In this work, the morphology of wheat starch granules in dough before and after baking was investigated using three-dimensional (3D) confocal laser scanning microscopy (CLSM), with a particular focus on gas cell walls (GCWs) of varying thickness and integrity (intact and partially ruptured) located in the loaf's core. The image stacks were acquired over depths ranging from 20.2 & micro;m to 40.8 & micro;m. Individual starch granules were segmented and analyzed in 3D, enabling quantitative characterization of their size and morphology. In total, 1131 granules were analyzed across the six samples. Compared to raw dough, the Feret diameter of starch granules increased by approximately 1.5-fold in thick GCWs, for both B-type (8 to 12 & micro;m) and A-type starch granules (24 to 36 & micro;m). Granule swelling was moderate and occurred preferentially along the direction of gluten extension within GCWs, providing direct evidence of a directional coupling between starch granule and gluten network deformation in a real dough system. Additionally, a small proportion of granules exhibited internal softening within a poorly extensible outer envelope; a comprehensive scheme has been produced to unravel the underlying mechanisms during baking. These results provide new quantitative and three-dimensional insights into starch behavior in the vicinity of gas cell walls and contribute to a better mechanistic understanding of bread structure development during baking.

During the baking of double-layered flatbreads, delamination occurs when the internal flow of water vapor within the flatbread's porous structure significantly exceeds its escape to the outside. This makes the gas permeability of the dough a critical factor in pressure build-up. Data on bread crust and the layers in doublelayered flatbreads are scarce but essential for a better understanding of the baking process. This study investigated the gas permeability of double-layered flatbreads baked at 300 degrees C for varying durations, with and without steam injection, using a specially designed two-chamber experimental apparatus. The study revealed low permeability values in both the upper and lower flatbread layers (ranging from 6 x 10-17 to 3 x 10-15 m2). During delamination, the detachment of the upper layer leads to a more rapid increase in its permeability. The experimental work was complemented by numerical simulations to assess the relevance of permeability to gas generation in the dough. The simulations confirmed the primary role of water vapor compared to carbon dioxide in pressure build-up. The permeability must be below 8 x 10-13 m2, regardless of oven temperature (100, 200, 300, 400 degrees C), to allow for a relative pressure increase in excess of 10 kPa. The greater the permeability, the longer the time taken for pressure to build up, all the more so when the temperature was low.

The objective of this study was to evaluate the ability of a single compression-relaxation test to qualify the softening behaviour of four different starch samples - normal wheat, waxy wheat, normal rice and waxy rice - during heating and within a range of water content (WC) close to that of starch in bread dough during baking (moderate WC). Temperatures were selected according to the gelatinization behaviour of the different starches. Compression-relaxation tests at 10 % strain were performed to identify the step-responses of the materials. The fast compression minimised the relaxation effect during the test, enabling Young's modulus and dynamic viscosity to be determined independently. Young's modulus values ranged from 130 to 1700 kPa, while dynamic viscosity values varied from 7 x 106 to 7 x 108 Pa s across all samples analysed. Microscopic observations and DSC confirmed that starch granules swelled and gelatinized to different extents depending on local liquid water availability. Waxy starches softened more markedly, with Young's modulus approaching that of gluten at 80 degrees C. These findings demonstrate the effectiveness of compression-relaxation tests in assessing starch softening and provide a basis for modelling stress distribution around granules during gas cell expansion.

The delamination of layers (or 'puffing' effect) in flatbreads is mainly governed by heat transfer encouraging manufacturers and small-scale bakers to bake at high temperatures (up to 350 degrees C) and apply direct heat. The use of high baking temperatures to achieve delamination of layers during baking has raised concerns regarding excessive energy consumption, as well as considerations of food quality and safety. This study investigated use of a partial vacuum to enhance the double layering of flatbreads at low temperatures (160 and 200 degrees C) and for different dough thicknesses (2 and 4 mm). Temperature probes and camera were used to monitor the dynamics of deformation and energy transfer under a partial vacuum. To complement this evaluation, final quality attributes (water content, texture test, color and alkaline water retention capacity) were then analyzed for the most promising baking profiles. It is clearly demonstrated by this study that double-layered flatbread can be successfully par-baked or baked at 200 degrees C through the application of low pressure (similar to 50 kPa) to 2 mm thick dough. The role of the partial vacuum in bringing about an earlier boiling point is highlighted, which could boost the evaporative flow, complementing the direct effects of low pressure on gas expansion and desolubilization, and facilitate the delamination of layers.

