| Average shear rate ( ga&d2; ) of a twin-screw co-rotating extruder was investigated to calculate shear history---a factor to model the mechanical effect (R S) of extrusion. Fluids with different flow behavior indices, which represent different feed materials, were extruded at degrees of fill ranging from 0.4--1.0. Using mixer viscometry assumptions, ga&d2; was estimated and then modeled as a function of screw speed and degree of fill.; To calculate extrusion thermal effects on thiamin degradation ( Rbeta), kinetic parameters were obtained by heating flour, mixed with thiamin, at temperatures >100°C in a shearless environment. Two methods (atmospheric and controlled-pressure) to obtain the parameters were compared. The results from the two methods were not statistically different. The parameters obtained from the controlled-pressure method had lower standard error; for 25% moisture content flour, activation energy was 121.0 kJ/g-mol and reaction rate at 80°C was 9.69E-5 min-1.; For extrusion, the same thiamin flour mixture was extruded at screw speeds of 100--300 rpm at two temperatures profiles. R beta was calculated using the thiamin kinetic parameters. RS was calculated by mathematically removing Rbeta from the total thiamin retention. At higher temperature, thermal effects predominated over mechanical effects for thiamin loss, while at lower temperature, mechanical effects slightly prevailed over thermal effects. Mechanical effects caused 28.9% to 64.5% of total thiamin loss. |
