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In bakery products, quality attributes such as texture, colour, softness and springiness of the crumb are important attributes for consumer’s perception. These attributes mainly set up during the baking stage and are influenced by baking conditions. Many physical and chemical changes occur to lead to a porous structure. The objective of this work is to follow the setting of a bread structure with focuses on the cellular microstructure of sandwich bread type by X-ray micro-tomography (µCT).
Samples like fermented dough continue to evolve even during the µCT scanning entangling the possibility to get focused images. An alternative is to scan them in the frozen state. A sample holder was designed (Chevallier et al., 2016) in order perform a fast freezing inside a Dewar filled with solid CO2. Samples included dough, bread and partially baked samples at 48 °C, 68 °C, and 98 °C, to identify critical stages of the structure development and fixation. Then, the frozen samples could be scanned without getting defrosted for acquisition times varying from 3 to 5 min.
The porosity and the size of the gas cells were analyzed from reconstructed sections after the determination of the minimum representative volume of interest and an automatic thresholding to get binary images. Porosity was determined from the voxel ratio (void volume in the volume of interest) in the 3D image analysis. The size of the gas cells was derived from the local thickness calculation (Hildebrand and Rüegsegger, 1997) i.e. the diameter of the largest sphere which encloses a point in the void and which is entirely bounded within the solid surfaces.
Dough samples exhibit a homogeneous distribution of small gas cells. When temperature rises in the sample during baking, pores sizes evolve until the structural fixation which occurred near 68 °C, marking the transition between pore expansion and stabilization. This fixation is caused by starch gelatinization and protein denaturation which are well-known key processes for dough setting and gas retention capacity.
A methodology has been successfully developed for the study of dough samples in the frozen state. It allowed the acquisition of 3D images with a higher resolution by reducing the movements inside the samples. This is a powerful tool in characterizing the microstructure of dough and its transformation in bread during baking. It gives access to the 3D structure that can be analyzed and gives access to the porosity, the size distributions of the pores and the matrix to describe the cellular structure. The 3D model that can be built from the data set can be a really helpful tool for different applications and, particularly, the simulation of the heat and mass transfers occurring during baking and the understanding of the implementation of structure during food processing.
| References | 1. Chevallier, S., Rouaud, O., Jury, V. (2016). X-ray microtomography to study the baking of bread. A dynamic approach to follow crust and crumb formation. Proceedings of the Brucker microCT user meeting, Luxembourg, Luxembourg. 2. Hildebrand, T, Rüegsegger P, “A new method for the model-independent assessment of thickness in three-dimensional images” Journal of Microscopy, 67-75, 1997. |
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| Country | France |
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