Specific Mechanical Energy (SME) in Thermoplastic Starch Production: Factors Affecting SME and its Implications

Extrusion-cooking is a widely used technique for processing plant starch into thermoplastic starch (TPS). The specific mechanical energy (SME) consumption is a significant factor in determining the amount of energy required to obtain a product mass unit using this technique. The SME is the energy required per unit mass of the processed material and is expressed in kilowatt-hours per kilogram (kWh/kg).

To measure the power consumption of an extruder, a standard wattmeter connected to the extruder drive unit is used. The results are then converted into an index of SME consumption using Levin’s equation. This equation uses variables such as screw rotations, maximal screw rotations, power, engine loading, and extruder capacity to calculate the SME value.

Research conducted by Bindzus et al. on wheat, maize, and rice starch processed in a twin-screw extruder found SME values spread over a range of 0.081 to 0.365 kWh/kg. Brummer et al. reported that maize starch extrusion on a twin-screw extruder had SME values ranging from 0.1 to 0.25 kWh/kg, which is related to material moisture to a small degree. Potato starch extrusion, as investigated by Della Valle et al. on a twin-screw extruder, showed SME values ranging from 0.1 to 0.32 kWh/kg. Wiedmann and Strobel recorded SMEs ranging from 0.1 to 0.55 kWh/kg in their investigations into the extrusion of wheat TPS, depending on material moisture content.

Mitrus found that material composition and extruder screw rotation speed have a substantial impact on SME during TPS processing. An increase in glycerol content led to a decrease in SME, while an increase in extruder screw rotation speed caused the SME to rise.

The investigations revealed that changes in SME during TPS production are affected by material blend moisture content. The higher the material moisture, the higher the SME values. The highest SME values recorded were related to potato starch of 20% moisture and 20% glycerol content. All of the starch types investigated were found to show lower SME values with increasing glycerol contents in the mixtures.

Furthermore, it was found that the SME decreases with extrusion repetition in repeatedly extruded potato TPS. The higher the number of extrusions, the lower the SME becomes. This result suggests that a certain amount of crystallographic structure is destroyed during processing. Unprocessed, half-crystalline starch needs a higher energetic input to be melted and processed than thermoplastic starch of amorphous structure, which has no crystallographic structures at all.

In conclusion, the specific mechanical energy consumption is a crucial factor in determining the amount of energy required to obtain a product mass unit using extrusion-cooking for processing plant starch into TPS. The SME values are affected by various factors such as material moisture, glycerol content, extruder screw rotation speed, and extrusion repetition. The investigations reveal that the SME values of different starch types vary significantly and depend on the material composition and process conditions.

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