On Food and Cooking

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.