Kinetics of bubble growth in wheat flour dough during proofing studied by computed X-ray micro-tomography

Abstract

Bread doughs of various compositions were prepared and studied by computed X-ray microtomography (XRT) with high resolution (5 μm). Their cellular structure was followed during the last stage of proofing, from 40 min to 180 min, to focus on the films separating bubbles and their possible coalescence. Image analysis allowed to determine the porosity, the connectivity index, the bubble size and the cell walls thickness distributions which were fitted by usual mathematical functions. Images showed that different compositions led to different cellular structures. The kinetics of porosity and connectivity had sigmoid shapes with 0.7 and 1 as asymptotic values, respectively. For an average formula, mean cell width grew from 410 μm to 675 μm and the mean cell wall thickness grew from 200 μm to 230 μm. At the end of proofing, most bubbles were interconnected, i.e. they were separated by films of thickness lower than 5 μm. Changes of recipe led to different structures, which were discussed through the influence of the liquid fraction and of the sugar content on the coalescence of bubbles. These results suggested that at the end of fermentation, the dough could be considered as a three phase co-continuous medium: gas/liquid/viscoelastic matrix.