**2. Fabrication of thermoplastic starch**

The fusion of mixture of native starch granules with sufficient amount of plasticizer, at lower temperature than their degradation, leads to a starchy material consisting of entangled polysaccharide chains. This material is called thermoplastic starch (TPS). Most common methods used for the preparation of TPS are extrusion and solvent casting method. For that various types of plasticizers such as glycerol, water, urea formamide etc are used during the formulation. The characteristics of mechanical work and flow (temperature, pressure, residence time, energy) during the preparation of TPS are known and modeled [Agassant et al., 1996, Vergnes et al., 1998]. A specific mechanical energy (SME) higher than 300kJ/g is necessary to achieve a complete destructuration of starch granules. The presence of low molecular mass (plasticizers) raises the threshold energy similar to the action of sugars in the cereal products [Fan et al., 1996]. The SME transmitted as shear, leads to the breakdown of starch grains by fragmentation, and once the melt phase obtained, it is accompanied by a moderate depolymerization, particularly in the amylopectin [Barron et al., 2002].

The native starch can be transformed to TPS by several treatments which destroys its granular structure. Physical treatment requires simultaneous action of temperature, shear and lower water content during extrusion. Initially amorphous regions of starch granules are more accessible to water followed by its crystalline zones. The amount of water should be sufficient to hydrate these starch molecules which ultimately results in its gelatinization. Also for obtaining the melt phase of plasticized TPS, additional energy is required to destroy the residual crystalline structures. This mechanical treatment reduces the crystallinity of starch granules. Figure 1 shows that the viscosity of plasticized starch is of the same order and magnitude as those of synthetic thermoplastics at lower temperatures [Martin et al., 2003].

Fig. 1. Flow curves of thermoplastic materials at 200°C and TPS from smooth pea and wheat at 125°C
