Extrusion

By

Nitesh Yadav

www.facebook.com/nitesyadav

Extrusion

Extrusion cooking is the process of cooking moistened starchy and/or proteinous

food material by a combination of heat, pressure and mechanical shear. Extrusion is a

process which combines several unit operations including mixing, cooking, kneading,

shearing, shaping and forming. Food extrusion machines use single or twin screws to

transport, mix, knead, shear, shape, and/or cook multiple ingredients into a uniform food

product by forcing the ingredient mix through shaped dies to produce specific shapes and

lengths. Extrusion provides the foundation for continuous production. Food extruders are

used to produce pasta and other cold formed products, cereals, snacks, pet food, feed,

confectionery products (including chewing gum, licorice), modified starches for soup,

baby food, and instant foods, beverage bases, and texturized vegetable proteins. Single-

screw extruders have been in use for continuous cooking and in the forming of ready-to-

eat (RTE) cereals as a one-step process since the 1960s. Twin-screw extruders were in

common use in food production by the 1980s.

The principles of operation are similar in all types: raw materials are fed into the

extruder barrel and the screw(s) they convey the food along it. Further down the barrel, a

smaller flight restricts the volume and increase the resistance to movement of the food.

As a result, it fills the barrel and the space between the screw flights and becomes

compressed. 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.

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, squirals or shells can be formed. Typical products

include a variety of low density, expanded snack foods and ready to eat (RTE) puffed

cereals.

Classification of Extruders:

Method of operation (cold extruders and extruder cookers)

Method of construction (single and twin screw extruders)

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

temperature 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. However, 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.

Basic Components of a Food Extruder

While each manufacturer uses its own names for extruder parts, the basic

components of a food extrusion line include:

A feeding unit or storage bin above the extruder that meters the raw ingredients

into the extruder.

Pre-conditioner: an assembly sometimes used to adjust the moisture content and

temperature of ingredients (may partially or completely cook them before entering

the extruder).

Extruder barrel: a pipe-like retainer in which the screw turns. The barrel may be

built in sections that contain their own heating or cooling sleeve.

Screw: the core of any extruder, the screw conveys the product through the

extruder. The flight of the screw pushes product forward. The screw determines

not only the quality of the product but also the output of the extruder. Extruders

can use either a single screw or twin screws.

Vent: an opening in the barrel before the die plate that allows pressure and steam

to be removed.

Die plate: final assembly for shaping the product as it leaves the extruder.

Cutting station: ensures precise and consistent shape and size of product.

Gear box, motor, and controls

Most extruders used for food processing are constructed of stainless steel for

sanitation.

Popularity Gained in Extrusion:

Versatility. A very wide variety of products are possible by changing the

ingredients, the operating conditions of the extruder and the shape of the dies.

Many extruded foods cannot be easily produced by other method.

Reduced costs. Extrusion has lower processing costs and higher productivity than

other cooking or forming processes. Some traditional processes, including

manufacture of cornflakes and frankfurters, are more efficient and cheaper when

replaced by extrusion.

High production rates and automatic production. Extruders operate

continuously and have high throughputs. For example, production rates of up to

315 kgh-1 for snack foods, 1200kgh-1 for low density cereals and 9000 kgh-1 for

dry expanded pet foods are possible.

Product quality. Extrusion involves high temperatures applied for a short time

and the limited heat treatment therefore retains many heat sensitive components.

No process effluents. Extrusion is a low moisture process that does not produce

process effluents. This eliminates water treatment costs and does not create

problems of environmental pollution.

Extrusion involves simultaneous mixing, kneading, and cooking, it cause a large number

of complex changes to a food, including hydration of starches and proteins,

homogenization, gelation, shearing, melting of fats, denaturation or re-orientation of

proteins, plastification and expansion of food structure. Use of the extrusion cooking

process gives the following food benefits:

Starch gelatinization

Protein denaturation

Inactivation of raw food enzymes

Destruction of naturally occurring toxins

Diminishing of microorganisms in the final product

Operating characteristics:

Temperature

Pressure

Diameter of die apparatus

Shear rate (shear rate is influenced by the internal design of barrel, its length and

the speed and geometry of the screw).

Extrusion in the food industry within industry as it can:

1. Work continuously

2. Act as a heat exchanger (i.e. heat or cool)

3. Handle viscous materials (i.e. thick mixtures); produce a range of different

products (textures and shapes).

Advantages of Extrusion in Food Processing

The advantage of extrusion is that it produces a more homogeneous and consistent

cooking process, which leads to a final product of higher quality with minimum

waste.

Extrusion is a very efficient process, since all steps can be done in-line: mixing,

cooking, forming, cooling, and cutting.

Many extruders with modular designs allow changing from one product to

another product, color, or shape to be done within minutes—a significant process

efficiency.

Twin-screw extruders are largely self-cleaning, an advantage from the viewpoint

of both sanitation and labor savings.

Type of snack foods

Cheese puffs/Cheese curls

Cheese balls

Corn chips and Tortilla chips

Crispy rice

Crackers

Confectionary

Extruded bread

Co-extruded snack

Bugles snack

Fryums

Pet snack

Dog chews/treats

Fish feed

Potato chips

Bread crumbs

Uses of extrusion in industry- Pasta

1. Pasta production relies mainly on the process of extrusion, e.g. macaroni

2. Low temperatures and pressure are needed to keep the pasta from cooking.

Uses of extrusion in industry- Snacks

1. The production of snack products is a rapidly growing area. High temperatures

and pressures are necessary to produce the snacks.

