**4. Influence on product quality**

Extrusion-cooking is a process widely used in the food industry to manufacture snacks, crackers and expanded cereals. The degree of expansion in extruded products is an important characteristic which relates to the texture and sensory properties of extrudates (Lue et al., 1990). Extrusion-cooking has an important influence on product quality, emphasizing features like expansion, texture, shelf-life, colour and flavour.

For extrusion cooking, changes in ingredients such as sugar, salt and fibre, or processing parameters like screw design or speed and temperature, can affect extrusion system variables and product characteristics such as texture, structure, expansion and sensory attributes (Mendonça et al., 2000).

Products obtained with high temperatures and short extrusion process times normally present a porous, open structure, what confers to them a "crunchy" texture (Barrett, 2003).

Expansion occurs in both radial and axial directions, at different degrees, depending on the viscoelastic properties of the melt. Vaporization of moisture and cooling of the extrudate serve to bring the product from a molten to a rubbery state; and further drying is usually used to produce the brittle, fracturable texture typical of these products (Barrett, 2003).

Colour in extruded products is influenced by temperature, raw material composition, residence time, pressure and shear force (Guy, 2001; Mercier et al., 2001). During the extrusion process, several reactions happen and they in general affect the colour of the products. Among the most important, the most common are non-enzymatic browning (Maillard and caramelization) and pigment degradation. The process conditions normally used in thermoplastic extrusion (high temperature and low moisture content) are known to favour the reaction among reducing sugars and amino acids, that results in the formation of coloured compounds and the reduction of the amino acid lysine. If the browning is too intense, colours and flavours can be produced. Besides, changes in colour during the extrusion process can be an indicator to evaluate the intensity of the process in terms of chemical and nutritional changes (Ilo et al., 1999).

protein–protein interactions and lower viscosity. At relatively lower moisture contents, higher barrel temperatures (140 to 180°C) result in better textures. At higher moisture levels, temperature needs to be decreased as moisture flash-off may cause considerable water loss

High moisture levels combined with elevated temperatures yield extrudates that are very soft and not self-supporting after the die. However, a specially designed die which provides cooling at this section will increase the viscosity of the hot extrudate before exiting, contributing to the correct elasticity and fluidity required for texturization (Noguchi, 1998). The temperature at which solidification occurs is related to the plasticization temperature.

Low moisture (15 to 30%) extrusion tends to result in processes with greater generation of mechanical energy and products with lower density, while high moisture (50 to 70%) extrusion results in products with higher density, and is normally used in pellet production

Extrusion-cooking is a process widely used in the food industry to manufacture snacks, crackers and expanded cereals. The degree of expansion in extruded products is an important characteristic which relates to the texture and sensory properties of extrudates (Lue et al., 1990). Extrusion-cooking has an important influence on product quality,

For extrusion cooking, changes in ingredients such as sugar, salt and fibre, or processing parameters like screw design or speed and temperature, can affect extrusion system variables and product characteristics such as texture, structure, expansion and sensory

Products obtained with high temperatures and short extrusion process times normally present a porous, open structure, what confers to them a "crunchy" texture (Barrett, 2003). Expansion occurs in both radial and axial directions, at different degrees, depending on the viscoelastic properties of the melt. Vaporization of moisture and cooling of the extrudate serve to bring the product from a molten to a rubbery state; and further drying is usually used to produce the brittle, fracturable texture typical of these products (Barrett, 2003).

Colour in extruded products is influenced by temperature, raw material composition, residence time, pressure and shear force (Guy, 2001; Mercier et al., 2001). During the extrusion process, several reactions happen and they in general affect the colour of the products. Among the most important, the most common are non-enzymatic browning (Maillard and caramelization) and pigment degradation. The process conditions normally used in thermoplastic extrusion (high temperature and low moisture content) are known to favour the reaction among reducing sugars and amino acids, that results in the formation of coloured compounds and the reduction of the amino acid lysine. If the browning is too intense, colours and flavours can be produced. Besides, changes in colour during the extrusion process can be an indicator to evaluate the intensity of the process in terms of

emphasizing features like expansion, texture, shelf-life, colour and flavour.

if a cooling die is not used (Thiebaud et al., 1996).

(Guy, 2001).

**4. Influence on product quality** 

attributes (Mendonça et al., 2000).

chemical and nutritional changes (Ilo et al., 1999).
