**3. Thermoplastic extrusion process**

El-Dash (1982) defined the process of thermoplastic extrusion as being a continuous process in which mechanical friction is combined with thermal heating in order to continuously mix, plasticize and gelatinize the starch, denature protein materials, restructuring them for the obtaining of products with new textures and shapes.

Single-screw cooking extruders were developed in the 1940's to make puffed snacks from cereal flours or grits. An expanding demand for precooked cereals and starches required machines with larger capacity, so extruders with a nominal capacity of 5 ton per hour were developed in the 1960's, with numerous news applications: snacks, infant feeding, pet foods, etc. In the 1970's products containing more than one component were developed, such as egg rolls and ravioli for coextrusion. Then, the use of two extruders in series, the first for cooking and the second one for forming and structuring, resulted in several products. At the end of the 1970's, the use of twin-screw extruders for food processing that expanded the range of application began (Mercier & Feillet, 1975; Linko et al., 1981; Harper, 1979). Finally, the extruders are meant for specific markets, as it will be seen for extruders intended for the productions of biodegradable packagings (Flores et al., 2010; Mandrogón et al., 2009).

Food extruders are generally available with segmented screws and barrel section, facilitating total control over the configuration of the machine to get a combination of various process parameters. Extrusion cooking is a high-pressure operation that provides sudden expansion of the processed products. The physical characteristics of the extrudate reflect the effectiveness of the process and suitability of ingredients (Patil et al., 2005).

Beneficial effects include destruction of antinutritional factors, gelatinization of starch, protein denaturation/texturization, increased soluble dietary fibre and reduction of lipid oxidation. But Maillard reactions between protein and sugars can reduce the nutritional value of the protein, depending on the raw material types, their composition and process conditions. Heatlabile vitamins may be lost to varying extents (Singh et al., 2007, Patil et al., 2005).

Some of the use applications of the extrusion process to physically and/or chemically modify starch both in the areas of food and biopolymers will be presented below.
