**3.1 Raw materials**

The most used raw materials in the extrusion process are starch and protein based materials. The structure of the extruded products may be formed from starch or protein polymers. Most products, such as breakfast cereals, snacks and biscuits are formed from starch, while protein is used to produce products that have meat-like characteristics and that are used either as full or partial replacements for meat in ready meals, dried foods and many pet food products (Guy, 2001).

In general, the chemical or physicochemical changes in biopolymers that can occur during extrusion cooking include: binding, cleavage, loss of native conformation, fragment recombination and thermal degradation. The composition of raw materials can be altered by physical losses including leakage of oil and evaporation of water and volatile compounds at the die. Since most chemical reactions occur in the high-pressure zone of the barrel, thermally labile compounds such as flavours and vitamins may be injected immediately before the die to minimize exposure to heat and shear (Riaz, 2000).

The structure of an extruded product is created by forming a fluid melt from a polymer and blowing bubbles of water vapour into the fluid to form a foam. The bubbles rapidly expand as the superheated water is released very quickly at atmospheric pressure. In the extruded structure, the fluid melt of the polymers forms the cell walls of the gas bubbles. After gas expansion, the rapid drop of temperature caused by water evaporation and the rapid rise in viscosity due to moisture loss, solidifies the cell structure. The rapid increase in viscosity is followed by the formation of a glassy state. Starch polymers are very good at this function and also expand well. Structure forming polymers must have a minimum molecular weight sufficient to give enough fluid viscosity to prevent or control shrinkages of an extrudate after it reaches its maximum expansion (Guy, 2001).

Thermoplastic Extrusion in Food Processing 273

and less time for the transformation of the melt in the shearing section (high-pressure zone). On the other hand, in certain cereals, such as hard wheat, durum wheat, vitreous flint maize and some varieties of barley there is a strong bonding between the starch granules and the protein layers forming a hard particle of flour that requires more energy to break down and will generate more heat in the extruder. Therefore, if high expansion is required in a low moisture product, finely milled forms of harder endosperm types will give excellent results. If the product requires low to medium expansion, some of the hard material may be replaced by soft flour; and for low expansion in a dense product such as breading crumb,

Inside the extruder, starch goes through several stages. First, the initial moisture content is very important to define the desired product type. Once inside the extruder, and at relatively high temperatures, the starch granules melt and become soft, besides changing their structure that is compressed to a flattened form (Guy, 2001). The application of heat, the action of shear on the starch granule and water content destroy the organized molecular structure, also resulting in molecular hydrolysis of the material (Mercier et al., 1998). The starch polymers are then dispersed and degraded to form a continuous fluid melt. The fluid polymer continuum retains water vapour bubbles and stretches during extrudate expansion until the rupture of cell structure. The starch polymer cell walls recoil and stiffen as they cool to stabilize the extrudate structure. Finally, the starch polymer becomes glassy as moisture is removed, forming a hard brittle texture (Guy, 2001). The final expanded product presents air cells that are formed due to superheated water vapour pressure. When the temperature of the extrudate is reduced below its glass transition temperature (Tg), it

According to Colonna et al. (1998), maximum expansion degree is closely related to starch content. Maximum expansion is obtained with pure starches (an increase of 500% in product diameter), followed by whole grains (400%) and with lower expansions for seeds or germ (150-200%); the starch content of these products is 100, 65-78, 40-50 and 0-10, respectively.

Proteins are biopolymers with a great number of chemical groups when compared to polysaccharides and are therefore more reactive (Mitchel & Areas, 1992) and undergo many changes during the extrusion process, with the most important being denaturation (Camire, 2000). Proteins are formed from chains of amino acids and have a wide range of physical sizes and forms in native raw materials. Proteins in general are classified, with respect to their solubility, in albumins, globulins, prolamines and glutelins with solubility in water, saline solution, alcohol solution and acid or alkaline solutions, respectively (Pereda et al.,

During extrusion, disulfide bonds are broken and may re-form. Electrostatic and hydrophobic interactions favour the formation of insoluble aggregates. The creation of new peptide bonds during extrusion is controversial. High molecular weight proteins can

Enzymes, also proteins, lose their activity after being submitted to the extrusion process due to high temperatures and shear. Also, proteins lose their solubility in water and saline

soft flour may be used (Guy, 2001).

**3.3 Proteins** 

2005).

solidifies and maintains its expanded form (Riaz, 2000).

dissociate into smaller subunits (Guy, 2001).

The minimum starch content for expansion is 60-70% (Riaz, 2000).
