Structural characteristics of corn starches extruded with soy protein isolate or wheat gluten

Commercially available corn starches containing 0, 25, 50 and 70% amylose were extruded with 10, 20 and 30% soy protein isolate (SPI) or wheat gluten (WG) at 22% moisture content (dry basis) in a C.W. Brabender single screw laboratory extruder using a 140°C barrel temperature and a 140 rpm screw speed. True, solid and bulk densities; percent total, closed and open pores; and shear strengths of the extrudates were determined. The microstructures of the extrudates were studied by scanning electron microscopy (SEM). The total pores of the extrudates were affected significantly (p < F=0.0001) by type of protein (SPI or WG) and starch amylose. The open or closed pores, were affected by protein type only. The interaction between amylose and protein contents was highly significant <(p < F=0.0001). In general, the total pores and bulk densities were higher for WG-starch extrudates compared to SPI-starch extrudates. These values decreased as amylose content increased from 0 to 25% and then increased thereafter. The open pores, on the other hand, increased with increasing protein content from 10 to 20% and then decreased. Extrudates containing WG had higher shear strengths than those containing SPI.