| In this study, the separate thermal and mechanical effects from extrusion cooking were quantified and used to develop a predictive model for anthocyanin degradation. Grape pomace (anthocyanin source) mixed with wheat pastry flour (1:3, w/w dry basis) was extruded at screw speeds of 50, 100, 200, and 400 rpm at dough moisture content of 30%, at both 95°C and 125°C die temperatures. Thermal effect was investigated separately (isothermal and nonisothermal experiments) by heating the same mixture in steel cells in an oil bath at 80, 105, and 145°C. Anthocyanin degradation followed a pseudo first-order reaction. The rate constant and activation energy were k80°C = 2.81 x 10-4 s-1 and Delta E = 75,273 J/g-mol, respectively. Anthocyanin loss from extrusion ranged from 38--47% at 125°C die temperature, of which mechanical loss ranged from 1% and 63% at 50 rpm and 400 rpm, respectively. For extrusion at 95°C die temperature, total loss accounted for 29--35%, where mechanical loss accounted for 51% at 50 rpm to 66% at 400 rpm. An empirical equation was developed for mechanical loss versus shear history. The equation for mechanical loss was statistically different between the two extrusion temperatures. |
