Investigation of the Nucleation During Extrusion Cooking of Corn Starch by A Novel Nucleation Die

Abstract

In this paper, a novel method for the investigation of nucleation in expanding starch-based melts is introduced. Therefore, corn starch was extruded by a twin screw extruder. A long slit die, which facilitated the formation of a nucleation area to extend nucleation, was mounted at the exit of the extruder. The temperature of the nucleation die (160–190 °C), the moisture content of the starch melt (17–27 %), and the slit height (4 and 10 mm) were varied to investigate the effect of pressure profile and shear rates on nucleation. The pore size distributions of the samples were determined by image analysis of the cross-sections. The influence of temperature, moisture content, and slit height on nucleation was investigated by analysis of the resulting pore size distribution. We found two totally different types of product: a product with few big pores and a product with small and evenly distributed pores. In general, small pores were achieved when the vapor pressure was reached already at the earlier stages of the nucleation die. The addition of nucleating agents, such as droplets or solid particles did not change the appearance of the products significantly. Applying a nucleation area such as introduced in our study enables further investigation of the mechanisms of nucleation.