Briefly describe the application of extrusion technology in Food Technology

1. FOOD EXTRUSION
Abdul Rehman
APRIL 11, 2020

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ABSTRACT
Today’s lifestyles are vastly different from those of the past, due to increasing in number of single
person households and gender equality rights have lead to changes in food preparation and
consumption habits. i.e., modernization and globalization made peoples very hurry and no time to
cook their own food, especially during morning. A positive outcome of this has been made rapid
advances in food technology not only these factors and also the increasing demand for non-meat,
high-protein products made a highly profitable ventures in the food industry. Now a day’s
consumers do choice for nutritionally rich, therapeutic benefits and for attractiveness especially in
case of baby foods. Thus, to obtain above mentioned designer food, Food extrusion technology is
used.
Extrusion-technology is gaining increasing popularity in the global agro-food processing industry,
particularly in the food and feed sectors. Extrusion cooking technologies are used for cereal and
protein processing in food. Today their processing functions may include, conveying, mixing,
shearing, separation, heating or cooling, shaping, co-extrusion, venting, volatiles and moisture,
flavor generation, encapsulation and sterilization. They can be relatively low temperatures, as with
pasta, spaghetti, noodles etc. keeping in this view, the report describes in detail about the types of
extruders, physic-chemical changes occurring during extrusion process and recent developments
in food industry regarding extrusion process.
This technology has an important role in the food industry as efficient manufacturing processes.
Their main role was developed for conveying and shaping fluid forms of processed raw materials,
such as doughs and pastes. Extrusion cooking technologies are used for cereal and protein
processing in the food and, closely related, pet-foods and feeds sectors. The processing units have
evolved from simple conveying devices to become very sophisticated in the last decade. Today,
their processing functions may include conveying, mixing, shearing, separation, heating or cooling,
shaping, co-extrusion, venting volatiles and moisture, flavour generation, encapsulation and
sterilisation. They can be used for processing at relatively low temperatures, as with pasta and half-
product pellet doughs, or at very high ones with flatbreads and extruded snacks. The pressures used
in extruders to control shaping, to keep water in a superheated liquid state and to increase shearing
forces in certain screw types, may vary from around 15 to over 200 atmospheres.

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TABLE OF CONTENTS
1. Introduction………………………………………………………………………………03
2. Importance in Food Industry…………….…………………………………………….…03
3. Classification……………………………………………………………………………..05
4. Principle of operation…………………………………………………………………….06
5. Applications in Food Industry…………………………………………………………...09
5.1 Breakfast Cereals
5.2 Snack Food
6. Conclusion……………………………………………………………………………….13
Recommendations

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INTRODUCTION
Extrusion of foods is an emerging technology for the food industries to process and market a large
number of products of varying size, shape, texture, and taste. Extrusion cooking technology has
led to production of wide variety of products like pasta, breakfast cereals, bread crumbs, biscuits,
crackers, croutons, baby foods, snack foods, confectionery items, chewing gum, texturized
vegetable protein (TVP), modified starch, pet foods, dried soups, dry beverage mixes etc. The
functional properties of extruded foods play an important role for their acceptability which include
water absorption, water solubility, oil absorption indexes, expansion index, bulk density and
viscosity of the dough. The aim of this review is to give the detailed outlines about the potential
of extrusion technology in development of different types of products and the role of extrusion-
operating conditions and their effect on product development resulting in quality changes i.e.,
physical, chemical, and nutritional, experienced during the extrusion process.
Extrusion cooking, a process of forcing a material to flow under a variety of conditions through a
shaped hole (die) at a predetermined rate to achieve various products. Extrusion cooking of foods
has been practiced over 50 years. The food extruder which was initially limited to mixing and
forming macaroni and ready to eat cereal pellets is now considered a high temperature-short time
bioreactor that transforms raw ingredients into modified intermediate and finished products.
During extrusion thermal and shear energies are applied to raw food materials causing structural,
chemical, and nutritional transformations such as starch gelatinization and degradation, protein
denaturalization, lipid oxidation, degradation of vitamins, antinutritions and phytochemicals,
formation of flavors, increase of mineral bioavailability and dietary fiber solubility.
IMPORTANCE IN FOOD INDUSTRY
1. Extrusion technologies have an important role in the food industry as efficient manufacturing
processes. Their main role was developed for conveying and shaping fluid forms of processed raw
materials, such as doughs and pastes.
2. Extrusion cooking technologies are used for cereal and protein processing in the food and,
closely related, pet foods and feeds sectors. The processing units have evolved from simple
conveying devices to become very sophisticated in the last decade.

