use an electric mixer to beat the butter and shortening until creamy, about 3 minutes.

Whipped butter

Add the sugar, vanilla, and lemon zest, beating until fluffy, about 3 minutes.

Zesty

Add the egg and beat just until combined. On low speed, add the flour mixture in three additions, alternating with the ice water. You should start and end with the flour mixture. Scrape down the bowl, then mix again for just a few seconds.

Whipping

Use a clean bowl to whip the egg whites with the cream of tartar until they form soft peaks. With a spatula, gently fold the egg whites into the batter.

Butter, line with parchment paper, and re-butter three 8-inch cake pans. Divide the batter among the three pans and bake at 325 degrees F for 40 to 45 minutes, until the edges are lightly brown and a toothpick comes out clean. Let the cakes cool for 20 minutes in the pans, then gently invert them and cool completely.

Three cakes

The Curd

3/4 c. fresh lemon juice (from about 6 lemons)

Zest of 2 lemons

2 eggs

7 egg yolks

3/4 c. sugar

4 T. butter

Pour the lemon juice over the zest and let sit for 10 minutes to soften the zest. Whisk the eggs, egg yolks, and sugar together, then add the lemon juice and lemon zest.

Lemons 'n eggs

Whisk just until combined. Place the egg mixture in the top bowl of a double boiler. Stir continuously until the mixture has thickened to about the consistency of pudding, about 6 minutes.

Curd

Take the bowl off the heat and whisk in the butter until the curd is smooth.

Mmm butter

The (butter)Cream

2 c. sugar

1/2 c. all-purpose flour

2 c. milk

1/2 c. heavy cream

2 c. unsalted butter, soft

1 1/3 t. vanilla extract

1/2 c. lemon curd

Whisk the sugar and flour together in a medium saucepan, then add the milk and cream. Cook over medium heat and stir occasionally until the mixture has come to a boil and thickened, about 20 minutes. Remove from heat and use an electric mixer to beat on high speed until the mixture has cooled. On low speed, mix in the butter until fully incorporated. On medium-high speed, continue to beat until the frosting is light and fluffy. Mix in the vanilla, then set aside a half cup of frosting for the cinnamon buttercream on the chocolate cake. Mix in the lemon curd.

Lemon cake assembly

Cut one of the cake layers into an oval with one pointed end and one end with a bump in it (think about the shape of a lemon cut in half: the bump is where the stem was attached).

Lemon shape

Cut the other two layers into similar shapes about half an inch and one inch smaller, respectively. Stack the layers and angle the edges so that the top of the layer underneath lines up with the bottom of the layer on top. For the top layer, angle the edges steeply to approximate the rounded side of a lemon.

Shaping

Lay the bottom layer on your presentation dish (I used a cutting board covered in foil because this cake was enormous) and slide wax paper underneath the edges to keep the dish clean. Spread lemon curd generously over the bottom layer, then cover with the middle layer, more curd, and the top layer. Cover the entire outside of the cake with a thin layer of frosting and place the whole thing in the freezer for about 10 minutes (this is called a crumb coat and will make your life a lot easier when doing the final frosting). Spread the lemon frosting over the cake with a spatula or a knife. Add the lemon skin look by poking the frosting all over with a fork.

Lemon cake

Chocolate cake with cinnamon buttercream and chocolate ganache

Chocolate cake

The Cake

1/4 c. unsweetened cocoa powder

1/3 c. + 1 T. hot water

1/4 c. sour cream

1 c. all-purpose flour

2/3 t. baking powder

1/3 t. baking soda

1/4 t. salt

1/4 c. butter, softened

3 T. vegetable shortening

1/2 c. sugar

1/3 c. dark brown sugar

1 egg

1 t. vanilla extract

Mix the hot water, cocoa powder, and sour cream, and set aside to cool. In a separate bowl, mix the flour, baking powder, baking soda, and salt. With an electric mixer, cream the butter and shortening on medium speed until light and fluffy, about 5 minutes. Add the sugars and beat for another 5 minutes. Beat in the egg, then the vanilla. Scrape down the bowl and beat for another 30 seconds.

Chocolate batter

Add the flour mixture in three additions, alternating with the cocoa mixture. You should begin and end with the flour.

Adding flour

Butter, line with parchment paper, and re-butter a pan. I used a bread pan to get the shape of voltmeter right, but this would be enough for one 8-inch cake pan. Bake in a 325 degree F oven for 35 to 40 minutes, until a toothpick comes out clean. Let the cake cool for 20 minutes in the pan, then remove and let it cool completely.

Poked

The Buttercream

Use the 1/2 cup of frosting you set aside from the lemon buttercream. Add 3/4 t. cinnamon and mix until incorporated.

The Ganache

4 oz. dark chocolate, coarsely chopped

1/4 c. heavy cream

2 T. light corn syrup

Put the chocolate in a heatproof bowl. Combine the heavy cream with corn syrup in a small saucepan and bring just to a boil. Pour the liquid over the chocolate and let stand for two minutes, then slowly stir until the chocolate is incorporated and the ganache is smooth.

Chocolate cake assembly

Cut three inches off the length of the chocolate loaf cake. Split the remaining cake in half vertically to form two layers. Place the bottom layer on the presentation dish (next to the lemon, if you’re going all the way with this) and place wax paper under the edges. Spread cinnamon buttercream over the cake, and cover with the upper layer. Pour chocolate ganache over the top, spreading it over the sides as well. Place the cake in the freezer to set the ganache.

Cut two inches off the length of the remaining piece of loaf. Cut the large piece in half vertically into two layers. Cover the top half in cinnamon buttercream and place it on the upper half of the ganache-covered cake. Dye the remaining buttercream blue (or another color of your choice) and use a pastry bag with a thin tip or a Ziploc bag with a corner cut off to pipe the markings for the voltage meter “screen.” Use the remaining frosting to cover a small square of chocolate cake and place a candle in it–your “lightbulb.”

Finished product

Complete the circuit with red licorice ropes.

Circuit complete