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Experimental Determination of Longitudinal Expansion During Extrusion of Starches

¹University of Nebraska, Department of Biological Systems Engineering, 212 L.W. Chase Hall, Lincoln, NE 68583-0730. ²University of Nebraska, Industrial Agricultural Products Center, 208 L.W. Chase Hall, Lincoln, NE 68583-0730. ³MGP Ingredients Inc., Atchison, KS 66002. ⁴Corresponding author. Phone: 1-402-472-1634. Fax: 1-402-472-6338. E-mail: mhanna1@unl.edu

Expansion during extrusion of starches is a unique phenomenon used extensively in the food industry. Sectional expansion index is usually determined as the ratio of the square of the diameter of the final extrudate to the square of diameter of the die through which the product is expelled. Longitudinal expansion index is calculated indirectly by doing a mass balance across the extruder and making an assumption for density of the dough melt in the die. The objective of this research was to determine the longitudinal expansion index experimentally using a die designed to measure the velocity of the melt dough inside the die. A tracer was induced to change the conductivity of the melt dough. The velocity inside the die was then determined by measuring the time of travel of a tracer with the help of electrical probes. Corn starch (25% amylose and 30% moisture content) was extruded in a twin-screw extruder at a barrel temperature of 140°C, screw speeds of 80, 120, and 160 rpm, and feed rates corresponding to the screw speeds varying from 7.35 to 13. 35 kg/hr. An inverse relationship was observed between the longitudinal expansion and the sectional expansion indices.