1-2 Extrusion

Extrusion and other terminal
agglomeration technologies
BY GALEN J. ROKEY AND BRIAN PLATTNER
REVIEWED AND EDITED BY ADAM FAHRENHOLZ, CHARLES STARK, AND CASSANDRA JONES
preconditioning step. The initial portion of this

Pellet mills and many other feed processing
chapter will not revisit the entire subject of
technologies result in an agglomerated or pelleted preconditioning; however, it examines the
feed. Recognized advantages of pelleted feed are as importance of preconditioners for an extrusion-
follows: based pelleting system. While the text may only
• Increased bulk density; mention one of these pelleting technologies, such as
• Less bridging in bins; extrusion, the reader should understand that the
• Less dust; principles apply to all systems. Preconditioning
• Reduced ingredient segregation; with steam and water has been associated with
• Less feed waste; extrusion cooking of feed products since the
• Increased nutrient density; inception of the extrusion cooking process.
• Improved palatability; Extrusion-cooked feed products whose production
• Increased nutrient availability; and processes successfully employ preconditioning
• Decreased microbiological activity. include petfoods, swine starter diets, full-fat soy,
aquatic feeds and other specialty animal feeds.
An alternative to conventional pelleting is the
extrusion process. Extrusion may be used either to The preconditioning step initiates the heating
improve upon an existing process, or to process by the addition of steam and water into the
manufacture feeds that a conventional pelleting dry mash. Uniform and complete moisture
system cannot. As formulated feeds become more penetration of the raw ingredients significantly
sophisticated to meet the specific physiological improves the stability of the extruder and enhances
needs of the animal and the expectations of the the final product quality. In addition, plasticizing
public, extrusion-based processing technologies will the raw material particles prior to extrusion reduces
continue to be a factor in this industry. Several the wear on equipment caused by the abrasive raw
items key to extrusion such as steam material particles. There are generally two
preconditioning, the extrusion process and situations where preconditioning should be
equipment, process parameters, and final products considered when producing extrusion-cooked and
are discussed in this chapter. Novel technologies, pelleted products. First, it should be used in
such as the Universal Pellet Cooker (UPC), and the conjunction with moist extrusion where material is
Sphere-izer Agglomeration System (SAS) are also extrusion cooked at in-barrel moisture contents
discussed. greater than 18%. Second, one should consider
using preconditioning in situations where the raw
material particles are difficult to hydrate, such as
Preconditioning large particles, or non-uniform particle size
Preconditioning is an integral part of any pelleting distributions. In general, any extrusion process that
or extrusion system. As one begins to examine would benefit from higher moisture and longer
alternatives to the conventional pelleting process, retention time will be enhanced by preconditioning.
they quickly realize the added importance of the
Feed Pelleting Reference Guide Section 1: Introduction
Chapter 2: Extrusion and Other Technologies
Because preconditioning is recognized as being preconditioners being used in the extrusion cooking
important to producing premium products and industry today are double (DC) preconditioners,
operating an efficient extrusion cooking process, it differential diameter/differential speed (DDC)
is important that the basic principles of the preconditioners, and high intensity (HIP)
preconditioning process are well understood. The preconditioners (see Figure 2-2). The single-shafted
three objectives accomplished during the preconditioner, as found in most traditional
preconditioning process are: hydration of raw pelleting systems was also utilized to a large extent
material particles; heating of raw material particles; in the past in extrustion, but the double-shafted
and mixing of materials added to the preconditioner conditioners represent today’s technology.
in separate streams. This is accomplished in a
preconditioner by holding the materials in a moist,
warm environment for sufficient time and with Figure 2-2. Types of atmospheric
sufficient mixing. This process results in the raw preconditioners.
material particles being plasticized by the steam and
water in the environment. In practice, the objective
is to completely plasticize the raw material particles
in order to eliminate any dry core as illustrated in
Figure 2-1.
Figure 2-1. The objective of preconditioning is to Double Conditioner

eliminate the unplasticized core in the raw
material particles.
Differential Diameter Conditioner
Preconditioning hardware and operation

