Anne-Laure Réguerre

In situ Fast X-ray Tomography Study of the Evolution of Cellular Structure in Bread Dough During Proving and Baking

The use of fast in-situ X-ray computed micro-tomography with careful 3D image analysis has allowed the development of gas cell structure during dough fermentation to be followed, within a volume close to 0.1 cm 3 with a resolution of 15 μm. Yeast and liquid (water +oil) contents were modified in the dough composition. The evolution of dough was followed until the void volume fraction reached 0.7, after an inflection point. The gas cell and cell wall size distributions were determined by granulometry and their initial values were respectively 180 and 240-300 μm. The evolution of the mean cell wall size displayed a minimum plateau (180-240 μm) in a time interval [ t 1 , t 2 ]. Before t 1 , bubbles grow freely and beyond t 2 , coalescence appears to prevail, as shown by the irregular structure depicted by the cell size distribution. The value of t 1 is found to increase by a factor of 2 when the concentration of yeast is reduced by 2, and replacing 5% water by 10% oil also increases t 1 . Both changes of composition lead to more irregular cellular structures. Similar analyses were performed during the baking step, which allowed the interval of temperature for crumb setting to be determined, and showed that little change of cellular structure occurred during baking.

Determining the Cellular Structure of Two Cereal Food Foams by X-ray Micro-Tomography

The cellular structure of two products, an extruded breakfast cereal and a short dough biscuit, was characterized by two different X-ray micro computed tomographic systems. Acquisitions were made by a compact desktop system Skyscan 1174 (Bruker μCT, Belgium) and at the European Synchrotron Radiation Facility (ESRF, beamline ID19, France) at different resolutions (voxel size of 6.5xa0μm, 7.5xa0μm, 16.2xa0μm and 25.8xa0μm). 3D images were processed for the density, the connectivity index and the granulometry of cells and cell walls. These experiments underlined the importance of the resolution for determination of quantitative measurements such as densities and thicknesses. The median width calculated for the cell walls distribution in the biscuit dropped from 141 to 50xa0μm when the voxel size changed from 25.8 to 7.5xa0μm. Images well showed that even though the food products had close values of porosity 0.6 and 0.7 for biscuit and extruded breakfast cereal respectively, their cellular structures were very different. The biscuit had small cells (median value of the distribution varied from 125 to 152xa0μm, according to resolution) and larger cell walls (50–141xa0μm) than the extrudate (32–109xa0μm) which, on the contrary, exhibited very large cells (307–400xa0μm). Beyond methodological issues, these differences could be clearly attributed to the differences of compositions and processes.

Multi-scale structural changes of starch and proteins during pea flour extrusion

Dehulled yellow pea flour (48.2% starch, 23.4% proteins, d.b.), was processed by a twin-screw extruder at various moisture contents MC (18-35% w.b.), product temperature T (115-165u202f°C), and specific mechanical energy SME (50-1200u202fkJ/kg). Structural changes of extruded pea flour were determined at different scales by measurements of density (expansion), crystallinity (X-ray diffraction), gelatinisation enthalpy (DSC), starch solubility in water and protein solubility in SDS and DTE (SE-HPLC). Foam density dropped from 820 to 85u202fkg/m3 with increase in SME and T (R2u202f≥u202f0.78). DSC and XRD results showed that starch was amorphous whatever extrusion conditions. Its solubility in water augmented up to 50%. Increasing temperature from 115 to 165u202f°C decreased proteins soluble in SDS from 95 to 35% (R2u202f=u202f0.83) of total proteins, whereas the proteins soluble in DTE increased from 5 to 45% (R2u202f=u202f0.75) of total proteins. These trends could be described by sigmoid models, which allowed determining onset temperatures for changes of protein solubility in the interval [125, 146u202f°C], whatever moisture content. The SME impact on protein solubility followed similar trends. These results suggest the creation of protein network by SS bonds, implicating larger SDS-insoluble protein aggregates, as a result of increasing T and SME, accompanied by creation of covalent bonds other than SS ones. CSLM images suggested that extruded pea flour had a composite morphology that changed from dispersed small protein aggregates to a bi-continuous matrix of large protein aggregates and amorphous starch. This morphology would govern the expansion of pea flour by extrusion.