Colloids: Foams

Light and airy but nevertheless decadent, chocolate mousse is a dessert worthy of its reputation.  Though it is somewhat finnicky, the result is completely worth the effort.  And with a little bit of knowledge about the chemistry behind this confection, it becomes very easy to whip up the perfect dessert.

To begin, a colloid occurs when one substance is microscopically dispersed in another substance.  To put it into perspective, colloids are the midpoint between a solution and a mixture.  This is generally occurs when particles dispersed are between 1 and 1000 nanometers large.  There are different types of colloids, depending on the states of matter of each component.  The type we created, a foam, occurs when a gas (air) is trapped in tiny bubbles in a liquid (egg whites).

To begin, we melted butter, coffee and chocolate together in a double boiled to create a ganache.  We then set this aside to cool while we prepared the eggs.

We separated the eggs, making sure not to let the whites come into contact with any fats.  This would have prevented the proteins from forming bubbles to their full potential.

We then added cream of tartar to the egg whites (as an acid, it stabilizes them so they can take on more volume) as well as salt and a bit of sugar.  Molly whipped them up into nice stiff peaks while I dealt with the yolks.

To the yolks I added sugar and a bit of water.  I stirred the mixture over a double boiler to dissolve the sugar and cook the eggs, and then transferred the bowl into a larger bowl of ice water for cooling.  As the temperature came down, I beat the mixture on high until it was cool, thick, and frothy.

At that point we had two colloids: the yolk foam and the white meringue.  We folded the two together, taking care not to destroy the carefully created air pockets.

Finally, we folded in the ganache mixture.  The result war a silken, pillowy soft chocolate mousse.   We let the mousse set in the fridge before digging in, and meanwhile we concocted our sourdough starter (more details to come later).

Our colloidal experience was wildly successful; we constructed and maintained a perfect amount of microscopic air pockets for a delicate mousse, and the mixture came together seamlessly.  In all, it was a delectable day.

Here's the recipe we used:

Chocolate Mousse

Six to eight servings

• 6 ounces (170g) bittersweet or semisweet chocolate, chopped

• 6 ounces (170g) unsalted butter, cut into small pieces

• 1/4 cup (60ml) dark-brewed coffee

• 4 large eggs, separated

• 2/3 cup (170g), plus 1 tablespoon sugar

• 2 tablespoons (30ml) dark rum

• 1 tablespoon (15ml) water

• pinch of salt

• 1/2 teaspoon vanilla extract

Adapted from Mastering the Art of French Cooking (Knopf) by Julia Child.

I tried to reduce the amount of butter in the recipe and found it wasn’t nearly as good. Since I’m not one to argue with Julia, I stuck close to the recipe tweaking it just slightly.

1. Heat a saucepan one-third full with hot water, and in a bowl set on top, melt together the chocolate, butter and coffee, stirring over the barely simmering water, until smooth. Remove from heat.

2. Fill a large bowl with ice water and set aside.

3. In a bowl large enough to nest securely on the saucepan of simmering water, whisk the yolks of the eggs with the 2/3 cup of sugar, rum, and water for about 3 minutes until the mixture is thick, like runny mayonnaise. (You can also use a handheld electric mixer.)

3. Remove from heat and place the bowl of whipped egg yolks within the bowl of ice water and beat until cool and thick, as shown in the photo above. Then fold the chocolate mixture into the egg yolks.

4. In a separate bowl, beat the egg whites with the salt until frothy. Continue to beat until they start to hold their shape. Whip in the tablespoon of sugar and continue to beat until thick and shiny, but not completely stiff, then the vanilla.

5. Fold one-third of the beaten egg whites into the chocolate mixture, then fold in the remainder of the whites just until incorporated, but don’t overdo it or the mousse will lose volume.

6. Transfer the mousse to a serving bowl or divide into serving dishes, and refrigerate for at least 4 hours, until firm.

For more on the chemistry of the meringue, refer to the foams post!