A small bubble-inflation device was developed for use in conjunction with a Low-to-Mid Range Magnification Digital Microscope. This device made it possible to observe the bi-extension of dough and gluten films at the top of a bubble under conditions that mimicked as closely as possible the typical GCW thicknesses, strains, strain rates and gaseous ambience found in dough during everyday proving or baking. For this stage, experiments were carried out at 20 degrees C. Stress-strain relationships were measured at strain rates ranging from 0.0006 to 0.09 s-1. The reduction in size yielded higher stress measurements compared to the results obtained from usual inflation devices (Alveograph (R)). Strain hardening in dough and hydrated gluten was observed regardless of the strain rate applied. A first trial using image analysis of the top of the bubble during deformation revealed useful information on the locations of the dough constituents and made it possible to reevaluate strain and strain-rate. The Hencky strain and stress at rupture were found to be of the order of 1.23 and 50-100 kPa respectively. This work illustrates the potential benefits of linking image analysis to a bubble-inflation device for the analysis of the mechanisms involved in the mechanical rupture of dough films.

Double-layered flat bread features an impressive oven rise and delamination during baking, leading to the formation of an internal pocket capable of holding various solid foods - a key quality criterion for consumers. These breads are unique in their baking method, which requires specialized ovens and high temperatures between 350 and 550 degrees C. Use of high baking temperatures to achieve the double layering development (called delamination) during baking has raised concerns over excessive energy consumption. In this study, whether or not the application of fermentation (affecting the accumulation of dissolved CO2) and baking temperature (which affects gas generation) were varied and the impacts on delamination were evaluated. To complement this evaluation, dough water content and temperature were monitored during baking. In a novel manner, this study characterized water distribution in flat bread, explaining the surprising decrease of water loss with increasing baking temperature. Our study has clearly shown water vapor to be the prime cause of delamination. The role of water pressure in dough inflation and in causing the edges to detach from the deck and the heterogeneity of flat bread water content were highlighted and a concept map proposed.

During the different steps of bread-making, changes in the microstructure of the dough, particularly in the gas cell walls (GCW), have a major influence on the final bread crumb texture. Investigation of the spatial conformation of GCWs is still a challenge because it requires both high resolutions and 3D depth imaging. The originality of the present work lies in the use of label-free non-destructive multiphoton microscopy (NLOM) to image the 3D structure of GCWs, shedding light on their behavior and organization in wheat bread dough. We demonstrated that second and third harmonic generation (SHG, THG) allow imaging, respectively, of starch granules and interfaces in bread dough, while the gluten matrix was detected via two-photon excitation fluorescence (TPEF). Last, a distinction between the gluten network and starch granules was achieved using gluten endogenous fluorescence (EF) imaging, while the position, size, and 3D orientation of starch granules in GCWs were determined from harmonic imaging, made possible by the acquisition of backward and forward SHG with linear polarization. These innovative experiments highlight the strengths of NLOM for a label-free characterization of bread dough microstructure for the first time, in order to understand the role of starch granules in dough stabilization.

Background: This literature review describes the evolution during baking of the three main components in dough (starch, proteins, and the aqueous phase) in order to understand what causes gas cells to open. To date, most of the literature has focused on the role played by proteins, gluten having received most attention in the last decades (strain hardening properties, ability to stretch without rupturing etc.). The possible role of a liquid lamella has more recently been proposed. While a number of articles directly evidence its existence, indirect results also provide proof of its presence. The role of starch in the mechanisms of gas cell stabilization/destabilization has been little considered. The multiple actions of starch described in this review may offer an explanation for this. Scope and approach: The authors have set out to consider all phases and to understand how they may interact during baking in such a way as to lead eventually to gas cell wall rupture. Key findings and conclusions: The four most likely situations are presented and discussed: gluten with poor ability to stretch: rupture occurs too early during baking. gluten with poor ability to stretch but assisted by a liquid lamella: rupture is delayed; extent of delay is dependent on starch's sorption of water. gluten with good ability to stretch, starch granules soften early during baking but do not fuse (ideal situation): structure opens late in baking when loaf is able to sustain its own weight. too many fusing starch granules: gas cell walls fail to rupture and loaf shrinks during cooling.

