Ice cream samples were periodically removed from a batch freezer to follow air incorporation and development of the air cell size distribution, as measured with an optical microscopy technique. The effects of operating conditions (freezing, not freezing, and freezing part way) and formulation (fat, emulsifier and stabilizer content) on development of air cells were studied. Air cell size decreased progressively with time during batch freezing. Whipping alone (no freezing) did not result in decrease in air cell size, indicating that freezing was necessary for stabilization of small air cells. Fat level and emulsifier content had no effect on air cell size, whereas stabilizer addition led to smaller initial air cells. The reduction in air cell size was correlated to changes in apparent viscosity of the slurry being formed within the freezer. (C) 2002 Elsevier Science Ltd. All rights reserved.