EVALUATION OF DIE SWELL AND VOLUMETRIC EXPANSION IN CORN MEAL EXTRUDATES

A method to determine die swell in the presence of volumetric expansio n due to moisture vaporization in extruded food products is proposed. The method is applied to twin screw extrusion of corn meal in a study to investigate the effects of extruder die parameter (exit capillary l ength and entrance cone angle), screw speed, and specific feeding load on die swell and volumetric expansion. The die swell results are comp ared with known effects for nonexpanded polymers. Die swell tended to decrease with increasing exit length, but had only a weak response to entrance angle. Increasing specific feeding load (SFL) increased die s well, where as increasing screw speed produced increases or decreases in die swell, depending on the SFL. Die swells ranged from approximate ly 1.2 to 1.7, accounted for approximately 10 to 22% of the total radi al expansion, and for approximately 18 to 38% of the extrudate cross s ectional area. Volumetric expansion increased with increasing die exit length, but was insensitive to entrance angle with a slight concave u pward response. Increasing screw speed produced large increases in vol umetric expansion, while increasing SFL produced only slight increases . Observations about die swell compare favorably with previous studies of nonexpanded polymers, suggesting that the proposed analysis method may be useful for food viscoelasticity evaluation studies.