Cooking for Geeks: chocolate port (not pork!) cake


My book, Cooking for Geeks (O'Reilly Media, 2010), looks at the science behind what happens in the kitchen and uses recipes to illustrate the principles. Take egg whites: why do they whisk up into a foam? Here's a short excerpt on the food science, followed by a recipe for chocolate port cake. Port, not pork! Every time I say this out loud, people think I'm talking about chocolate pork cake. While that might be delicious, chocolate port cake is really, really good, and it's easy to make, too. If you're afraid of baking but want to try making a dessert for a special romantic occasion, give this a try.

Whisked egg whites are the Styrofoam of the culinary world: besides acting as space fillers in cakes, waffles, and soufflés and as "insulators" in desserts like lemon meringue pie, when overcooked, they taste about the same as Styrofoam, too. All metaphors aside though, egg whites are much more forgiving than many cooks realize. With a little attention spent on understanding the chemistry and a bit of experimentation, egg-white foams are easy to master.

The key to understanding egg whites is to understand how foams themselves work. Whisking egg whites turns them into a light, airy foam by trapping air bubbles in a mesh of denatured proteins. Since regions of the proteins that make up egg whites are hydrophobic — literally, water-fearing — they normally curl up and form tight little balls to avoid interacting with the water. But when whisked, those regions of the proteins are slammed against air bubbles and unfold, and as more and more proteins are knocked against an air bubble, they form a layer around the bubble and essentially trap it in the liquid, creating a foam that's stable.

Oils — especially from egg yolks or any trace oils present in the whisking bowl — prevent egg whites from being whisked into a foam because they're also able to interact with the hydrophobic sections of the proteins. Water and sugar don't interfere with the formation of protein-based foams for the same reason.

Once the air bubbles are encapsulated by the proteins in the egg white, it takes quite a bit of effort to get them to break. Exposing the whites to any oil before whisking is a problem; even a trace amount of fat from a small amount of stray egg yolk will interfere with the creation of the foam. But once the eggs are whisked, they're much more resilient. Try this experiment: whisk an egg white to soft peaks, then add 1⁄2 teaspoon (5g) olive oil and continue to whisk. It might surprise you how long it takes before the oil starts to noticeably interact with the foam, and even then, that the foam remains mostly stable.

Chocolate Port Cake

You'll need a small saucepan, two clean bowls, a whisk, and a round baking pan or springform pan, 6-8" / 15-20 cm.

In the saucepan (over a burner set to low heat), melt and mix together, but do not boil:

– 1/2 cup (125g) port (either tawny or ruby)

– 1/2 cup (114g) butter

Once butter is melted, turn off heat, remove pan from burner, and add:

– 3 oz (85g) bittersweet chocolate, chopped into small pieces to facilitate melting

Leave the chocolate to melt in the port/butter mixture.

In two bowls, separate:

– 4 large (240g) eggs

Make sure to use a clean glass or metal bowl for the egg whites, and be careful not to get any egg yolk into the whites. Whisk the egg whites to stiff peaks.

In the bowl with the egg yolks, add:

– 1 cup (195g) granulated sugar

Whisk the egg yolks and sugar together until thoroughly combined. The yolks and sugar should become a slightly lighter yellow after whisking for a minute or so. Pour the chocolate mixture into the egg yolk/sugar mixture and whisk to thoroughly combine.

Using a lat wooden spoon or lat spatula, add to the chocolate mixture and fold in (but do not overstir!):
– 3/4 cup (100g) all-purpose flour

Then fold in the egg whites in thirds. That is, transfer about a third of the whisked egg whites into the chocolate mixture, mix together, and then repeat twice more. Don't worry about getting the whites perfectly incorporated, although the batter should be relatively well mixed together.

Grease your cake pan with butter and line the bottom with parchment paper, so as to make removing the cake from the pan easier. Transfer the mix to the cake pan and bake in an oven preheated to 350°F / 175°C until a toothpick or knife, when poked into the center, comes out clean, around 30 minutes.

Let cool for at least 10 to 15 minutes, until the edges have pulled away from the sides, then remove from pan. Dust with powdered sugar (you can use a strainer for this: place a few spoonfuls of powdered sugar in the strainer and then jog it with your hand above the cake).

P.S. When working with chocolate in baking, don't just substitute, say, 80% bittersweet chocolate for a semisweet bar. In addition to differences in sugar, the two types of chocolate have different quantities of cocoa fat, and recipes that rely on the fat level will need to be adjusted accordingly.

See also Cooking for Geeks: an interview with Adam Savage