Extrusion of foods
Practical Action
The two factors that control the type and quality of foods that are produced by hot extrusion are the
operating conditions in the extruder barrel and the mixture of ingredients that is used.
Operating conditions
The important operating conditions are the temperature and pressure in the barrel, the diameter of
the die apertures and the product shear rate. The shear rate is influenced by the speed and geometry
of the screw (size, number, pitch and diameter of the flights), and by the internal design of the
barrel, including grooves in the barrel, or restrictions (known variously as ‘throttle rings’, ‘kneading
discs’, or ‘shearlocks’). Additional heating may be provided by a steam-jacketed barrel, a steam-
heated screw, or electric heating elements around the barrel.
‘High-shear’ extruders have high screw speeds and shallow flights to create the high pressures and
temperatures needed to make expanded products; ‘medium-shear’ extruders are used to make
texturised proteins and semi-moist foods; and ‘low shear’ extruders have a deep-flighted screw that
operates at low speeds in a smooth barrel to create low pressures for forming meat products or gums.
The selection of the correct type of extruder for a particular application should take account of the
types of ingredients and the properties required in the product (e.g. its bulk density, texture, colour
and other sensory properties) and the required production rate.
Dies have different shaped holes (e.g. round holes to produce rods, square holes for bars, or slots to
produce sheets), or they may produce more complex shapes. The temperature and moisture content
of the food and the extent of shearing in the barrel control the amount of expansion of the product
and hence its texture. Some products require the dies to be heated to give the required degree of
expansion, whereas others have cooled dies to reduce expansion. There are therefore a very large
number of potential combinations of equipment design features and small-scale processors should
seek advice from extruder manufacturers before purchasing a machine.
Ingredient mixture
Different mixtures of ingredients produce completely different products when the same operating
conditions are used in the same extruder. This is because starch, proteins, moisture and other
ingredients (e.g. oil or an emulsifier) have different effects on the structure and texture of the
extruded food. For example, starch or proteins create a three-dimensional structure that contains the
other ingredients. Starches from cereal or legume flours (e.g. maize, wheat, rice, barley, pea, bean),
or from tuber flours (e.g. potato, cassava, tapioca) are used for extruded breakfast cereals,
snackfoods, pasta and biscuits. Proteins from soybeans, sunflower seeds, rapeseed, or gluten from
wheat, are used to make meat-like products such as texturised vegetable protein.
When making extruded snackfoods from cereal or potato starch, the process operates at high
temperatures (130 - 180oC) to produce a ‘fluid melt’ that contains gelatinised starch and
superheated water vapour. When this leaves the extruder it expands to form hard, porous, brittle
snackfood products. Flavourings and/or colourings are sprayed onto the food after it is extruded.
Snackfoods can also be made from extruded ‘preforms’ or ‘half-products’. These are small, dense
extruded pellets that are sold to other processors to make the final snackfood by frying or toasting
the pellets. When half-products are heated, they soften and the residual moisture in the pellets turns
to steam, which rapidly expands the pellets to produce the snackfood
Extruded weaning foods are produced as flakes or pellets from a mixture of cereal and legume flours
that have the correct protein and energy content for growing children. The extruded products may
also be fortified with minerals and vitamins. In use, the products are ground to a powder and mixed
with hot water to form a porridge that is fed to children. The high temperatures used in the extruder
ensure that products are safe and have a shelf life in excess of 12 months when packed in
moistureproof and airtight packaging. The process is used for both commercial weaning foods and
foods used as emergency or supplementary foods by development agencies.
Extrusion cooking is also used to produce sugar confectionery products such as liquorice, toffee,
fudge and boiled sweets from sugar, glucose and starch. Hard-boiled sweets are produced from sugar
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