Chilling and freezing of part-baked bread. Part I: An MRI signal analysis Original Research Article

The overall objective of the present work is to locally assess physical changes taking place during the pre-chilling and freezing of part-baked bread and attempt to relate them to the appearance of crust flakes after final baking at retail. To achieve this objective, it was first necessary to develop an MRI device for the continuous non-invasive monitoring of the pre-chilling and freezing steps and to focus on interpreting the MRI signal as precisely as possible. There are many known sources of variation in the MRI signal during the process, including local changes in temperature, water content, porosity, ice content and molecular structure (e.g. under gelification). The present paper presents and discusses the results of this preliminary work. NMR and MRI experiments with non-yeasted dough revealed that during the pre-chilling step the MRI signal in a given voxel was mainly proportional to the local density (or inversely proportional to the local porosity) of the bread and to a lesser extent to the local liquid water content of the crumb, as water vapor is not visible with MRI. The MRI signal is assumed to decrease proportionally to the local ice content during the freezing step.