Paper No.

: 02

Paper Title: PRINCIPLES OF THE FOOD PROCESSING & PRESERVATION

Module – 29: Food Extrusion

Paper Coordinator: Dr. P. Narender Raju,

Scientist, National Dairy Research Institute, Karnal, Haryana
Content Writer: Dr. Kuna Aparna,
Asst. Professor, Prof. Jayashankar Telangana State
Agricultural University, Hyderabad
 Module 29

FOOD EXTRUSION
 Introduction
• Food extrusion is a form of extrusion used in food
processing.
• It is a process by which a set of mixed ingredients
are forced through an opening in a perforated
plate or die with a design specific to the food, and
is then cut to a specified size by blades.
• The machine which forces the mix through the
die is an extruder, and the mix is known as
the extrudate.
• The extruder consists of a large, rotating screw
tightly fitting within a stationary barrel, at the
end of which is the die.
• Extrusion enables mass production of food via a
continuous, efficient system that ensures uniformity of
the final product.
• Food products manufactured using extrusion usually
have a high starch content.
• Examples: pasta, breads (croutons, bread sticks,
and flat breads), many breakfast cereals and ready-to-
eat snacks, confectionery, pre-made cookie dough,
some baby foods, full-fat soy, textured vegetable
protein, some beverages, and dry and semi-moist pet
foods
• Types of extrusion: There are two types of extrusion
processes.
– Cold extrusion
– Hot extrusion (or extrusion cooking)
• Extruder: This equipment has a screw inside a barrel that
conveys materials along the barrel and kneads the food into a
semi-solid, plasticized 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.

• 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.

• 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.

• Different sizes of equipment are available, from small manual

machines that are used in micro-scale production and food service
outlets, to larger electric machines.

• The equipment has a mixing chamber, extruder barrel and a die for
the desired pasta shape.

• Shapes are cut to the appropriate length as they emerge from the
die, except rigatoni, which is extruded in long lengths and then cut
to the correct size (straight for rigatoni or angled for penne rigati).

• Pasta is cooked immediately in food service outlets, or dried by

processors for retail sale. It can also be frozen for up to six months.
Hot extrusion
• Extruder-cookers may be single- or twin-screw
machines.

• Twin-screw machines have approximately twice

the capital and maintenance costs of single screw
machines and are unlikely to be affordable by
most small-scale processors as compared to
single-screw extruders.

• 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 & Process of extrusion
• 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.
• In the extrusion process, raw materials are first ground
to the correct particle size, usually the consistency of
coarse flour.
• The dry mix is passed through a pre-conditioner, in
which other ingredients are added depending on the
target product; these may be liquid sugar, fats,
dyes, meats or water.
• Steam is injected to start the cooking process, and the
preconditioned mix (extrudate) is then passed through
an extruder.
• The extruder's rotating screw forces the extrudate
toward the die, through which it then passes.
• The amount of time the extrudate is in the extruder is
the residence time.
• The extruded product usually puffs and changes
texture as it is extruded because of the reduction of
forces and release of moisture and heat.

• The extent to which it does so is known as

the expansion ratio.

• The extrudate is cut to the desired length by blades

at the output of the extruder, which rotate about the
die openings at a specific speed.

• The product is then cooled and dried, becoming rigid

while maintaining porosity.
• Many food extrusion processes involve a high
temperature over a short time.

• Important factors of the extrusion process are the

composition of the extrudate, screw length and
rotating speed, barrel temperature and moisture, die
shape, and rotating speed of the blades.

• These are controlled based on the desired product to

ensure uniformity of the output.

• Moisture is the most important of these factors, and

affects the mix viscosity, acting to plasticize the
extrudate.
• Increasing moisture will decrease viscosity, torque, and product
temperature, and increase bulk density.
• This will also reduce the pressure at the die. Most extrusion
processes for food processing maintain a moisture level below
40%, that is low to intermediate moisture.
• The amount of salt in the extrudate may determine the colour and
texture of some extruded products.
• The expansion ratio and airiness of the product depend on the salt
concentration in the extrudate, possibly as a result of a chemical
reaction between the salt and the starches in the extrudate.
• Colour changes as a result of salt concentration may be caused by
"the ability of salt to change the water activity of the extrudate and
thus change the rate of browning reactions".
• Salt is also used to distribute minor ingredients, such as food
colours and flavours, after extrusion; these are more evenly
distributed over the product's surface after being mixed with salt.
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.

