Description of Extrudate Characteristics in Relation to the Shear Stress of Plasticised Starches Determined In-line

Abstract

The behaviour of wheat, corn and rice starch as well as of mixtures thereof was investigated with the aid of the shear stress of material plasticised under extrusion cooking conditions. The shear stress was determined in a laboratory extruder using an in-line viscometer. The extrusion experiments were performed using a combination of a fractionated factorial design and mixture experiments. It was shown for each starch and mixture of starches that a functional dependency existed between the extrusion process parameters and the shear stress of the plasticised materials. A second relationship was found between the shear stress of the plasticised materials and the product characteristics of the extrudates. With respect to the extrusion cooking of various types of starch, it could thus been demonstrated that it is possible to predict the development of product characteristics by determining the shear stress of plasticised materials in-line. Furthermore, regression analysis showed that the levels of the test ranges for the relationship between the shear stress and the process parameters are dependent on the starch or starch mixtures used. Molecular degradation of the starch polymers of the different types of starch and their mixtures resulted in extrudates with different functional properties. This also applied to extrudates with the same mean molecular weight. Extrudates made of wheat starch had a much higher cold paste viscosity than extruded rice or corn starches. Extrudates made of corn starch and those made of mixtures of rice, corn and wheat starch had a higher cold water solubility and a lower sediment volume than those made of wheat or rice starch.