The trick is to mix the two materials as evenly as possible while losing as little air as possible Typically, between a quarter and half of the foam volume is lost at this stage The traditional method of mixing base and foam is to vigorously stir a quarter of the foam into the base to lighten it, then use a spatula to “fold” the two together by repeatedly scooping some base, cutting vertically through the foam, and depositing the base along the cut surface Why laboriously fold rather than quickly stir? Because the rough mass of starch, fats, and other foreign matter in the base pops bubbles, and the more you rub the bubbles against such a mass, the more bubbles you lose Simply stirring continuously grates the two phases together and causes a substantial loss of air Folding has the advantage of disturbing the foam only along the surface where the base is being deposited, and that surface is only disturbed for a single stroke The result is minimal grating of bubbles against mix, and maximal bubble survival Despite the usual cookbook direction to fold the whites and base together quickly, it’s best to fold slowly The disruptive shear force felt by a given bubble is proportional to the velocity at which it’s being pushed along the base The slower your spatula moves, the less damage it will do to the foam The one exception to the folding rules is the soufflé made with a fruit puree or juice cooked with sugar to a thick syrup Such a base can be poured onto the foam as it’s beaten — a soufflé version of the Italian meringue — and will actually increase the mix volume Preparing and Filling the Soufflé Dish Ever since La Chapelle’s timbale of cream, soufflé dishes have been prepared in two steps: first the interior is buttered, and then coated with sugar for a sweet soufflé, with breadcrumbs or ... felt by a given bubble is proportional to the velocity at which it’s being pushed along the base The slower your spatula moves, the less damage it will do to the foam The one exception to the folding rules is the soufflé made with a fruit puree or juice... base can be poured onto the foam as it’s beaten — a soufflé version of the Italian meringue — and will actually increase the mix volume Preparing and Filling the Soufflé Dish Ever since La Chapelle’s timbale of cream, soufflé.. .The result is minimal grating of bubbles against mix, and maximal bubble survival Despite the usual cookbook direction to fold the whites and base together quickly, it’s best to fold slowly The disruptive shear force