| To determine the separate thermal effects of extrusion, isothermal and nonisothermal experiments were conducted by heating wheat flour with 0.4616% (w/w) food-grade β-carotene in a shearless environment at 78, 138, and 149°C, at 28% (w/w) moisture content. To determine the total (thermal plus mechanical) effects of extrusion on retention of trans-β-carotene, the same mixture was extruded at 30/50/70/90/110°C and 50/70/90/110/130°C at screw speeds of 200, 250, 300, and 400 rpm on two separate days with a dough moisture content ranging from 30–36% (w/w). In the isothermal experiments at 78°C, retention did not change significantly. However, at 138°C for up to 60 minutes, the calculated first-order reaction rate constant was 6.83 × 10−5 s−1. The rate constant, 3.56 × 10−4 s−1 , and activation energy, 18.82 kJ/g·mol, obtained from the nonisothermal experiments were used to calculate trans-β-carotene retention due to thermal effects during extrusion. For both temperature profiles, total trans-β-carotene retention ranged from 58–97%. Thermal effects accounted for less than 5% of the loss, showing that mechanical effects were the predominant cause of trans-β-carotene degradation. A linear statistical model worked well for separate days of extrusion; however, an exponential model was more effective for combined days of extrusion. Overall, food-grade β-carotene was more stable than other sources of β-carotene. |
