Effect of processing conditions on the physicochemical and structural characteristics of pregelatinised starch–fatty acid–glycerol extrudates

Abstract

The possibility of using physically modified starch in combination with glycerol and fatty acids to produce biodegradable materials with tailor made functional properties was explored. For this purpose pregelatinised starch prepared by drying native maize starch aqueous dispersions in a pilot double drum drier, was extruded in the presence of glycerol with and without the addition of fatty acid potassium salts in a pilot twin screw cooker extruder equipped with a custom built slit die rheometer. The processing conditions employed were: heating at either 80 or 100 °C in all heated sections of the extruder and rotational screw speeds either 80 or 115 or 145 or 210 rpm. The in line melt viscosity measurements showed that the flow behavior of the starch–glycerol systems was significantly influenced by the addition of either myristic or palmitic acids. The examination of physicochemical properties of the extrudates indicated that the addition of fatty acids affected the functionality of starch–glycerol systems. That is, the moisture content, the bulk density and the water solubility index of the fatty acid containing starch–glycerol systems were higher than those of the corresponding control ones whereas the expansion ratio of the fatty acid containing extrudates was lower than that of the control ones. Three point bending tests revealed that the higher the screw speed the lower the modulus of elasticity attained by the samples. Structural studies carried out using X-ray diffraction analysis, indicated that the degree of crystallinity of starch–glycerol extrudates was lower than that of their starch–glycerol–fatty acid counterparts. The experimental results are discussed in the light of conformational changes occurring during extrusion due to starch–fatty acid interactions and their impact on the structural changes which have taken place in the starch matrix of the extrudates.