Analysis of mass transfer mechanisms during drying of extruded pasta

Kurt Martin Waananen, Purdue University

Abstract

The first section of this thesis identifies key characteristics of drying models including controlling process resistances, internal mechanisms of moisture movement, and methods of model coefficient determination, model solution, and model validation. Similarities and differences between previous work are noted, and strategies for future drying model development are discussed. Experimental studies were conducted to identify mechanisms of moisture movement in foods during drying. Heat transfer studies indicated that the center temperature for 5.56 mm diameter pasta cylinders approached the dry bulb air temperature within 3$\sp\circ$C in less than ten minutes, supporting the assumption that pasta drying can be considered an isothermal process. Analysis of tests conducted with 3.18, 4.76, and 5.56 mm diameter pasta cylinders at 71$\sp\circ$C indicated that mass transfer data were consistent with control by internal diffusion. The drying characteristics of unpuffed samples were not affected by total pressure suggesting that the mechanisms of internal moisture movement is best considered a liquid or adsorbed phase diffusion phenomenon. At 202 kPa, vapor phase diffusion was found to contribute approximately 10% of the moisture flux in puffed samples at 71$\sp\circ$C and 17% of the flux at 55$\sp\circ$. Effective moisture diffusivities for unpuffed pasta as a function of moisture content were obtained at temperatures of 40, 55, 71, 90, 105, and 120$\sp\circ$C. Diffusivities increased as temperature and moisture content increased, with values ranging from 8 to 106 $\times$ 10$\sp{-12}$ m$\sp2$/s. Diffusivities for puffed pasta at temperatures of 40, 55, 71, and 105$\sp\circ$C ranged from 36 to 221 $\times$ 10$\sp{-12}$ m$\sp2$/s. An isothermal diffusion model for a cylinder was formulated and solved by finite-difference techniques. The model was found to be an effective tool to describe drying curves at temperatures below the boiling point of water. Pasta cylinders were dried at temperatures ranging from 90 to 120$\sp\circ$C and total pressures between 55 and 202 kPa. The experimental results provided strong evidence that a bulk flow mass transfer mechanism is important in drying of unpuffed pasta at temperatures above the boiling point of water.

Degree

Ph.D.

Advisors

Okos, Purdue University.

Subject Area

Agricultural engineering|Food science

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