The effects of rheological properties on mass transfer and stress development in pasta -type products during drying
Abstract
The overall objective of this research was to determine the effects of variations in rheological properties on final product quality as judged by the extent of breakage. The overall objective was achieved through three sub-objectives: (1) to qualitatively determine the rheological properties as functions of temperature and moisture content, (2) to study the effects of rheological properties on drying curves, moisture profiles, and shrinkage during drying and equilibration using the thermomechanical model (TMM), and (3) to predict differences in stress development and failure between products during drying and equilibration using the elastic-plastic stress model. Rheological properties were measured for four pasta-type products made from each of two flours, bread and durum, by each of two forming methods, sheeting and extrusion. Moduli of extruded products were greater than those of sheeted products. Master curves were developed for each product and used in the mass transfer and stress models. The TMM, with the incorporation of restricted shrinkage based on the ratio of the modulus to the glassy modulus, predicted mass transfer in pasta-type products, and the criteria for saturated drying was determined to be a modulus ratio less than 0.6. Moisture profiles were non-Fickian indicating that the theological properties influenced drying kinetics. The durum extruded pasta-type product was predicted to have greater moisture loss and ultimate shrinkage than the bread sheeted product. A sensitivity study revealed that the glassy diffusion coefficient and the factor for the multilayer moisture in the GAB isotherm had the greatest influence on drying curves. Glassy strain, based on deviation from ideal collapse, was used to predict strain in the transition and glassy states, and failure was predicted to occur when tensile stress was greater than measured ultimate stress. Minimal compressive stresses were predicted to develop during drying, but large tensile stresses leading to failure were predicted to develop during equilibration. Variations in raw materials and processing affected final product quality in terms of collapse and breakage.
Degree
Ph.D.
Advisors
Okos, Purdue University.
Subject Area
Agricultural engineering|Food science
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