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3. Today, their processing functions may include conveying, mixing, shearing, separation, heating
or cooling, shaping, co-extrusion, venting volatiles and moisture, flavour generation,
encapsulation and sterilization.
4. They can be used for processing at relatively low temperatures, as with pasta and half product
pellet doughs, or at very high ones with flatbreads and extruded snacks. The pressures used in
extruders to control shaping, to keep water in a superheated liquid state and to increase shearing
forces in certain screw types, may vary from around 15 to over 200 atmospheres.
CLASSIFICATION
Figure 1 Classification of Extruders
Single-screw extruders
The equipment consists of a cylindrical screw that rotates in a grooved cylindrical barrel, made
from hard alloys or hardened stainless steel to withstand the frictional wear. The length to diameter
ratio of the barrel is between 2:1 and 25:1. The pitch and diameter of the screw, the number of
flights and the clearance between the flights and the barrel can each be adjusted to change the
performance of the extruder. The screw is driven by a variable speed electric motor that is
sufficiently powerful to pump the food against the pressure generated in the barrel. The screw
speed is one of the main factors that influences the performance of the extruder: it affects the
residence time of the product, the amount of frictional heat generated, heat transfer rates and the
shearing forces on the product. Typical screw speeds are 150–600 rpm, depending on the
Extruder
No of
Screws
Single-
screw
Twin-
screw
Corotating
Counter-
rotating
Operation
Hot
Extrusion
Cold
Extrusion

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application. Compression is achieved in the extruder barrel by back pressure, created by the die
and by:
 increasing the diameter of the screw and decreasing the screw pitch
 using a tapered barrel with a constant or decreasing screw pitch
 placing restrictions in the screw flights.
Die pressures vary from around 2000X103 Pa for low viscosity products to 17000X103 Pa for
expanded snack foods. Single-screw extruders can be classified according to the extent of shearing
action on the food into:
 High shear. High speeds and shallow flights create high pressures and temperatures that
are needed to make breakfast cereals and expanded snackfoods.
 Medium shear. For breadings, texturised proteins and semi-moist petfoods.
 Low shear. Deep flights and low speeds create low pressures for forming pasta, meat
products and gums.
Twin-screw extruders
The screws in twin-screw extruders rotate within a ‘figure of 8’ shaped bore in the barrel. Screw
length to diameter ratios are between 10:1 and 25:1. Extruders are classified according to the
direction of rotation and the way in which the screws intermesh. Co-rotating intermeshing screws,
which are self-wiping (the flights of one screw sweep food from the adjacent screw) are most
commonly found in food-processing applications. The spacing between the flights can be adjusted
so that large spaces initially convey the material to the cooking section and then smaller spaces
compress the plasticised mass before extrusion through an inter-changeable die. One of the main
advantages of twin-screw extruders is the greater flexibility of operation that is possible by
changing the degree of intermeshing of the screws, the number of flights or the angle of pitch of
the screw. ‘Kneading discs’ can also be fitted to the screws so that the product passes between and
through the discs to increase the kneading action. Their operating characteristics are described as:
Twin-screw extruders have the following advantages:
 The throughput is independent of feed-rate, and fluctuations in production rate can be
accommodated by the positive displacement action of the screws. In contrast, a single
screw must be full of material to operate effectively. The positive displacement also
produces higher rates of heat transfer and better control of heat transfer than a single screw
does.
 Twin-screw machines handle oily, sticky or very wet materials, or other products that slip
in a single screw. The limitations for single- and twin-screw machines are respectively 4%
and 20% fat, 10% and 40% sugar, and 30% and 65% moisture. There is therefore greater
flexibility in operation using different raw materials

