Starchy Product: Coupling SME and Viscosity for a Better Prediction

The extrusion of starchy products is a tricky process as it modifies the basic structure of the material (loss of the grain structure, degradation of amylose and amylopectin chains…).

The objective of this study is to predict with Ludovic® the starch transformation considering the influence of the Specific Mechanical Energy (SME) on the intrinsic viscosity.

The material is a wheat starch composed of 73.1% amylopectin, 26% amylose, 0.7% lipid and 0.2% protein.
SME was calculated with and without coupling between SME and viscosity.
No coupling means that SME and viscosity are calculated independently along the screw, i.e. assuming a constant value average value of SME (325 kWh/t) for estimating the viscosity with the viscous law.
Despite the fact that intrinsic viscosity [η] and SME are usually related by exponential or power law relationships, intrinsic viscosity [η] can be correctly estimated by a simple linear relationship (in our case, in the range 200-800 kWh/t(Figure 1).

intrinsic viscosity vs SME

Without coupling, simulation provides correct tendencies, but the calculated SME is overestimated (Figure 2a) whatever the conditions, apparently because a too moderate SME value was chosen to calculate the viscosity. An overestimation of SME drives to an underestimation of intrinsic viscosity (Figure 2b).

SME and intrinsic viscosity_calculated vs experimental

With coupling, simulation results are largely improved, SME and intrinsic viscosity follow the same tendencies, but much closer to the experimental values (Figures 2a and 2b).

The objective of TSE simulation is to enable the prediction of starch transformation.
The advantage to take into account the specific effect of SME on starch viscosity is clearly shown in this figure. Whatever the processing conditions, the starch transformation is estimated by simulation with an error inferior to 10% (when it could reach more than 30% without coupling).
This coupling option in Ludovic® can also be adapted to other materials in order to increase the accuracy on the viscosity/SME prediction.

*based on « Importance of Coupling Between Specific Energy and Viscosity in the Modeling of Twin Screw Extrusion of Starchy Products » by F. Berzin, A. Tara, L. Tighzert and B. Vergnes (Polymer Engineering and Science – 2010)

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