**2.7.1 Single-screw extruders**

Single-screw extruders are the most common extruders applied in the food industry. The classification of single-screw extruders can be defined based on process or equipment parameters such as: conditioning moisture content (dry or wet), solid or segmented screw, desired degree of shear and heat source. From a practical point of view, the main classification used considers the degree of shear and the heat source (Riaz, 2000).

Regarding screw configuration, there are screws made up of only one piece or screws of multiple pieces. Single element screws may present different configurations: (i) screw with constant depth and flight – straight –; (ii) screw with constant flight and variable depth – tapered – (conical from the feeding extremity to the die extremity); (iii) screw with a reduction in depth just after feeding, becoming constant at the end – tapered-straight – and (iv) screw with flight openings – interrupted flight – to increase shear force due to the increase in leakage flow and turbulence of the material (El-Dash, 1981).

Screws of multiple elements can be built up to desired configuration due to the great number of possible formats, varying screw flight and depth. Usually this type of screw is divided into five sections, where the first section presents wide flight and great depth with the objective of homogenizing and conveying the material. In the second section, also known as the intermediate section, there is a reduction in parallel screw flight (or adjacent screw flight) and depth, resulting in even greater mixing of the material and beginning of shear, while the material is transported to the next section. The third section is responsible for an increase in shear and pressure, promoting structural changes in the material, which passes to the viscoelastic state. The increase in shear force in the third section can be reached with interruptions in screw flight favouring material turbulence. In the fourth section, due

Thermoplastic Extrusion in Food Processing 271

When the material enters the barrel, the ingredients are thoroughly mixed before further processing in the other zones of the extruder. In this initial step, the screw is designed with a large screw channel depth to provide enough space between the root of the screw and the barrel for sufficient mixing to take place, and often, the screws are reverse-threaded to permit intensive mixing and longer residence times before delivery. In the next zone, the diameter of the root increases rapidly while the channel depth becomes shallower in order to provide material cooking, thus increasing the pressure applied to the product, and the starchy content of food is gelatinized and the proteinaceous material denatured (El-Dash,

When needed, after the cooking zone, the material is forced to the depressurizing zone were the screw root diameter is much smaller, while the channel depth is much deeper than in the previous zone. To promote pressure reduction, a depressurizing valve can be opened to atmospheric pressure or to a vacuum pump. In the forming zone, the diameter of the screw root increases, reducing channel depth and resulting in an increase in the pressure applied. This pressure must be high enough to permit the extrusion of the product in the appropriate

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

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

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

1981).

form through the die (El-Dash, 1981).

**3.1 Raw materials** 

products (Guy, 2001).

**3. Raw materials and changes in major components** 

before the die to minimize exposure to heat and shear (Riaz, 2000).

after it reaches its maximum expansion (Guy, 2001).

to the small clearance available for the material, there is high shear and an increase in the temperature of the molten mass, resulting in cooking of the product. In the last section, due to an even greater reduction in screw flight and depth, shear and heat generation promote final cooking of the product (El-Dash, 1981).

Extrusion conditions when using extruders that have single screws of multiple elements can be controlled varying the number of sections, screw configuration in each section and through the inclusion of shearlocks (El-Dash, 1981).

Single-screw extruders can be classified in four different types based on the degree of shear, as follows:

Cold forming extruders – operate with moderate conditioning moisture contents (30 - 40%), low shear and smooth internal barrel surface, deep flight and low screw speed. These are not used for thermoplastic extrusion. They are used to form compact products such as pasta, cookies, pastry doughs, processed meats and certain candies (Riaz, 2000).

High-pressure forming extruders – operate with low shear, grooved barrel and compression screw. They are used to produce pre-gelatinized flours and pellets (for post expansion by hot air or frying) (Riaz, 2000). The latter are considered 3rd generation products.

Low-shear cooking extruders – operate with moderate degree of shear, high compression screw and grooved barrel (straight or helicoidal) to favour mixing. Usually involve external heating (steam jacket or electric resistance) to improve cooking with the objective of pasteurization, enzymatic inactivation, protein denaturation and/or starch gelatinization (Riaz, 2000).

Collet extruders – operate with high shear, grooved barrel and screw with an increase in compression (through multiple shallow flights). Commonly used to produce expanded snacks from corn grits. Conditioning moisture content must be low (12 - 14%) and temperature high (150 – 175°C), resulting in partial dextrinization and gelatinization of starch. Due to the high pressure formed within the extruder, when exiting the die there is immediate expansion of the material (Riaz, 2000).
