Influence of Food Extruder Die Dimensions on Extruded Products Expansion

Abstract

This paper studied the effects of food extruder die dimensions on the extrudate expansion indices using twin-screw extruder. The extruder has 59.6 mm screw diameter and screw L/D ratio of 20. The die dimensions considered are die length, die diameter and die temperature. The feed material used is yellow corn flour. Dies with diameter ranging from 2.5 – 5.0 were used to determine the effect of die nozzle diameter on extrudate expansion. Results obtained showed that radial expansion of the extrudate decreased with increasing die diameter. An average reduction of about 42% in radial expansion was recorded for a difference of 2 mm in die diameter. The overall expansion of the extrudate was not affected by changes in die diameter for all the diameters and lengths of die studied. On the contrary, increase in die diameter caused increase in the longitudinal expansion of the extrudate. The longitudinal (axial) expansion obtained with a 3mm die diameter was 0.6, which increased to 1.2 for a 4mm die diameter. The regression model shows that SME decreased with increasing die diameter. Dies with length ranging from 16 – 54mm with a constant nozzle diameter of 3mm were used to determine the effects of die nozzle length on extrudate expansion. The die length significantly influenced axial expansion of extrudate. Axial expansion increased with increase in die length. A linear regression model was developed to relate die length to extruder SME. The quadratic effect is negligible in this case and as such was not included in the regression equation. Effects of die nozzle L/D on radial expansion of extrudate during extrusion were also studied. There was an initial sharp increase in the radial expansion of the extrudate from 8.9 to 13 as the nozzle L/D ratio increased from 5.33 to 6.4. The value then dropped to 10.5. At higher L/D ratio (above 6.4), the expansion decreased. At this high L/D ratio corresponding to a high extrusion pressure, the residence times and shear rates were high enough to cleave the starch molecules and thus reduce expansion. Experimental results on the effects of die diameter and die length on flow characteristics revealed that when all other extruder conditions are kept constant, increasing the diameter of the die nozzle causes increase in the volumetric flow rate of extrudate. Flow rate decrease proportionally with increasing die length.