A study of the crust setting and its effect on heat and mass transport and expansion during bread baking

The final properties of crust like thickness, crunchiness, or newly formed compounds contribute greatly to the final appreciation of bread products by the consumer. Surprisingly, the mechanisms underlying crust formation during baking received little attention so far, although the subject is gaining interest. The objectives of this study were to (i) better understand these mechanisms, (ii) assess the physical effects of crust on the baking process itself. A 1D baking model previously validated against experiments helped in achieving the latter objective (Wagner et al, 2006). By simulation, when the crust set early while the dough core still did not warm, the late expansion at core induced the compression of crumb located beneath the crust. Ruptured, but not already stiffened cell walls were required for compression. The use of a fabric lid permitted to experimentally limit the expansion at different final heights, hence at earlier times, without disturbing mass transfer between the dough and the oven. Both experiment with MRI and simulation showed that the densest crumb regions moved towards the bread core with a delayed crust setting. All this demonstrated that the competition between heat transport to the core and crust formation during baking greatly affected the final profile in crumb porosity (Zhang et al, 2007). Both experiment using infra-red and simulation showed that profiles in porosity also affected total water loss (WL), which is of technological interest again. High porosity in crust decreased WL since it implied a thicker crust in turn limiting the rate at which heat penetrates to the vaporization front. Conversely, high porosity in crumb favored water transport towards the core by evapo-condensation-diffusion, which in turn enhanced the crust thickening and limited WL by the above-mechanism. In a next step related to the objective (i), heat and mass transport and biochemical reactions were investigated in the surface layers of dough during baking. Heating and drying rates were changed by varying dough geometry, oven temperature or steam injection. Crust thickness and profiles in water content were measured from samples after repeated interruptions of baking. Dough stiffening was monitored by DMTA, reproducing the heating and drying rates previously observed. All these data aimed at a thorough formalization of the mechanisms governing crust formation in a 2D baking model, as well as its validation. Preliminary results will be presented together with this perspective.