3.2. Bulk density (BD)
The experimental values of bulk density of extrudates under different designed extrusion conditions are presented in Table 2.
BD ranged from 0.423 to 0.864 g/ml for the extruded products. The response surface plots were prepared (not shown to reduce the manuscript size; instead the one for ER which causes the bulk density to be lower is presented next) to evaluate and visualize the effect of feed moisture and protein content effect on BD under different temperatures. Results showed that at the low barrel temperature (140 C), increasing protein content and decreasing moisture content resulted in a higher BD. At high barrel temperature (180 C), increasing protein content lead to decrease the BD.
At middle barrel temperature (160 C), the minimum BD area appeared in the middle part of the figure. Soy protein isolate (SPI) constructs small uniform pores in the extruded products after being squeezed out of the die as soy protein isolate can work as high quality emulsifiers between hydrophilic materials and hydrophobic materials by exposing the hydrophilic
groups and hydrophobic groups to their respective phases. The thickness of the wall of the pores become thinner when
the amounts of soy protein isolate increase and the soy protein isolate absorbs high amounts ofwater. Thus, it is logical to expect an increasing bulk density of the extrudates with increasing protein and feed moisture contents. Starch gelatinization during extrusion processing has a big influence on bulk density of extrudates. The low