Chocolate Mousse Day

What the day is

Chocolate Mousse Day is observed on April 3. A mousse is a foam, and chocolate turns out to be an awkward material to build a foam from.

Origin and background

A foam is gas held in a liquid by something that stops the bubbles merging. In a classic mousse that something is protein or fat. Whipped egg white builds a film of unfolded protein around every bubble, while whipped cream relies on partly crystallized fat gathering at the bubble surfaces. Most recipes use one or both, with chocolate for flavor and for firmness once it is chilled.

The awkwardness is chocolate's relationship with water. Melted chocolate is cocoa solids and sugar suspended in cocoa butter, and a few drops of water make it seize into a stiff grainy paste, because the water wets the sugar and cocoa particles and glues them into clumps while the fat stays stubbornly separate. This is why every recipe insists on a dry bowl.

The counterintuitive part is that more water fixes it. Add enough and the particles disperse properly into a continuous water phase, and the mixture goes smooth and fluid again. A French chemist demonstrated the consequence: chocolate melted with water alone, then whipped over ice as it cools, sets into a mousse with the texture of the classic version and no egg or cream in it whatsoever. The cocoa butter crystallizing as it chills does the work that dairy fat would otherwise do.

It fails in a specific way too. Overwhipping drives the fat crystals together until the foam collapses into something grainy, and the remedy is to melt it and whip it again, which works about half the time.

Temperature handles most of the rest. Chocolate poured into a cold mixture sets instantly into hard flecks, which is why recipes insist on folding warm into cool and never the reverse.

How to take part

• Keep every bowl and whisk bone dry until water goes in deliberately.
• Try the chocolate-and-water version, which needs only two ingredients.
• Stop whipping earlier than feels right.
• Fold warm chocolate into a cool mixture, never the other way around.