What are the mechanics of folding?
The goal of folding is to incorporate delicate, airy ingredients such as whipped cream or beaten egg whites into heavy, dense ingredients such as egg yolks, custard, or chocolate without causing deflation. The tools required for folding are a balloon whisk and a large, flexible rubber spatula.
In the test kitchen, we like to start the process by lightening the heavier ingredients with one-quarter or one-third of the whipped mixture. A balloon whisk is ideal for the task: Its tines cut into and loosen the heavier mixture, allowing the whipped mixture to be integrated more readily. Next, the remaining whipped mixture can be easily incorporated into the lightened mixture. For this round of folding, we preserve the airiness of the dessert by using a rubber spatula, which is gentler than a whisk.
To demonstrate the importance of folding, we made two kinds of lemon soufflés and our Triple-Chocolate Mousse Cake using three methods: incorporating the whipped ingredients in two additions as specified in the recipes; folding the whipped ingredients in all at once; and finally, vigorously stirring in the whipped ingredients in one addition. The results were not surprising. When the beaten egg whites or whipped cream was incorporated in two batches, the soufflés and mousse were perfectly smooth and light. When we ignored the two-step process and folded everything together at once, the desserts were not quite as ethereal. Finally, strong-armed stirring produced a lumpy, dense end-product.
Our recommendation: Don’t cut corners when it comes to folding. Take your time and use a light hand to gradually incorporate whipped cream or beaten egg whites into heavier ingredients.