destruction of micro-organisms and some toxic compounds, grinding, hydration,
expansion, shearing, mixing and shaping [RIAZ].
Different kinds of extruder are existing: single screw extruder, twin screw extruder and
some special types like “turbo-extruder” and “multi-screw- or planetary roller
extruder” [SCHUCHMANN, 2005].
Extruder normally consists of a dosing and entering area with a high volume, where
raw materials are not yet compacted and they can be removed quickly. The basic
product mechanical handling is achieved by a screw construction, in which the free
product volume decreases along the screw from the dosing to the nozzle. Dry or prehumidified
solid raw materials are added in the first casing part and then liquid
ingredients (like water) follow [SCHUCHMANN, 2005].
In the kneading area, the free product volume continues to decrease by decreasing
screw pitch and the product is compacted. In this area, also steam can be injected or it
can be degassed [SCHUCHMANN, 2005].
In the cooking extrusion the boiling zone follows and the product is further
compressed. The product density can reach up to 1800 g/l in front of the nozzle.
[SCHUCHMANN, 2005]. Then, the product is conveyed through the nozzle, formed
there and possibly expanded. In the production of dense pellets, a further forming
zone follows the boiling zone, in which temperature and pressure are reduced, before
the product is forced through the nozzle [SCHUCHMANN, 2005].