EXTRUSION OF FOODS
There are two types of extrusion process: cold extrusion, which mixes and shapes foods such as
biscuit dough and pasta without cooking them; and hot extrusion (or extrusion cooking), which is
used to produce a wide range of products, including crisp snackfoods, sugar confectionery and soya-
based weaning foods. Both use equipment known as an ‘extruder’. This equipment has a screw
inside a barrel that conveys materials along the barrel and kneads the food into a semi-solid,
plasticised mass. In cold extruders the material is not heated but simply formed into shapes
(including rods, tubes, strips or shells), when it is forced through openings in a ‘die’ at the discharge
end of the barrel. In extruder-cookers the material is heated by friction and/or supplementary heaters
in the barrel and it emerges from the dies under pressure. Some snackfood products expand rapidly
and have a light, crisp texture, caused by steam being flashed off due to the sudden pressure drop
when they emerge from the die.
Figure. 1: Mixing pasta dough, extruded spaghetti, dies for pasta
machines, pasta extruder, (Fords Food Equipment at
www.freshpastamachines.co.uk/lillo_due_pasta_machine.html )
Cold-extruded products are preserved by chilling, baking or drying,
whereas extrusion cooking destroys contaminating micro-organisms
and the dry products have a long shelf life. They are packaged to
prevent them picking up moisture and to prevent oxidation during
storage. Cold extruders are suitable for all scales of operation from
household- to small-scale, but extruder-cookers are much more
expensive and are likely to only be affordable by larger-scale
producers.
Cold extrusion
The main application of cold extruders is in pasta production,
although similar machines are used to form biscuit dough into
different shapes. A pasta extruder (Figure 1) is used to make many
different types of pasta (Table 1) using dough made from durum
wheat flour (or ‘semolina’) and eggs. Coloured pasta can also be
made by adding tomato purée or spinach paste.
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