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Title:Microstructural and fluid transport properties of potatoes during frying
Author(s):Alam, Tanjila
Advisor(s):Takhar, Pawan
Department / Program:Engineering Administration
Discipline:Agricultural & Biological Engineering
Degree Granting Institution:University of Illinois at Urbana-Champaign
Abstract:Microstructural and transport properties are the key determinants to understand the product quality, oil uptake and moisture loss during frying of foods. Besides, permeability is also an important physical property for studying the transport mechanisms of porous foods when pressure driven flow is involved. Food processes like frying, drying, solvent extraction, microwave heating, baking, membrane separation and reverse osmosis have significant pressure development. Experiments were conducted to investigate the microsructural changes during frying of potato disc and determine the oil and water permeability of potato disc at different temperatures. The complex microstructural changes and mass transfer mechanisms in potato discs during frying were analyzed. The potato discs (thickness of $\approx$1.65mm) were fried at 190$^{\circ}$C for 0, 20, 40, 60 and 80 s and X-ray micro-computed tomography ($\mu$CT) was used for three-dimensional (3D) imaging of microstructure of porous potato discs for different frying durations. Total porosity, pore size distribution, oil content and air content of potato discs were calculated from resulting 3D data sets. Oil and air content measured by analysis of micro-CT images followed trends similar to Soxtec and Pycnometer methods, respectively. Image analysis showed a significant change in pore size distribution as a function of frying time. Frying time was also observed to have an effect on tortuosity, which is an important microstructural transport property. Tortuosity was measured by path length ratio method from 3D data sets obtained from image analysis. A linear inverse relationship was observed between porosity and tortuosity where tortuosity decreased with the increase of porosity. During frying, oil content increased with the decrease of tortuosity. This phenomenon indicated that the lower tortuosity created a less complicated and sinuous path, thus resulting in less resistance to oil penetration. Micro-CT technique can serve as an effective tool for elucidating microstructure of fried foods, and can provide complementary information to conventional lab techniques. The permeability values of water and oil through porous potato discs were determined by developing an engineering model using Darcy's law relation, mass balance equation and poroelasticity relations at temperatures within 25$^o$C - 80$^o$C. A finite element package (COMSOL Multiphysics ver. 5.0, Burlington, MA) was used to solve these equations to obtain the velocity profiles at different pressure levels. The water permeability values determined from the velocity profiles were in the range of (2 to 4) x$10^{-15}$ $m^2$ for the temperature range from $70^oC$ to $80^oC$ and the oil permeability were in the range of (2 to 12)x$10^{-15}$ $m^2$ for temperature range from $25^oC$ to $70^oC$ under the applied pressure of 138 to 348 kPa. The predicted velocity-pressure gradient curves for obtaining permeability values showed good agreement with the experimental velocity-pressure gradient data points. An Arrhenius model was developed to represent the relationship between permeability, fluid content (water or oil) and temperature. In case of pressure driven water flow, swelling occurred in the potato disc below 50$^o$C and compression occurred above this temperature, while in case of oil, compression occurred at all temperatures. The velocity-pressure gradient behavior of oil and water through potato disc exhibited deviation from Darcy's law due to threshold pressure gradient.
Issue Date:2015-12-09
Rights Information:Copyright 2015 Tanjila Alam
Date Available in IDEALS:2016-03-02
Date Deposited:2015-12

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