**2. Equipments**

The use of thermoplastic extrusion in food processing is facilitated by the dynamism of extruders, which can be divided into two types: single-screw and twin-screw extruders (Riaz, 2000).

Extruders are composed of five main parts: (i) the pre-conditioning system; (ii) the feeding system; (iii) the screw or worm; (iv) the barrel; (v) the die and the cutting mechanism (El-Dash, 1981), which can be seen in Figure 1. Also, they can vary with respect to screw, barrel and die configuration. The selection of each of these items will depend on the raw material used and the final product desired (Riaz, 2000).

Fig. 1. Schematic representation of an extruder including its main parts and zones

In the extrusion process, the dry or pre-conditioned material (generally between 15 and 30% moisture content) is fed to the extruder through a screw feeder, reaching the feeding zone. The screw in this zone presents greater depth and pitch of the worm flight, and has as main function the transportation and homogenizing of the raw material. The material is conducted from the feeding zone to the compression zone. In the compression zone, there is a reduction in screw depth and pitch, with a consequent increase in shear rate, temperature (110 - 180ºC) and pressure (20 – 30 atm). In this zone, the conversion from a solid material to a fluid melt starts to occur. In the subsequent high pressure zone, the screw has its depth and pitch reduced even more, resulting in higher shear and maximum heat generation. Thus, the extruded mass reaches maximum temperature and pressure and a reduction in viscosity immediately before exiting the extruder (Fellows, 2000; Riaz, 2000). The material, under high pressure, is expelled through the die and, in contact with ambient pressure, expands to its final format and cools rapidly through water flash-off (Fellow, 2000). In material that is not previously conditioned, water is added, in liquid or vapour form, during the process (El-Dash, 1981). The product that leaves the extruder is generally submitted to a drying process, reaching values close to 3% moisture content, as is the case of extruded snacks (Riaz, 2000).

#### **2.1 Pre-conditioning**

266 Thermoplastic Elastomers

The use of thermoplastic extrusion in food processing is facilitated by the dynamism of extruders, which can be divided into two types: single-screw and twin-screw extruders

Extruders are composed of five main parts: (i) the pre-conditioning system; (ii) the feeding system; (iii) the screw or worm; (iv) the barrel; (v) the die and the cutting mechanism (El-Dash, 1981), which can be seen in Figure 1. Also, they can vary with respect to screw, barrel and die configuration. The selection of each of these items will depend on the raw material

Fig. 1. Schematic representation of an extruder including its main parts and zones

In the extrusion process, the dry or pre-conditioned material (generally between 15 and 30% moisture content) is fed to the extruder through a screw feeder, reaching the feeding zone. The screw in this zone presents greater depth and pitch of the worm flight, and has as main function the transportation and homogenizing of the raw material. The material is conducted from the feeding zone to the compression zone. In the compression zone, there is a reduction in screw depth and pitch, with a consequent increase in shear rate, temperature (110 - 180ºC) and pressure (20 – 30 atm). In this zone, the conversion from a solid material to a fluid melt starts to occur. In the subsequent high pressure zone, the screw has its depth and pitch reduced even more, resulting in higher shear and maximum heat generation. Thus, the extruded mass reaches maximum temperature and pressure and a reduction in viscosity immediately before exiting the extruder (Fellows, 2000; Riaz, 2000). The material, under high pressure, is expelled through the die and, in contact with ambient pressure, expands to its final format and cools rapidly through water flash-off (Fellow, 2000). In material that is not previously conditioned, water is added, in liquid or vapour form, during the process (El-Dash, 1981). The product that leaves the extruder is generally submitted to a drying process, reaching values close to 3% moisture content, as is the case of extruded

**2. Equipments** 

snacks (Riaz, 2000).

used and the final product desired (Riaz, 2000).

(Riaz, 2000).

Pre-conditioning with steam or water has always been an important part of the extrusion process. Recent research has shown that efficient throughput of the extruder is almost doubled if the starting material is pre-conditioned with steam or water (Guy, 2001). There are many applications of extruded cooked food products where pre-conditioning plays a key role in the overall extrusion process. These products include direct expanded and flaked breakfast cereals, pre-cooked pasta, textured vegetable proteins, meat analogues, extruded bread crumbs, and third-generation snacks.

