Measurement of the physical change in starch during extrusion cooking

The measurement of the degree of processing in extrusion cooking is extremely important for the control of quality in extruded products. For starch-rich formulations, it has been shown that both the concentration and the physical form of the starch are important factors in processing. They dominate the rheology of the fluid melt inside the extruder and the expansion phenomena, which serve to create structure in extruded products, This paper reviews the development of starch in an extruder and examines methods, which may be used to measure the physical characteristics of the starch for quality control purposes. In particular it describes the development studies for new methods based on rapid spectroscopic techniques undertaken by a consortium of European research workers, which may be used for both on-line and in-line quality control. NIR powder spectroscopy of ground extrudates and NIR transflectance spectroscopy from within the hot melt fluid inside the die head cavity of an extruder were both found to provide the basis for new methods.