Effect of processing history on the functional and structural characteristics of starch–fatty acid extrudates

Normal maize starch was extruded with and without the addition of fatty acid potassium salts in a twin screw cooker extruder equipped with a slit die rheometer, at 100, 120, 140 or 160 °C barrel temperatures and at screw speeds 20, 91, 161 or 227 rpm. The in line melt viscosity measurements showed that the flow behaviour of the starch system was influenced by the addition of either myristic or palmitic acids used. The examination of functional properties of the extrudates such as bulk density, water solubility index, expansion ratio, degree of gelatinization and water adsorptivity indicated that the addition of fatty acids affected the functionality of starch systems. That is, the extrudate texture became more cohesive and compact whereas the water solubility exhibited by the system was decreased. Structural studies indicated that the crystallinity of starch–fatty acid extrudates was higher than that of the starch systems without fatty acid whereas all extrudates exhibited high glass transition temperatures and their structure was very rigid and brittle.