Aeration of bread dough during mixing: Effect of mixing dough at reduced pressure

The effect of mixing bread doughs under partial vacuum, as practiced in the Chorleywood breadmaking process, was studied. The void fraction of air and bubble size distributions in doughs were measured, as well as baked loaf volumes, from doughs mixed in several Tweedy-type mixers. Both the void fraction of air and loaf volumes decreased as mixing pressure was reduced. Bubble size distributions did not appear to vary significantly, but the number of bubbles per unit of dough volume decreased with reduced mixing pressure. A mechanism explaining how mixing at reduced pressure produces fewer bubbles and how fewer bubbles translate into a finer crumb structure in the baked loaf is presented. The proposed mechanism suggests that in addition to entrainment, disentrainment, and bubble breakup, a fourth aeration process occurs during dough mixing: bubble compression, arising from the viscoelastic nature of the dough. Evidence is presented that indicates bubble compression occurred during mixing; during subsequent relaxation of the dough, a 65% volumetric expansion of the bubbles was observed, with a time constant of 20 sec.