• 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,
snack foods, 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.
• 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 high temperatures used in the extruder ensure that products

are safe and have a shelf life in excess of 12 months when packed in
moisture proof 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 and corn syrup with
added colours, acids and flavours.
 Extruded products
• Extrusion has enabled the production of new processed
food products and "revolutionized many conventional
snack manufacturing processes".

• The various types of food products manufactured by

extrusion typically have a high starch content.
– Directly expanded types include breakfast cereals and corn
curls, and are made in high temperature, low moisture
conditions under high shear.
– Unexpanded products include pasta, which is produced at
intermediate moisture (about 40%) and low temperature.
– Texturized products include meat analogues, which are made
using plant proteins ("textured vegetable protein") and a long
die to "impart a fibrous, meat-like structure to the
extrudate", and fish paste.
– Confectionery made via extrusion includes chewing
gum, liquorice, and toffee.
• Some processed cheeses and cheese analogues
are also made by extrusion.

• Other food products often produced by extrusion

include some breads (croutons, bread sticks,
and flat breads), various ready-to-eat snacks, pre-
made cookie dough, some baby foods,
some beverages, and dry and semi-moist pet
foods.

• Specific examples include macaroni, jelly

beans, sevai, and some french fries. Extrusion is
also used to modify starch and to pellet animal
feed.
Effects of extrusion on nutritional quality
The extrusion process results in "chemical reactions that occur within
the extruder barrel and at the die". Extrusion has the following
effects:
• Destruction of certain naturally occurring toxins
• Reduction of microorganisms in the final product
• Slight increase of iron-bioavailability
• Creation of insulin-desensitizing starches (a potential risk-factor for
developing diabetes)
• Loss of lysine, an essential amino acid necessary for developmental
growth and nitrogen management
• Simplification of complex starches, increasing rates of tooth decay
• Increase of glycemic index of the processed food, as the "extrusion
process significantly increased the availability of carbohydrates for
digestion"
• Destruction of Vitamin A (beta-carotene)
• Denaturation of proteins.
 Conclusion
• Recent research publications indicate that use of non-traditional
cereal flours, such as amaranth, buckwheat or millet, may be used
to reduce the glycemic index of breakfast cereals produced by
extrusion.

• The extrudate using these cereal flours exhibits a higher bulk and
product density, had a similar expansion ratio, and had "a significant
reduction in readily digestible carbohydrates and slowly digestible
carbohydrates".

• Another research work states that replacing 5% to 15% of

the wheat flour and white flour with dietary fibre in the extrudate
breakfast cereal mix significantly reduces "the rate and extent of
carbohydrate hydrolysis of the extruded products", which increased
the level of slowly digested carbohydrates and reduced the level of
quickly digested carbohydrates.
 Suggested readings
• Harper, J.M. (1978). "Food extrusion". Critical Reviews in Food
Science and Nutrition 11 (2): 155 -215.

• Karwe, Mukund V. (2008). "Food extrusion". Food

Engineering 3. Oxford Eolss Publishers Co Ltd.

• Riaz, Mian N. (2000). Extruders in Food Applications. CRC

Press.

• Camire, M.E. (1998). "Chemical changes during extrusion

cooking. Recent advances.". Advances in Experimental
Medicine and Biology 434: 109–121.

• Guy, Robin (2001). Extrusion Cooking: Technologies and

Applications. Cambridge: Woodhead Publishing. pp. 111–116.
 Glossary
• Extrusion is a process by which a set of mixed ingredients are forced through
an opening in a perforated plate or die with a design specific to the food, and
is then cut to a specified size by blades.

• Cold extrusion, is a process which mixes and shapes foods such as biscuit
dough and pasta without cooking them.

• Hot extrusion (or extrusion cooking), is a process which is used to produce a

wide range of products, including crisp snack foods, sugar confectionery and
soya-based weaning foods.

• Directly expanded snacks include breakfast cereals and corn curls, and are
made in high temperature, low moisture conditions under high shear.

• Unexpanded snacks include pasta, which is produced at intermediate

moisture (about 40%) and low temperature.

• Texturized snacks include meat analogues, which are made using plant
proteins ("textured vegetable protein") and a long die to "impart a fibrous,
meat-like structure to the extrudate".