By Harold McGee
Once the mix has been prepared, it’s prechilled to speed the subsequent freezing. It’s then frozen as rapidly as possible in a container with coolant-chilled walls. The mix is stirred to expose it evenly to the cold walls, to incorporate some air, and above all to produce a smooth texture. Slow cooling of an unstirred mix—“quiescent cooling”—causes the formation of relatively few ice crystals that grow to a large size, grow together into clumps, and give a coarse, icy texture. Rapid cooling with stirring causes the quick production of many “seed” crystals which, because they share the available water molecules among themselves, cannot grow as large as a smaller population could; the agitation also helps prevent several crystals from growing into each other and forming a cluster that the tongue might notice. And many small crystals give a smooth, velvety consistency.