Selecting an extruder can be a confusing process for buyers. Many options are available in the marketplace when selecting extrusion systems for product. For example: is a single- or twin-screw extruder required? Should it be a "wet" or "dry" extruder? Should it have internal steam locks or a single face die plate? Should it have continuous or interrupted flights, and so on. Appropriate selection depends on several factors.

1. Digital Re-print November | December 2013
Single or twin-screw extruder:
what are the options?
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3. FEATURE
Single or twin-screw
extruder:
what are the options?
by Dr Mian N Riaz, director, Food Protein R&D Center, Nutrition and
Food Science Department, Texas A&M University, USA
S
electing an extruder can be a confusing
process for buyers. Many options are
available in the marketplace when
selecting extrusion systems for product.
For example: is a single- or twin-screw
extruder required? Should it be a "wet"
or "dry" extruder? Should it have internal
steam locks or a single face die plate?
Should it have continuous or interrupted
flights, and so on. Appropriate selection
depends on several factors:
• Physical and sensory properties of the
end product
• Formula ingredients: their physical
nature (i.e. whether the product
will utilise high levels of fresh meat),
moisture content, whether they are
constantly available or only during
certain seasons, and the potential for
occasionally using substitute ingredients.
• What kind of product do you want to
extrude? Is it food grade, or feed or
pet food? Should each piece be multicoloured or centre-filled? Is the shape
general, exotic, or detailed? What is the
target bulk density? In the case of feed,
how much fat needs to be added to the
formula? How much can be applied to
the surface?
• What is the production rate? The size
of an extruder depends on market size,
since extruders function best when
operating at full throughput per hour.
• What is the energy source? For product
heating, will steam or electricity be
more economical where the extrusion
plant is built? If it is a small operation in
a developing country, would a tractor
power take-off drive be more suitable?
• What about capital availability and the
recovery date target? Would a used
extruder fill a start-up operation’s needs
better?
16 | november - december 2013
Segmented screw/barrel singlescrew “wet” extruders
Segmented screw/barrel single-screw
extruders are the most widely applied cooking extrusion design in the food, pet food
and feed processing industries. “Wet” means
that steam and water can be injected into
the barrel during processing. Typically, the
barrels of these machines are also equipped
with heating and cooling jackets. They can
process more tonnage of extruded products
than any other extruder design, and can produce a range of products, from fully cooked,
light-density corn snacks, to dense, partially
cooked and formed pastas.
A typical single-screw extruder consists of
a live bin, feeding screw, preconditioning cylinder, extruder barrel, die and knife. The live
bin provides a buffer of raw material so the
extruder can operate without interruption.
Typically, the height of raw material in the
bin is kept within defined limits by high- and
low-mounted sensors which activate a conveyor supplying the bin. The bin is designed
to prevent the bridging of its contents, and
the blocking of the feed screw leading to
the preconditioner. The speed of the feed
screw to the conditioner or extruder must
be variable, in order to ensure a continuous
uniform supply of raw material. This is crucial
to ensure the consistent and uniform operation of the extruder.
Because single-screw extruders have a
relatively poor mixing ability, they are usually supplied with premixed material, which
often has also been preconditioned with
added steam and water. Generally, preconditioning prior to extrusion enhances extrusion processes which benefit from higher
moisture content and longer equilibration
time. Preconditioning of the raw material
typically improves the life of wear parts in
the extruder several times over. Although
the weight of ingredients in the extrusion
system is increased, preconditioners are relatively
inexpensive to build for the
volume they hold and the time they
add to the process. Perhaps most importantly, product quality can be greatly improved
by preconditioning the raw ingredients.
Application
The first major commercial application
of the single-screw extruder in the food
processing industry was the conversion of
semolina flour into pasta using solid screws.
This low-shear, low-temperature forming
process first found commercial production
in the 1920s and 1930s, and remains a
standard process even though equipment
has improved. Several recent developments
in the single-screw extruder have further
increased its efficiency and versatility. A brief
list of the products made by single-screw
extruders can be found in Table 1.
Twin-screw extruders
Recent years have seen increasing
requirements for new products with intricate
shapes and small sizes, which are beyond the
capabilities of single-screw systems. Twinscrew extruders can fill some of these needs.
The term “twin-screw” applies to extruders with two screws of equal length, both
placed inside the same barrel. Twin-screw
extruders are much more complicated than
single-screw extruders, but at the same time
provide much more flexibility and better
control. Twin-screw extruders are generally
categorised according to the direction of
screw rotation and to the degree to which
the screw intermeshes.
In the counter-rotating position the
extruder rotates in the opposite direction,
whereas in the co-rotating position the
screw rotates in the same direction. These
two categories can be further subdivided
&feed millinG technoloGy
Grain

