How the drying rate at bread crust can affect its viscosity? | Comment le séchage de croûte du pain affecte sa viscosité ?

Water evaporates very fast from the surface layers of dough, enhanced by high heating rates at the beginning of baking. The rheological properties of the surface layers play an important role during the baking process, especially in local and overall expansion as well as water loss. Therefore, the aim of this study was to determine the rheological properties of bread dough in the heat-moisture dynamics of the baking process, with the view of largely exploring the crust water content-temperature domain covered by usual variations in the oven air temperature and velocity, steam injection, radiation and others different baking conditions. The dynamic mechanical thermal analysis (DMTA) method was retained since temperature could be controlled up to values encountered in a baking oven. Tensile rather than compression mode was retained since all the faces of the sample were exposed to dehydration and solicitation of the dough strip. Screening of the water content-temperature domain with the rheometer oven was achieved by varying the heating rate. Temperature and water content were monitored during DMTA measurements. Different heating rates, from 1 to 30°C/min, in dough strips mimicking bread crust were successfully reproduced in the oven of a DMTA analyzer. At high heating rates (15-30°C/min) and below the temperature of water ebullition, the present study showed that dough viscosity behaved as if water content was kept constant. The heat-moisture dynamics generated by these high heating rates were close to those encountered in crust in most baking conditions (oven temperature of about 200°C and higher). These results confirmed the only temperature-dependency of mechanical properties formerly made by models of transport and deformation during baking. For lower heating rates, starch gelatinization was not accompanied by an increase in viscosity as for higher rates, but rather a slow down; the decrease in water content appeared to govern the general increase in viscosity.