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Figure 2 Single and twin screw extruders
PRINCIPLE OF OPERATION
Extruders come in a wide variety of sizes, shapes and methods of operation. There are hydraulic
ram extruders, in which a piston forces dough through a die, roller types, in which two rolls rotate
towards each other to force dough through a die, and screw types (both single and twin) in which
a rotating screw pumps dough through a die. Extruders are used in food processes because they
provide mechanical energy for cooking and melting; by pumping they force dough through a
shaped opening, and in some cases, they can be used for intense mixing for dispersion and
homogenisation of ingredients.
Figure 3 Corotating and Counterrotating twin screw extruders

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However, the principles of operation are similar in all types:
Raw materials are fed into the extruder barrel
convey the food down the barrel
smaller flights restrict the volume and increase the resistance to movement of the food
it fills the barrel and the spaces between the screw flights
End product is compressed
Figure 4 A Typical Food Extruder
As it moves further along the barrel, the screw kneads the material into a semi-solid, plasticized
mass. If the food is heated above 100ºC the process is known as extrusion cooking (or hot
extrusion). Here, frictional heat and any additional heating that is used cause the temperature to
rise rapidly. The food is then passed to the section of the barrel having the smallest flights, where
pressure and shearing is further increased.
Screw(s)

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Finally, it is forced through one or more restricted openings (dies) at the discharge end of the barrel
as the food emerges under pressure from the die, it expands to the final shape and cools rapidly as
moisture is flashed off as steam. A variety of shapes, including rods, spheres, doughnuts, tubes,
strips, squirls or shells can be formed. Typical products include a wide variety of low density,
expanded snack foods and ready-to-eat (RTE) puffed cereals.
Cold extrusion, in which the temperature of the food remains at ambient is used to mix and shape
foods such as pasta and meat products. Low pressure extrusion, at temperatures below 100ºC, is
used to produce, for example, liquorice, fish pastes, surimi and pet foods.
Extrusion cooking is a high-temperature short-time (HTST) process which reduces microbial
contamination and inactivates enzymes. The main method of preservation of both hot- and cold-
extruded foods is by the low water activity of the product (0.1–0.4), and for semi-moist products
in particular, by the packaging materials that are used.
Extrusion cooking has gained in popularity over the time due to following advantages:
• Versatility: a wide range of products, many of which cannot be produced easily by any other
process, is possible by changing the ingredients, extruder operating conditions and dies
• Cost: extrusion has lower processing costs and higher productivity than other cooking and
forming processes
• Productivity: extruders can operate continuously with high throughput.
• Product quality: extrusion cooking involves high temperatures applied for a short time, retaining
many heat sensitive components of a food
• Environmentally-friendly: as a low-moisture process, extrusion cooking does not produce
significant process effluents, reducing water treatment costs and levels of environmental pollution.

