Cross-linking starch at various moisture contents by phosphate substitution in an extruder

Starch was extruded with sodium trimetaphosphate and sodium hydroxide in a single-screw extruder with barrel and die temperature of 130 °C at three screw speeds of 40, 90, and 140 rpm and three moisture contents of 40, 55, and 70%. Time required for phosphorylation of starch in an extruder was found to be less than 2 min, and cross-linking of starch by phosphorus, which was incorporated into starch, was confirmed by paste viscosity of extruded starch. Cross-linking starch with 2.5% sodium trimetaphosphate did not significantly affect water absorption index, but reduced water solubility index so replacement of the process including mixing of starch with synthetic polymer in the extruder with another process including cross-linking of starch and then mixing with synthetic polymer can result in composite with lower solubility of starch. On the other hand, increasing moisture content of starch reduced both water absorption and solubility index of extrudates.