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The effect of thermomechanical treatment on starch breakdown and the consequences for process design van den Einde, R.M.; Bolsius, A.; van Soest, J.J.G.; Janssen, L.P.B.M.; der Goot, A.J.van; Boom, R.M.

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Recent understanding of the mechanisms underlying the changes in molecular weight of starch as a function of process parameters during thermomechanical treatment, for example extrusion, holds promise towards more effective optimization of thermomechanical processes according to the desired modification of molecular weight. This paper summarizes recent advances in theoretical understanding and experimental methods. Empirical observations from extruder experiments can be better understood on this basis. The current status in this field has opened up potential for developing new equipment in which the main process parameters, which normally occur simultaneously, can be separated and optimized. This will create a much wider window for product-oriented process design and for new products. Figure 1-Molecular weight distribution of native (a) and heated-sheared waxy corn starch (b) (Van den Einde and others 2003b).

An in‐line method was applied to determine local residence time distribution (RTD) at two places in a completely filled corotating twin screw extruder. Axial mixing effects of different types of elements were evaluated. Paddles +90° induced flow patterns that appear to be circular, both upstream and downstream, whereas paddles −30° induced flow patterns that appear to be circular and mainly upstream. Transport and single‐lead elements induced backflow. The results could be explained from pressure differences and direction of drag flow in the elements. All elements were characterized using an equivalent additional mixing length λ, dependent on the type of element and its position in the extruder, which may be practically useful for extrusion design. POLYM. ENG. SCI. 45:271–278, 2005. © 2005 Society of Plastics Engineers.