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APPLICATIONS IN FOOD INDUSTRY
Extrusion technology has led to production of a wide variety of cereal-based foods, protein
supplements, and sausage products. Presently, several products are developed by extrusion i.e.
pasta, breakfast cereals, bread crumbs, biscuits, crackers, croutons, baby foods, snack foods,
confectionery items, chewing gum, texturized vegetable protein, modified starch, pet foods, dried
soups, and dry beverage mixes. Extrusion cooking is becoming popular over other common
processing methods due to its automated control, high capacity, continuous operation, high
productivity, versatility, adaptability, energy efficiency, low cost. Moreover, it also enables design
and development of new food products, high product quality, unique product shapes and
characteristics, energy savings and no effluent generation. Extrusion cooking also helps in
modifying the structure, improving the solubility, swelling power, water hydration viscosity and
water holding capacity. It also increases the soluble fiber content of fibrous materials such as plant
cell-wall rich materials, brans and hulls of various cereals and legumes.
BREAKFAST CEREALS
Extrusion-cooking technology has played a very important and decisive role in the innovation and
development of breakfast cereal products. The extrusion-cooking process is a new cooking concept
– thermomechanical HTST (high temperature short time) cooking – that makes particular use of
mechanical processing of the material and is therefore an original alternative to classical
hydrothermal cooking. Then, it allows continuous cooking of a large range of recipes with various
cereals to produce different shapes and textures at satisfactory cost. Two types of extrusion-cooked
breakfast cereals can thus be found on the market:
 Directly expandedextrusion-cookedbreakfast cereals. Cereal flours and/or grits are cooked
with ingredients and with a very low moisture content (usually below 20%). The process
may use single- or twin-screw extruders, the configuration and operating characteristics of
which generally lead to highly mechanical cooking.
 Pellet-to-flakes extrusion-cooked breakfast cereals. Cereal flours and/or gritsare cooked
with ingredients and at a moisture level in the range of 22–26%. They are usually processed
in twin-screw extruders, the configuration and operating characteristics of which lead to a
lower mechanical component of cooking, reinforcing the thermal component as opposed
to the previous processing conditions.

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Table 1 The major directly expanded breakfast cereal available in the market
SNACK FOOD
The term snack foods covers a wide range of food products. However, the general range of snack
foods will include products such as nuts, biscuits and merge into confectionery and meat products,
with major snack foods as popcorn, potato chips or crisps and baked or fried snacks and starch
based snacks.
The major types of snack foods:
 Raw cut vegetable snacks
 Formed dough products from potato derivatives
 Formed dough products from maize derivatives
 Half product or pellet snacks
 Directly expanded extruded snacks
 Popcorn or puffed wheat
 Related processes, such as snack biscuits and breadsticks
The processes used to create snack foods are very important consideration with the basic
description of most important phenomena with three stages:
Formation of a dough

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(by hydration of starch polymers so that they form a fluid mass that can be shaped into the
individual snack pieces)
Heating of the dough mass
(water is superheated and released rapidly as vapour within the dough mass for puffing)
Snack is stabilised
(by being dried to low moisture levels to form a hard-brittle structure)

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CONCLUSION
Extruders permit the production of many foods of nutritional importance. The ability of extruders
to blend diverse ingredient in novel foods can be utilised in the development of functional foods.
Traditional snacks or breakfast cereal can be enhanced by the addition of extra fibre or whole grain
flour as ingredients. During extrusion, transformed into palatable cereal based products that also
promote beneficial physiological effects. The technique is well adapted in creating food products
with desired shape and texture with low economic costs.

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RECOMMENDATIONS
Extruded products are one of the fastest-growing segments of the food industry, and for years,
extrusion has been a mainstay of producing new and creative products. Extrusion cooking
produces a wide range of finished products from inexpensive raw materials with minimum
processing time. Extrusion is a highly versatile unit operation that can be applied to a variety of
food processes. Extruders can be used to cook, form, mix, texturize, and shape food products under
conditions that favor quality retention, high productivity, and low cost. This paper provides the
evident knowledge as to how the various products and waste by products can be utilized in an
efficient manner for manufacturing a nutritional product with primary emphasis on changes that
are experienced during the extrusion process at the physical and nutritional level. Thus, extrusion
cooking will remain as an integral part of manufacturing of wholesome processed foods. In future,
research should be emphasized in the area of development of improved extrusion technology
which is less dependent on the nature of raw materials being used. Also, value addition and by
product waste utilization in the extrusion should be encouraged in future in order to produce
healthy extruded products. Extrusion cooking has a potential for becoming the most important
food processing technology in future. Further readings can be:
 Extrusion, Food Processing Technology, P J Fellows
 Extrusion Cooking, Technology & Application, Robert Guy