Chilling and freezing of part-baked bread. Part II: Experimental assessment of water phase changes and structure collapse Original Research Article

The first objective of the present work was to assess physical changes taking place during the pre-chilling and freezing of part-baked bread, by means of MRI. The second objective was to relate them to the appearance of crust flakes in certain circumstances after final baking at retail. The intensity of the MRI signal decreased in all voxels once water crystallization started. The decrease was preceded by a plateau (signal of constant intensity) except in the case of certain voxels under the upper crust, which exhibited an increase in signal intensity of varying magnitude. Neither the dry matter measurements of samples taken from different locations in the loaf nor the analysis of MRI signal intensity during prolonged storage gave significant evidence of any local increase in water content following possible vapor condensation on cooling. The increase in MRI signal intensity observed in particular voxels immediately under the upper crust was attributed to loaf collapse and subsequent densification of the crumb at local scale. This interpretation led the authors to propose a hypothetical mechanism relating flaking to collapse. If the loaf cannot deform in response to the compression of the gas phase on cooling, tensile forces will develop within its structure and make it more sensitive to mechanical shocks during transportation or to high hydrothermal stresses on final baking. This would finally result in some form of breakdown such as the detachment of parts of the crust and flaking phenomena. The theory is supported by MRI data showing that loaves with a lower level of deformation (i.e. of densification) were those most susceptible to flaking. This mechanistic theory is consistent with trends previously reported in the literature.