Thermophysical properties evolution of French partly baked bread during freezing (original) (raw)
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Food Research International, 2007
The thermal conductivity of food is usually measured in isothermal conditions with a line-heat source probe. The thermal conductivity of bread during the baking phase is function of temperature and is an important parameter to assess. This work aims at (i) developing a measurement method of thermal conductivity in pseudo-non-isothermal conditions, usable during the part baked bread thawing-baking phase, and (ii) establishing models to predict the change of the thermal conductivity as function of the temperature. The line-heat source probe method has been used, the probe temperature being corrected by subtracting the temperature increase due to the baking. Values obtained in pseudo-non-isothermal conditions during the thawing-baking phase are similar to these ones obtained in isothermal conditions. A parallel model has been successfully used to fit the experimental values of the thermal conductivity of bread during the baking phase.
Heat and mass transfer in par-baked bread during freezing
Food Research International, 2004
To understand the phenomena governing the heat and mass transfers during the freezing of par-baked bread, simulation of freezing in an infinite two-layer cylinder was performed. A fixed grid finite difference method was used in the solution of simultaneous heat and moisture transfer (SHMT) equations according to Lees' three-level scheme. The model accommodates the effects of temperature dependent variables such as apparent specific heat, enthalpy, thermal conductivity, and water activity, and predicts the temperature profiles and weight losses. The model was verified by freezing of par-backed breads with cylindrical shape within an experimental freezer. The RMSE between experimental results and model predictions is 0.505°C for surface temperature, and 1.8°C for center temperature. The precision on the loss water is 10%. These results indicate that the method could be successfully applied to SHMT operations in foods freezing.
Moisture diffusivity and water activity of part-baked bread at above and sub-freezing temperatures
International Journal of Food Science and Technology, 2006
Accurate values of the effective moisture diffusivity and water activity of foodstuffs are necessary in order to study mass transfer and the weight loss during freezing. This work focuses on the evaluation of the effective moisture diffusivity of part-baked bread and its crumb and crust and of the water activity of the part-baked bread crust. The effective diffusivities have been estimated by analyses of drying data on the bread and crumb samples. They were related to the temperature by the Arrhe´nius relationship. Water activity was experimentally determined at positive temperatures. Experimental data were modelled by the Guggenheim-Anderson-de Boer (GAB) model and a model deduced from the Clausius-Clapeyron equation. The results predicted by the Arrhe´nius and GAB models showed a good agreement with the experimental values. Finally, the moisture diffusivity and water activity in the subzero domain were obtained by extrapolation using an Arrhe´nius model and a GAB model, respectively.
Thermal conductivity measurements of a traditional fermented dough in the frozen state
Journal of Food …, 2007
In this study thermal conductivity of tarhana, a traditional Turkish fermented food product, was measured over the temperature range À25°C and +50°C using the needle probe method. Empirical equations for thermal conductivity calculations were obtained by application of statistical analysis to the results. Thermal conductivity of the samples was found to increase with temperature in the unfrozen state; in the frozen state it decreased with temperature. The probe method of conductivity measurement was found to give accurate and consistent results.
Assessment of thermal conductivity as a function of porosity in bread dough during proving
Food and Bioproducts Processing, 2009
During the proving process, yeast fermentation in the aqueous phase of dough produces carbon dioxide which diffuses through the aqueous phase to the gas cell nuclei entrapped during dough mixing. CO 2 evaporates to generate within gas cell nuclei an excess pressure that provides the driving force for dough expansion. As a result, the void fraction of the dough increases and thermal conductivity decreases. These two properties are of considerable interest because of their technological significance in breadmaking (specific volume and processing times). The aim of the present study was therefore to investigate experimentally the thermal conductivity and porosity evolutions of bread dough during the proving process. Apparent density and thermal conductivity of fermenting dough were experimentally determined as a function of fermentation time using the line-heat source probe and image analysis methods, respectively. Thermal conductivity values were observed in the range of 0.36-0.12 W m −1 K −1 and values of void fraction air between 6 and 78% at proving conditions of 35 • C and 95% HR for 100 min of fermentation time. According to experimental data, relationship between thermal conductivity and porosity ε could be adequately described by a nonlinear equation: (ε) = 0.421-0.134ε-0.219ε 0.5 .
Thermal Properties of Frozen Food: A Review Article
International Journal of Engineering Applied Sciences and Technology, 2020
Food is a mixture of solutes and water particles. The properties of the food material vary by the state of water particles present in them. The density, viscosity are some of the factors that attribute to the difference in thermal properties between frozen and unfrozen food. Several studies have been made in analyzing the thermal properties of frozen food of different varieties from frozen soup pouches to frozen meat. Several properties related to thermic properties such as thermal conductivity, latent heat, heat capacity is also under consideration. For instance, density is found using the known volume of the sample, porosity using water, and so on. Thus thermal properties of frozen food are of immense importance for developmental processes.
Estimation of thermal conductivity of short pastry biscuit at different baking stages
Journal of Agricultural Engineering, 2014
Thermal conductivity of a food material is an essential physical property in mathematical modelling and computer simulation of thermal processing. Effective thermal conductivity of non-homogeneous materials, such as food matrices, can be determined experimentally or mathematically. The aim of the following research was to compare the thermal conductivity of short pastry biscuits, at different baking stages (60-160 min), measured by a line heat source thermal conductivity probe and estimated through the use of thermo-physical models. The measures were carried out on whole biscuits and on powdered biscuits compressed into cylindrical cases. Thermal conductivity of the compacted material, at different baking times (and, consequently at different moisture content), was then used to feed parallel, series, Krischer and Maxwell-Eucken models.
Thermal diffusivity determination of pizza and puff pastry doughs at freezing temperatures
Journal of food processing and …, 2007
The thermal diffusivity of pizza and puff pastry doughs was measured in the temperature range from -35 to +15C by using time-temperature history method. Experimental data of thermal diffusivity were mathematically interpreted as a function of temperature by regression analysis in the frozen and unfrozen states. The thermal diffusivity of the frozen pizza and puff pastry doughs was greater than the unfrozen samples and decreased with temperature in the frozen state. Experimental values were compared to the prediction model and with previously documented values. A close agreement was found between the thermal diffusivity values determined experimentally and the values available in published literature.
3 Changes in Dough and Bread Structure as a Result of the Freezing Process
Advances in Heat Transfer Unit Operations: Baking and Freezing in Bread Making explains the latest understanding of heat transfer phenomena involved in the baking and freezing of bread and describes the most recent advanced techniques used to produce higher quality bread with a longer shelf life. Heat transfer phenomena occur during key bread-making stages (cold storage, resting, and fermentation) in which temperature and amount of heat transfer must be carefully controlled. This book combines the engineering and technological aspects of heat transfer operations and discusses how these operations interact with the bread making process; the book also discusses how baking and freezing influence the product quality. Divided into fourteen chapters, the book covers the basics of heat and mass transfer, fluid dynamics, and surface phenomena in bread-making industrial operations, mathematical modelling in porous systems, the estimation of thermo-physical properties related to bread making, design of equipment, and industrial applications.