A mathematical model was developed to increase the understanding of stress concentrations within a gas cell wall (GCW) in bread dough during baking. The GCW was composed of a single A-type wheat starch granule surrounded by various proportions of gluten typical of GCWs when about to rupture. Finite element simulations were carried out in 2D using linear viscoelasticity and visco-hyperelasticity. Strain orders of magnitude and rates relevant to dough during baking were applied as boundary conditions for two plausible sets of mechanical properties before and after protein coagulation and starch gelatinization (T < 50–60 °C and T > 70–80 °C). The average stress within the GCW was found to be strongly dependent on the starch fraction. Gluten-starch interactions influenced average stress values considerably when the starch fraction was greater than 11% v/v. The locations within the GCW where rupture was most likely to be initiated were identified by mapping maximal stress points using stress field and triaxiality analysis and the findings were discussed.

This paper proposes a model for bubble growth in semi-hard cheese coupling mechanical behaviour and mass transport. The modelling follows previous work centred on the mechanical aspects, and focuses in this paper on the mass transport phenomena. Data are compared to experimental results obtained on industrial-size cheeses, both under the rind and at core, and a sensitivity study is conducted to discuss the results. The model is in agreement with experiment at core, and underlines the great influence of the carbon dioxide production rate and the amount of cheese material surrounding the bubble on bubble growth. Under the rind, the model yielded poorer agreement, due to the fact that this region in the cheese is less homogeneous, and therefore with more intra- and inter-batch variation on the parameters that were characterized.

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This research aimed to study the effects of using a partial vacuum for bread baking on macromolecules and water distribution in gluten-free bread. Bread baking under partial vacuum results in greater oven rise and a larger gas fraction in the crumb. Because water's boiling point decreases under reduced pressure, it was expected that its distribution within the dough and its interactions with the others dough's constituents (mainly starch) would differ from those in bread baked under atmospheric pressure. Time-domain nuclear magnetic resonance was used, as it has the rare capacity to quantify both gelatinization and retrogradation of starch. Complementary rheological measurements made it possible to show that crumb Young's modulus was mostly influenced by the gas fraction whereas there was little change in starch gelatinization and retrogradation when dough was baked under partial vacuum. When insufficiently hydrated (48%), the volume of breads was practically the same whatever the baking process. Meanwhile, the nuclear magnetic resonance results suggested that amylose short-term crystallization (on cooling) is dependent on water content. In addition, crumb Young's modulus during storage at room temperature correlated with an increase in free induction decay signal intensity.

An experimental approach to partial baking of dough rolls at low temperature (< 105 degrees C) and partial vacuum (-20 kPa) was developed. The impact of various partial vacuum and heat transfers at the bottom of dough rolls during baking, on the oven-rise and the crumb structure of baked rolls was evaluated. Oven-rise and crumb structure obtained using partial vacuum were compared to those obtained using a commercial convection oven. A specially Magnetic Resonance Imager (MRI) designed oven was adapted with a temperature-controlled plate to modify the setting of the dough at the bottom of the roll. Dynamic and non-invasive assessment of the structure on the scale of the millimetre was carried out using Magnetic Resonance Imaging. The analysis of the mechanisms involved in the structure setting, on the local scale, was presented according to three stages identified in the oven-rise. No large gas cells and collapse were encountered at the initial lowering in pressure. Partial vacuum produced lower crumb density than that obtained at the atmospheric pressure. The setting of the periphery was involved in the partial collapse at the bottom at the later stage of baking and large rate-of deformations were involved in the formation of large gas cells.