5. Image courtesy of Wenger Manufacturing, Kansas, USA
FEATURE
according to the relative positions of the two
screws (intermeshing and non-intermeshing).
The non-intermeshing twin-screw
extruder is like two single-screw extruders
sitting side by side, with only a small portion
of the barrel in common. These types of
extruders depend on friction for extrusion,
just like single-screw extruders. In non-
intermeshed extruders, neither pumping nor
mixing is positive. Their design does not
provide a positive displacement action for
pumping the product forward.
In intermeshing twin-screw extruders, the
screws partially overlap each other in a figureeight barrel track, resulting in positive pumping, efficient mixing and self-wiping action
(although only in co-rotating machines: mixing is limited in counter-rotating machines).
This is what differentiates these types of
extruders from non-intermeshing and singlescrew machines. These extruders are like a
positive displacement pump, forcing material
in the barrel between the screws to move
toward the die by the rotation of the screw.
Co-rotating self-wiping types of extruders are most commonly used in the food
industry. When they were developed, these
extruders significantly increased the variety
of products that could be made using extrusion technology.
The twin-screw extruder consists of several sub-components very similar to singlescrew extruders (live bin, feeding screw,
preconditioning cylinder, extruder barrel,
jacketed heads and rotating screw). The
bearing assembly in twin-screw extruders
is much more complicated because more
components (such as drive and torque
dividing gears) are required. Twin-screw
extruders also have three processing zones:
feeding, kneading and a final processing zone
very similar to single-screw extruders. These
zones were described in the single-screw
extruder section above.
Applications
Twin-screw extruders became popular
in the food industry in the mid-1980s to
the mid-1990s. Originally developed for
processing plastics, food companies began
using twin-screw extruders for products
like sticky caramels and candies that could
not be made with single-screw machines.
Very soon, twin-screw extruders became
popular with food manufacturers for many
specialised food items.
However, new developments for singlescrew extruders are beginning to change the
relative advantages of the two technologies.
Variable speed drivers now give single-screw
extruders a flexibility approaching that of
twin-screw machines. Improved gravimetric
feed systems and mass flow meters allow
for the more precise measurement of recipe
components. These recent improvements,
along with systems for computer control,
have made it possible to process several
foods formerly made with twin-screw, possibly limiting the market for twin-screw
extruders.
Single-screw vs twin-screw
Single-screw extrusion has been successfully employed in food and feed production
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7. Image courtesy of Wenger Manufacturing, Kansas, USA
FEATURE
for the last 60 years. Because of consumer
demands for innovative food products in
the market, extruder manufacturers adopted and developed twin-screw extruders
around 30 years ago.
Twin-screw extruders have greater ability and flexibility for controlling both product and process parameters. They are a
flexible design, permitting easy cleaning and
rapid product changeover. Because of the
ability to better match the desired shear,
the twin-screw extruder has more control
over product variability. Screw speed can
also be used to compensate for some
variations in the properties of the starting
material. Because screw speed is such an
influential variable, the twin-screw extruder
is a better choice for plants producing a
wide variety of high-value products at low
volume.
Single-screw extruders limit formulas
to a 12-17 percent fat level. Fat content
above that level reduces friction due to
lubrication, and does not help the hardware transform mechanical energy into
heat for cooking purposes. On the other
hand, the fat level for recipes designed
for twin-screw extuders can be as high
as 18-22 percent and still maintain the
required mechanical energy. This is only
possible due to the greater number of
screw configuration options provided by
twin-screw extruders, compared to singlescrew machines.
In single-screw extruders, with the help
of steam injection a fat level as high as 17
percent can be achieved. However, the
addition of steam injection to twin-screw
extruders allows the product to be processed more consistently, which as a result
18 | november - december 2013
allows better binding of the fat and reduces
its leakage from the products during handling
and packaging.
Moisture content is very critical during
the extrusion process for starch gelatinisation and protein denaturation. The average moisture content of a typical formula
ranges from 20 to 28 percent. Moisture, in
the form of steam or water, is added to the
preconditioner and extruder barrel to help
soften raw ingredients and reduce their
abrasiveness. Twin-screw extruders have
the ability to run equally under narrow or
wide ranges of moisture.
Ultimately, processors should consider
using twin-screw extruders in situations
dealing with:
• Frequent product changeovers
• Products with a high internal fat
content (more than 17 percent)
• The addition of a high level of fresh
meat in the product (up to 35
percent)
• Uniform product size and shapes
• Ultra-small product sizes (less than 1.5
mm)
• Products made with low density
powders
• Special formulations (e.g. products
with high levels of protein and fibre,
or which require a high level of
moisture, or require the use of sticky
raw materials like soy isolates or wheat
gluten)
Table 1: Single-screw extruder products
Direct expanded corn snacks
Texturised vegetable protein
Ready-to-eat breakfast cereals
Full-fat soy
Pet foods
Floating and sinking aquatic feed
Baby foods
Rice bran stabilisation
Precooked or thermally modified starches, flours and grain
Breading
Table 2: Twin-screw extruder products
Co-extruded snacks and other food items
Precooked pasta
Noodles, spaghetti and macaroni
Third-generation snacks
Texturised vegetable protein (soy and wheat)
Semi-moist food
Pet treats
Meat analogue
Rice bran stabilisation
Multicolour food and snacks
Cereals and corn flakes
Corn chips and tortilla
Loose fill (packaging material) from starches
More
inforMtion:
www.tamu.edu
Ultra-fine aquatic feed
High fat aquatic feed (salmon)
Premium pet food (with fresh meat)
&feed millinG technoloGy
Grain

8. FEATURE
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november - december 2013 | 19
GFMT half page vertical 90 x 270 plus 3mm bleed not left.indd 1
30/11/2012 13:44:07

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November - December 2013
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Single or twinscrew extruder:
what are the options?
•
Animal feeding
in the future:
reaching genetic
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smarter nutrition?
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