Characterizing the cellular structure of bread crumb and crust as affected by heating rate using X-ray microtomography

The effect of two baking conditions 240 °C and 220 °C (corresponding to heating rates 7.39 and 6.11 °C/min respectively) on the cellular structure of bread was investigated using X-ray microtomography. A comparison between helium pycnometry and X-ray microtomography was carried out and confirmed the quality of analysis in 3-D. Porosity profiles were determined in the interface crust/crumb and showed higher porosity and lower density of the upper crust when increasing heating rate and baking with steaming. The porosity profile of the whole slice bread showed differences between breads baked at 220 °C and 240 °C; that can be explained by the non uniformity in local expansion during baking resulting in different areas of variable density. Higher density was found in the bottom of the slice due to compression forces during baking. However, the upper zone of the slice was more porous, in relation with the expansion. These differences influence the texture and led to different kinetics of staling. Results of tortuosity confirm that the relative path length is shorter along the height related to the expansion of the bread during baking. Additionally, the relative path length through the pores is shorter when baking at 240 °C than when baking at 220 °C, in relation with porosity.

► Effect of heating rates (baking 220 °C and 240 °C) on bread structure was studied by X-ray tomography.
► Porosity is higher and density lower for upper crust when increasing heating rate.
► Heterogeneity in local expansion explain a difference in porosity map for the two temperatures.
► Differences in density and porosity influence the texture and the staling.
► Tortuosity is lower in the Z-direction and also when baking at 240 °C.