Nonlinear Rheology of Food Materials

Merve Yildirim Erturk, Purdue University

Abstract

The inter/intramolecular interactions and associations between constituents determine the microstructure of food and its response to mechanical deformation and flow. The characterization of food rheology enables the design of efficient processing equipment, production of high-quality, stable end products, prediction of textural and sensorial attributes, and assurance of consumer acceptability. Foods are subjected to rapid and large deformations during processing operations and consumption. Dynamic oscillatory shear tests are carried out by subjecting food to a sinusoidal deformation (or stress) and probing the mechanical stress (or strain) and recording the response as a function of time. In the SAOS region, the mechanical response is in the form of a perfect sinusoidal curve and interpretation is straightforward as expected from a linear model. On the other hand, LAOS response requires complex mathematical relations to extract meaningful rheological parameters. In this dissertation, Fourier Transform-Chebyshev Decomposition (FTC) and Sequence of Physical Processes (SPP) methods were utilized to quantify the LAOS response of selected food materials. The objective of this study is to gain new insights into the nonlinear rheology and structural architecture of food materials. To offer insights into the microstructure– rheology relations, rheological measurements were accompanied by various techniques probing chemical interactions (FTIR), imaging (Cryo-SEM, SEM), quantitative network analysis, and molecular size (SDS-PAGE). This dissertation showed that LAOS rheology is highly correlated with the network structure of food shown by the quantitative network analysis utilizing SEM images. It is a powerful tool to detect the effect of small molecules on the nonlinear rheology of food (HMW-LMW glutenin ratio, gliadin for dough, fat content in yogurt, and amylopectin/amylose ratio of starch in a suspension). Nonlinear parameters were sensitive to structural changes occurring in dough structure during processing conditions including aging at room and elevated temperatures. Lastly, the SPP method enabling time-resolved interpretation of nonlinear rheology provided detailed transient microstructural interpretations whereas the FTC method gave static measures at specific strains in an oscillation cycle. Thus, nonlinear rheology of doughs with various gluten subfractions in MAOS and LAOS regions as well as shear thickening characteristic of starch suspensions with changing amylopectin/amylose ratio interpreted by the SPP method gave more sensitive results than the FTC method. The application of fundamental knowledge from this work can be a guide to evaluating the architecture and nonlinear rheology of food for the assurance of consumer acceptancy and the fabrication of efficient machinery by building more accurate mechanical models of complex food systems.

Degree

Ph.D.

Advisors

Kokini, Purdue University.

Subject Area

Materials science|Mathematics|Mechanics|Physics

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