The unidirectional extension of a bread dough gas cell wall (GCW) composed of a single starch granule surrounded by gluten was considered in order to analyse where stress concentrated the most. The impact of gluten/starch interactions (cohesion or non-cohesion) and rheological moduli ratio on stress concentration was numerically assessed in 2D and 3D using the linear and nonlinear Structural Materials Modules available in COMSOL Multiphysics. A first linear viscoelastic 1-element generalized Maxwell (1EGM) model which is theoretically suited for infinitesimal strains (<<0.1) and small strain rates was performed. A second 1-EGM hyper-elastic approach was performed to account for finite strain (large strain > 1). The two approaches were compared. The average of the Von Mises stress throughout the GCW was found to be equal using both approaches up to 0.08 strain. The Gluten/starch interactions were also modelled using the pair Thin Elastic layer (TEL) boundary condition to account for the gluten/starch interface without modelling a dedicated domain.

A 1D baking model was previously developed to improve the understanding of transport phenomena and bubble formation inside dough. Using this model, the present study focused on crust setting and its particular behavior. Simulation was used to study the influence of various parameters that govern crust setting and its characteristics such as thickness and porosity. On the basis of a literature review, the crust was defined as the dry zone, a region where the water content represents less than 0.60kg/kg of dry matter. The mechanisms responsible for total water loss and crust setting were proposed in paper I and validated for multiple operating conditions: water loss is essentially driven by heat transport and to a lesser extent by the mechanism of water transport by evaporation-condensation-diffusion. In the present study, a sensitivity analysis of parameters of different driving mechanisms was undertaken through which it was possible to establish a hierarchy of the key parameters from a technological point of view. Overall heat transfer coefficient at the top of the loaf and thermal conductivity, or more generally the ratio between top and bottom heat fluxes, were ranked as the main parameters affecting crust setting and the water loss, thus suggesting a possible control of the process through the parameters evaluated.

A two dimensional model of bread baking was developed including, for the first time, the dependence of dough viscosity on both temperature and moisture content, the carbon dioxide dissolved from liquid water together with gas generation from yeast at the beginning of baking and the shrinkage due to dough drying. Particular attention was paid to experimental validation of both overall and local variables such as local temperature, overall mass loss, and local moisture content, overall CO2 released into the oven, and overall deformation and local expansion or shrinkage. Sensitivity studies on generation of carbon dioxide, gravity, and shrinkage are presented to discuss their influences on bread geometry, porosity (reflecting the alveolar structure) and gas pressure. © 2016 American Institute of Chemical Engineers AIChE J, 62: 3847–3863, 2016

The aim of this study was to investigate the momentum transport occurring during bubble growth in a viscoelastic material. The mechanical behaviour was modelled with a 5 element Maxwell model, implemented with the finite elements method, and a sensitivity analysis of the model parameters was undertaken. Air was injected into the bubble and growth was monitored with pressure sensors and by the analysis of MRI image. The experiment was repeated three times. Each time mechanical parameters were characterised on the same material as that used for the experimental validation of the model. Simulations were conducted in geometric and boundary conditions as close as possible to those of the experiment, and yielded good agreement with the experimental results. (C) 2016 Elsevier Inc. All rights reserved.

In this study, a model of transport phenomena describes a single bubble growth in semi-hard cheese. Carbon dioxide production, its transport to the bubble interface, equilibrium laws and mechanics were coupled. Semi-hard cheese mainly behaves as elastic when loads are quickly applied to a piece of cheese like during chewing (few seconds). However, when slowly loaded with increasing gas pressure during ripening in warm room, the mechanical cheese behavior can be simply modelled as a viscous material (Grenier et al. [9]). It is true, as long as viscosity remains low compared to the rate of gas production. This paper investigates a wider range of viscosity (from core η = 6.32 × 107 Pa.s to rind η = 2.88 × 108 Pa.s) than that used in previous studies. FEM simulations have shown that higher viscosities encountered close to the rind of a cheese block can partly explain the increase in gas pressure within bubbles from the core to the rind (up to 3.4 kPa). These results confirm that mechanics does not really control the evolution of bubble volume in cheese. However, mechanics can explain greater pressure observed close to the rind even if gas production is lower than at core.