**2.7.2 Twin-screw extruders**

Twin-screw extruders are composed of two axis that rotate inside a single barrel; usually the internal surface of the barrel of twin-screw extruders is smooth. Depending on the position of the screws and their direction of rotation, four different types of configurations are possible: (i) co-rotating intermeshing screws; (ii) co-rotating non-intermeshing screws; (iii) counter-rotating intermeshing screws; and (iv) counter-rotating non-intermeshing screws. Conical intermeshing extruders also exist. Although intermeshing screws result in greater residence time of the material in the extruder, non-intermeshing screws cause greater degrees of shear, especially if they rotate in opposite directions. However, this type of extruder is little used in the food industry, even though they present more efficient displacement properties (El-Dash, 1981). The intermeshing configuration is more effective, as the two screws function as a positive pump, increasing the drag flow and reducing the slipping of material in the extruder. Non-intermeshing screws provide higher shear than intermeshing screws because of the open channel between them.

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

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

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

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,

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

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

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

Twin-screw extruders are composed of two axis that rotate inside a single barrel; usually the internal surface of the barrel of twin-screw extruders is smooth. Depending on the position of the screws and their direction of rotation, four different types of configurations are possible: (i) co-rotating intermeshing screws; (ii) co-rotating non-intermeshing screws; (iii) counter-rotating intermeshing screws; and (iv) counter-rotating non-intermeshing screws. Conical intermeshing extruders also exist. Although intermeshing screws result in greater residence time of the material in the extruder, non-intermeshing screws cause greater degrees of shear, especially if they rotate in opposite directions. However, this type of extruder is little used in the food industry, even though they present more efficient displacement properties (El-Dash, 1981). The intermeshing configuration is more effective, as the two screws function as a positive pump, increasing the drag flow and reducing the slipping of material in the extruder. Non-intermeshing screws provide higher shear than

cookies, pastry doughs, processed meats and certain candies (Riaz, 2000).

hot air or frying) (Riaz, 2000). The latter are considered 3rd generation products.

final cooking of the product (El-Dash, 1981).

as follows:

(Riaz, 2000).

through the inclusion of shearlocks (El-Dash, 1981).

immediate expansion of the material (Riaz, 2000).

intermeshing screws because of the open channel between them.

**2.7.2 Twin-screw extruders** 

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, 1981).

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