Raspberry Lemon Cake

The Art.

    Once again, it was Labmate George's birthday. And once again, he had an awesome baking idea. His birthday cake request this year was to combine raspberry and lemon flavors, which probably has something to do with this "Eating Seasonally" thing he does. As for myself, I always know when raspberries are in season because I have an unyielding bond with nature (and because they are $1 a carton at the store). I wanted to highlight the fruit flavors in the cake, so I made a lemon zest glaze and piled the raw raspberries on top. To save time, I used raspberry jam for the filling, but cut it with some salted butter to prevent it from being too sweet. Leaving the cake un-iced allows the pretty yellow cake layers and pink filling to show.

The Science.

    To add lemon flavor to desserts, lemon zest and lemon juice can both be used. But, how do you know when to use zest and when to use juice? My rule of thumb is to use lemon zest in the baked component (i.e. the cake), and lemon juice in the glazes, icings, etc. This is for two reasons. First, the acidity of the lemon juice significantly affects the texture of baked goods. It increases the activity of baking soda and baking powder, as well as interfering with the formation of gluten. This can be beneficial up to a point, making the cake have a finer crumb and delicate texture. However, too much lemon juice can make the structure too flimsy [1]. The second reason to use lemon zest in cake batter is because it has a much higher concentration of limonene, the essential oil that we associate with lemon flavor. While limonene makes up only 8% of the aromatic compounds in lemon juice, the lemon peel can have up to 70% limonene in its essential oils [2,3]. Adding lemon juice to cake batter does not contribute significantly to the flavor, but lemon zest can give the bright punch of citrusy flavor you are looking for, without affecting the texture of the cake.

The Recipe.

Lemon Cake:

2 1/4 cups cake flour
2 1/4 teaspoons baking powder
1/4 teaspoon salt
3/4 cup salted butter
1 1/2 cups sugar
4 eggs
1 1/2 teaspoons vanilla
1 cup buttermilk

Zest from two large lemons

    Preheat oven to 350^oF.  Coat a 9x13 metal pan with baking spray and line with parchment paper. In a medium bowl, comine flour, baking powder, and salt. Beat butter in large bowl on medium-high until fluffy, then gradually beat in sugar. Beat eggs in, one at a time. Stir vanilla into buttermilk. Alternately add flour mixture and buttermilk to butter/sugar/egg mixture while beating on low speed. Fold in lemon zest. Pour into prepared pan and bake on center rack for 20-25 minutes, or until a toothpick inserted into cake comes out clean. If the cake is crowned in the center, press it flat as shown in this post. Cool in pan for 10 minutes, then invert onto a cake board to cool completely.

Raspberry filling:

3/4 cup raspberry jam

3 tablespoons salted butter, room temperature

    Combine jam and butter in a small bowl, set aside until cake is cooled. Note: I prefer Full Circle Select Raspberry Jam. It has seeds, which some people don't like, however I think is made from better quality fruit and thus doesn't have as much added sugar. The flavor and texture of this jam is definitely superior to Smucker's, for example.

Lemon glaze:

Juice from one large lemon

1 teaspoon lemon zest

2 cups powdered sugar

    Combine lemon juice and zest in medium bowl. Gradually stir in powdered sugar. 

Assembly:

    Trim the edges of the cooled cake with a sharp, serrated knife. (This produced a cake that was 11.5 inches long and 8 inches wide for me). Cut the cake into equal thirds, making three pieces that are approximately 8 inches wide and 3.75 inches long. I actually used my tape measure and scored the cuts into the top of the cake before I cut all the way through. Measuring accurately and making clean cuts will ensure that your cake layers are even and level when you stack them.

    Spread half of the raspberry filling over one of the cake layers, then place the second cake layer (brown side up) on top. Cover with the remaining raspberry filling, then place the third cake layer (brown side down) on it. Drizzle glaze on top of the cake, then cover with raspberries. 

Cake can be stored in the refrigerator for one week. 

1. Galvin, Lori et al. Test Kitchen Favorites. Brookline, MA: America's Test Kitchen, 2006.

2. Tounsi, Moufida et al. "Juice components and antioxidant capacity of four Tunisian Citrus varieties." Journal of the Science of Food and Agriculture, 2011. Vol. 91(1): pp142-51.
3. Bourgou, Soumaya et al. "Changes of Peel Essential Oil Composition of Four Tunisian Citrus during Fruit Maturation." Scientific World Journal, 2012: 528593.