Denis Flick

Water loss and crust formation during bread baking, Part I: Interpretation aided by mathematical models with highlights on the role of local porosity

ABSTRACT This study aimed to achieve greater understanding of the contributions of different mechanisms of water transport to the overall amount of water lost by bread loaves during baking. The analysis was based on simulations from a previously published baking model which takes into account heat and mass transport as well as the expansion of the gaseous phase leading to oven rise. Liquid and gaseous diffusion, convective transport through the gaseous phase (Darcy’s law), and the diffusive transport of vapor (evaporation–condensation–diffusion) were considered. When only permeation (Darcy’s law) prevailed, the greater the porosity of the top dough surface, the lower and slower the water loss (WL): in fact, the greater the porosity, the thicker the top area. This in turn limited the rate at which heat penetrated the top layers down to the vaporization front and slowed WL. At earlier baking times, when the ebullition temperature was not reached uniformly through the crumb, the evaporation–condensation–diffusion was involved at the same time as permeation. The water flux by evaporation–condensation–diffusion was orientated toward the dough core; it accelerated the crust thickening and slowed WL. It appeared that the higher the porosity in the crumb beneath the crust, the higher the flux by evaporation–condensation–diffusion, and the lower the WL. Local porosity variations in the crust (top surface) and the crumb beneath the crust could be reproduced experimentally by constraining the total expansion of the loaf with a permeable fabric cover. Experimental and simulated trends in WL were consistent, supporting the understanding of WL proposed.

Two-Way Coupling of Fluid-flow, Heat-Transfer and Product Transformation during Heat Treatment of Starch Suspension Inside Tubular Exchanger

Abstract When a liquid product is subjected to thermal treatment under continuous flow, its transport properties can evolve as a consequence of changes occurring on the product constitution (continuous phase, particles…). This two-way coupling is implicitly considered when the residence time distribution (RTD) associated with the product is studied through experimental work. For a number of liquid food products, there is experimental evidence that the minimum residence time decreases as the transformation state progresses. In this study, modeling work is employed for assessing the influence of transformation on the RTD associated with a starch suspension under continuous heat treatment. After including the impact of starch granules swelling on the suspension viscosity, the minimum residence time decreases from 55 % to 41 % of the mean value. The role played by the transformation in driving the RTD cannot be neglected if the transport properties evolve with the product transformation state.

Effect of freeze-dryer design on heat transfer variability investigated using a 3D mathematical model

During the freeze-drying process, vials located at the border of the shelf usually present higher heat flow rates that result in higher product temperatures than vials in the center. This phenomenon, referred to as edge vial effect, can lead to product quality variability within the same batch of vials and between batches at different scales. Our objective was to investigate the effect of various freeze dryer design features on heat transfer variability. A 3D mathematical model previously developed in COMSOL Multiphysics and experimentally validated was used to simulate the heat transfer of a set of vials located at the edge and in the center of the shelf. The design features considered included the vials loading configurations, the thermal characteristics, and some relevant dimensions of the drying chamber geometry. The presence of the rail in the loading configuration and the value of the shelf emissivity strongly impacted the heat flow rates received by the vials. Conversely, the heat transfer was not significantly influenced by modifications of the thermal conductivity of the rail, the emissivity of the walls, or the geometry of the drying chamber. The model developed turned out to be a powerful tool for cycle development and scale-up.

Water loss and crust formation during bread baking, Part II: Technological insights from a sensitivity analysis of a numerical model of baking

ABSTRACT 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 0.60u2009kg/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.