2. The gelatinised starch ensures that when the mixture expands by going from high

pressure to low pressure it holds its expanded shape.

Extruded hollow pasta

Extrusion has found great application in food processing. Products such as pastas,

breakfast cereals, cookie dough, Murukku, Sevai, Idiappam, jalebi, french fries, baby

food, dry pet food and ready-to-eat snacks are mostly manufactured by extrusion. 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, where

other ingredients are added (liquid sugar, fats, dyes, meats and water depending on the

product being made), steam is also injected to start the cooking process. The

preconditioned mix is then passed through an extruder, and then forced through a die

where it is cut to the desired length. The cooking process takes place within the extruder

where the product produces its own friction and heat due to the pressure generated (10–

20 bars). The cooking process utilizes a process known as starch gelatinization. Extruders

using this process have a capacity from 1–25 tonnes per hour depending on design.

Twin-Screw Food Extruder

Twin screw extruder two screws, side by side, are placed within the extruder

barrel; they are either counter-rotating or co-rotating. The co-rotating units are used for

compounding materials where thorough mixing and high output rates are important. Twin

screw extruder unit resembles a positive displacement screw pump. It conveys the

material at low speeds with controlled shear. The positive action assures that all portions

of the material experience a uniform residence time.

Twin-screw extruders are widely used for the more demanding applications and consist

of two types:

1. Counter-rotating twin-screw extruders—move in opposing directions

2. Co-rotating twin-screw extruders—move in same direction

Counter-rotating twin-screw extruders are commonly used for processing

relatively non-viscous materials requiring low speeds and long residence times, such as

gum, jelly, and licorice confections. Co-rotating twin-screw extruders have broadened the

variety of products that can be made with extrusion technology and are commonly used

in the snack food industry. They provide a high degree of heat transfer but not forced

conveyance.

While twin-screw extruders may cost 50 to 150% more than single-screw

extruders, they offer a number of advantages. They handle viscous, oily, sticky, or very

wet materials that will slip in a single-screw extruder. They also offer positive pumping

action, reduced pulsation at the die, and less wear in the smaller parts of the machine than

a single-screw extruder. They feature non-pulsating feed and can handle a wide range of

particle size, whereas a single-screw extruder is generally limited to a specific range of

particle size.

Common food processing unit, mostly in baking industry.

Fast-speed bioreactor or with heating, cooling, compressing, mixing, evaporating,

cutting, and aerating in one unit.

Twin-screw is now becoming more common, as it is easier to manipulate a

number of parameters.

Single-Screw Food Extruder

Single-screw extruders typically consist of three zones: feeding zone, kneading

zone, cooling zone. In general, single-screw extruders have poor mixing ability and

therefore materials should be pre-mixed or pre-conditioned. A single-screw extruder’s

processing conditions can be controlled to achieve a variety of effects on temperature and

residence time. The residence time in the barrel can vary from 15 to 300 seconds,

determined by increasing or decreasing the speed of the shaft. Single-screw extruders are

limited in their ability to transport sticky and/or gummy raw materials, and materials that

become sticky during heat compression. A variety of fried and baked snacks are

processed using single-screw extruders.

The twin screw extruder can be run at lower speeds (20-60 rpm) than the single screw

extruders (100-400 rpm). It shows that the twin screw extruder has much better

conveyance capacity.

Low, Medium, and High Shear Stress Extruders

Extruders can also be classified into the categories of low, medium, and high

shear stress. “Shearing” is the working, mixing action that homogenizes the conveyed

material.

1. Low shear stress extruders (forming extruders) are used to densify material that is

generally high in moisture, such as pasta. They operate at a slow speed and

feature a long length-to-diameter ratio, imparting low levels of mechanical energy

per unit of throughput.

2. Medium shear stress extruders handle materials with a lower moisture level and

mechanical energy inputs are higher. Typical products processed are pet food,

aquatic feeds, and textured vegetable proteins.

3. High shear stress extruders are used for highly expanded products with low

moisture and bulk density levels. They feature the shortest length-to-diameter

ratio and extruder speeds and mechanical energy inputs are high.

Effect on foods:

1) Sensory characteristics

Extent of changes to starch determined by the operating conditions and

feed materials, produces the wide range of product textures that can be

achieved.

The HTST conditions in extrusion cooking have only minor effects on the

natural colour and flavour of foods.

Fading of colour due to product expansion, excessive heat or reactions

with proteins, reducing sugars or metal ions may be problem in some

extruded foods.

Added flavours are mixed with ingredients before cold extrusion, but this

is largely unsuccessful in extrusion cooking as the flavours are volatilesed

when the food emerges from the die.

Flavours are therefore more often applied to the surface of extruded foods

in the form of sprayed emulsions or viscous slurries. However, this may

cause stickiness in some products and hence require additional drying.

2) Nutritional value

Vitamin losses in extruded foods vary according to type of foods, the

moisture content, the temperature of processing and the holding time.

Losses are minimal in cold extrusion

The HTST conditions in extrusion cooking, and the rapid cooling as the

product emerges from the die, cause relatively small losses of most

vitamins and essential amino acids.

Losses of ascorbic acid and β-carotene are up to 50% depending upon the

time that the food is held at elevated temperatures

High temperature and presence of sugar cause Maillard browning and a

reduction in protein quality.

Low temperatures and low concentration of sugars results in increase in

protein digestibility, owing to rearrangement of the protein structure.

Destruction of anti-nutritional components in soya products improves the

nutritive value of texturized vegetable proteins.