Pre-conditioning is not applied to all extrusion processes. In general, this step is applied when moisture contents around 20 to 30% and long residence times of the material are used. Pre-conditioning favours uniform particle hydration, reduces retention times within the extruder and increases throughput, increasing the life of the equipment, due to a reduction in the wearing of barrel and screw components, also reducing the costs of energy involved in the process (Huber & Rokey, 1990). Depending on screw configuration, the residence time of the material inside the extruder can vary from 5 seconds to more than 2 minutes, with the average residence time of the material in the pre-conditioner being 3 minutes. Preconditioning occurs with the addition of hot water (80-90°C) or steam, through spray nozzles, with the use of steam reducing energy consumption of the equipment up to 60% during the process. The most commonly used pre-conditioners have 2 axis of different diameters and rotation speeds, guaranteeing a residence time between 2 to 4 minutes and a production capacity between 300 and 18,000 kg.h-1. When it is necessary to add melted fats or oils during pre-conditioning, it is best to do it at the end of the equipment, because if addition is done at the beginning, a coating may be formed over the particles, making water penetration more difficult. The main aim of pre-conditioning is to uniformly hydrate the raw material in order to eliminate any dry core (Strahm, 2000).

#### **2.2 Feeding system**

Most raw materials used in food extrusion are solid. The feeding system is normally composed of a holding bin where the material is loaded and the discharge of the material can occur through a vertical feeding screw, a horizontal feeding screw, a horizontal vibrating trough system, a disk feeder or a volumetric belt feeder. It is necessary to guarantee a constant and non-interrupted feeding of the raw materials into the extruder for an efficient and uniform functioning of the extrusion process (El-Dash, 1981). When liquids are added, they can be dosed using a rotameter, orifice and Venturi meters, positive displacement meter, magnetic flow meter or metering pumps (Chessari & Sellahewa, 2000).

#### **2.3 Screw**

The screw of the extruder is certainly its most important component, not only to determine cooking degree, gelatinization and dextrinization of starch and protein denaturation, but also to ensure final product quality. Screws can be mono-piece (composed of a unique piece) or multi-piece (composed of various elements) (El-Dash, 1981). Screw elements can vary in number and shapes, each segment is designed for a specific purpose. Some elements only convey raw or pre-conditioned material into the extruder barrel, while other segments compress and degas the feedstock. Others must promote kneading, backflow and shear.

Thermoplastic Extrusion in Food Processing 269

The die presents two main functions: give shape to the final product and promote resistance to material flow within the extruder permitting an increase in internal pressure. The die can present various designs and number of orifices (El-Dash, 1981). Dies may be designed to be highly restrictive, giving increased barrel fill, residence time and energy input. Die design and its effects on functional properties and quality of a final product are many times

The cutting mechanism must permit obtaining final products with uniform size. Product size is determined by the rotation speed of the cutting blades. This mechanism can be

Two types of extruders are used for food production: (i) single-screw extruders and (ii) twinscrew extruders. Single-screw extruders are the most common extruders used in the food industry. Twin-screw extruders are used for high-moisture extrusion, products that include higher quantities of components such as fibres, fats, etc. and more sophisticated products.

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

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

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

classification used considers the degree of shear and the heat source (Riaz, 2000).

increase in leakage flow and turbulence of the material (El-Dash, 1981).

**2.5 Die** 

overlooked.

**2.6 Cutting mechanism** 

**2.7 Types of extruders** 

**2.7.1 Single-screw extruders** 

horizontal or vertical (El-Dash, 1981).

Some kneading screws have interrupted flights to improve dispersive mixing, increase backflow, or increase mechanical energy dissipation into the extruder (Huber, 2001). Main characteristics of screw design include: (i) screw length; (ii) screw diameter; (iii) screw channel depth; (iv) screw channel width; (v) axial flight land width; (vi) clearance between screw and barrel; (vii) screw helix angle; (viii) leading flank angle; (ix) trailing flank angle; (x) screw pitch; (xi) direction of drag flow; (xii) direction of pressure flow and (xiii) direction of leakage flow (Figure 2) (El-Dash, 1981). The screw pitch (t) is the distance between corresponding points on adjacent thread profiles, and the number of parallel screw channels or leads (n) is defined as the number of screw pitches in the axis distance that the helix advances in one turn.

Fig. 2. Main characteristics of screw design

#### **2.4 Barrel or sleeves**

The barrel is divided into feeding, kneading and high pressure zones (Figure 1).

The sleeves surrounding the screw can be solid, but they are often jacketed to permit circulating of steam or superheated oil for heating or water or air for cooling, thus enabling the precise adjustment of the temperature in the various zones of the extruder. And most sleeves are equipped with pressure and temperature sensing and temperature control mechanisms as well (El-Dash, 1981).

In twin-screw extruders, the sleeves are usually smooth but can be constructed with longitudinal or helical grooves (Huber, 2000). In single-screw extruders, the sleeves are usually fluted on the inside, with either straight or spiral grooves. Parallel grooves are often cut or more often cast into the barrel. Spiral grooves provide high forward flow, while straight grooves hinder it. The latter thus result in a lower flow rate, but more mechanical shear. The clearance between the screw and its sleeve is usually kept to a minimum to reduce leakage flow (El-Dash, 1981).
