EFFECT OF POROSITY ON MOISTURE DIFFUSION DURING DRYING OF PASTA

Heat and mass transfer studies were conducted with pasta cylinders at different porosities, temperatures and pressures. Tests with pasta cyl inders were conducted at porosities of 6 and 26%, temperatures of 40, 55, 71 and 105 degrees C and total pressures of 77, 101 and 202 kPa. A nalysis of tests conducted with 3.18, 4.76 and 5.56 mm diameter dense pasta cylinders (6% porosity) at 71 degrees C indicated that mass tran sfer was consistent with control by internal diffusion. The drying not affected by total pressure, suggesting that the mechanism of internal moisture movement is best considered a liquid or adsorbed phase diffu sion phenomenon. Effective moisture diffusivities of dense pasta incre ased as temperature and moisture content increased, with values rangin g from 8 to 106 x 10(-12) m(2)/s. Diffusivities of porous pasta (26% p orosity) ranged from 36 to 221 x 10(-12) m(2)/s. Heat transfer studies with dense and porous pasta indicated that the center temperature for 5.56 mm diameter pasta cylinders approached the dry bulb air temperat ure within 3 degrees C in less than 10 min, supporting the assumption that pasta drying can be considered an isothermal process. A semi-empi rical model for moisture diffusivity in pasta that accounts for the ef fects of porosity, pressure and moisture binding was developed. The mo del agrees reasonable well with experimental data for dense pasta, and for porous pasta aat low and atmospheric pressures (77 kPa and 101 kP a). The model underpredicts the moisture diffusivity for porous pasta at high pressures (202 kPa). The model provides a reasonable estimate of moisture diffusivity in pasta. (C) 1996 Elsevier Science Limited.