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Extrusion of starch

Extrusion is a forming technique whereby a material is forced, by compression, through a shaped opening in a die to produce a continuous profile. The material can be ram- or (single- or twin-screw) screw-extruded. The latter technique is more widely used in converting raw polymer, usually in powder or pellet form, through a combination of mixing, shearing, shaping and forming processes into a saleable finished or semi-finished product, with a particular shape and/or texture only achievable via extrusion routes. The combined effect of shear, temperature and local shear-induced temperature changes inside the extruder screw channel make the thermomechanical process during screw extrusion unique. Such a process is used with relatively dry materials to plasticize food mass, reduce microbial load, denature enzymes, gelatinize starch, polymerize proteins and, most importantly, texturize the end product into a desirable form. Screw extrusion has proved to be a particularly very attractive process in the food industry, with the advantages of versatility, high productivity, low cost, energy efficiency and no effluents causing waste problems.

Extrusion expansion is a complex phenomenon which occurs during high-temperature, low-moisture cooking and is a consequence of several events including starch structural transformations and phase transitions, nucleation, extrudate swell, bubble growth, and bubble collapse, with bubble dynamics dominantly contributing to the expansion phenomenon as shown in Figure 1.

Figure 1 - Schematic diagram of extrudate expansion

The main factors that significantly influence extrusion expansion are summarized in Figure 2.

Figure 2 - Factors influencing extrudate expansion

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