The preconditioners utilized for the extrusion
industry are almost exclusively atmospheric
preconditioners (i.e., they operate at prevailing
atmospheric pressure). Their maximum operating
temperature is the boiling point of water at High Intensity Preconditioner
When compared to the single preconditioners,
input beyond this point will only result in the loss of double preconditioners have improved mixing and
moisture as steam. Atmospheric preconditioners are have a longer average retention time of up to 1.5
relatively simple to construct and have lower minutes for a similar throughput. As with single
manufacturing and maintenance costs associated preconditioners, they have beaters that are either
with them compared to other preconditioner types. permanently fixed to the shaft or that can be
The three basic types of atmospheric changed in terms of pitch and direction of
conveying. The two shafts of a double Table 2-2. Coefficient of Variation for Moisture
preconditioner usually counter-rotate such that Content in Different Preconditioner Designs
material is continuously interchanged between the Preconditioner Design CV (%)
two intermeshing chambers. HIP 2.65
DDC 4.96
The most recent technology in the industry are the DC (Double Cylinder) 6.66
DDC and HIP preconditioners, which have the best SC (Single Cylinder) 9.36
mixing characteristics combined with the longest
average retention times. Retention times of up to 2- The more uniform moisture distribution not only
4 minutes for throughputs comparable to those used improves extrusion stability for recipes that become
in double and single preconditioners can be sticky when hydrated, but contributes to more
expected. As with a double preconditioner, the two consistent destruction of biological contaminates
shafts of a differential diameter/differential speed (salmonella) using thermal critical control points.
preconditioner usually counter-rotate such that As in traditional pelleting, preconditioners are
material is continuously interchanged between the usually installed above the extruder barrel so that
two intermeshing chambers. The HIP gives an the preconditioned material falls directly into the
added layer of control as each shaft has an inlet of the extruder as depicted in Figure 2-3. In
independent drive. This allows the direction of shaft addition, there are other important installation
rotation and speed of the shaft to be varied allowing recommendations for proper functioning of the
direct operator control of the residence time and preconditioning hardware and process.
mixing intensity (see Table 2-1).
Figure 2-3. Preconditioner installed above an
Table 2-1. Effect of Speed on Residence Time extruder.
Side A Side B
Retention Time
Speed Speed (minutes)
(rpm) (rpm)
100 500 1.00
250 125 1.47
800 50 2.40
When analyzing the various preconditioner designs,

one of the most efficient ways is to compare their
coefficient of variation. The increased mixing
intensity of twin shafted and variable speed
preconditioners compared to other designs yields Water and water-based slurry addition
improved moisture distribution in the material Perhaps the single largest difference between a
discharging the preconditioner. Table 2-2 lists the conventional pellet mill and an extrusion-based
average coefficient of variation (CV) of moisture system is the internal addition at the preconditioner
content in four different preconditioner designs. To of water and water-based slurries including colors,
determine CV after stable conditions were achieved, fresh meats and molasses. Extrusion processes
samples were collected off the preconditioner at 15 operate at much higher moisture levels, and thus are
second intervals for a 2.5 minute time period and capable of handling these additional process streams
analyzed for moisture content. with ease. Water is added to the preconditioner
from the top of the preconditioning chamber close
to the raw material inlet. For best distribution of
water throughout the raw materials, spray nozzles Figure 2-4. Introducing materials to the
are used. Other water-based additives such as preconditioner.
molasses, digests and fresh meats can be added in
conjunction with the water or at any point in the
preconditioning process. It is recommended to add
these slurries as close to the inlet of the
preconditioner as possible to allow for optimal
hydration of the dry materials and uniform
incorporation of all the added streams. Adding these
streams near the discharge of the preconditioner
often causes clumping of the material and does not
allow enough time for the complete incorporation
into the dry mash.
When adding water-based slurries, it is especially

critical that the preconditioner be a dynamic mixing
Lipid addition
device, such as a twin-shafted preconditioner. Low
agitation devices, such as preconditioners which Fats are often used to assist with process control.
utilize tempering screws will not satisfactorily They are typically added in a liquid form as a
incorporate the slurry into the dry material. The separate stream. The point of addition is critical to
exiting material will often contain clumps of wet achieve cook, while maximizing the inclusion level
product, which causes instability in the extruder’s of fat. Fat is usually added near the discharge of the
operation. Other preconditioners, such as single- preconditioner to allow optimum preconditioning.
shaft preconditioners, do not have enough retention Fat tends to coat individual feed particles, hindering
time to allow the moisture from the slurry to moisture absorption and the transfer of thermal
completely incorporate into the dry mash. The energy to accomplish gelatinization. If substantial
relatively short mixing time results in clumps of amounts of fat are to be added (15-20%) during
high-moisture material mixed with relatively dry extrusion, a portion of the total fat may be injected
mash being delivered to the extruder. in the extruder barrel. Extending retention times in
the preconditioner is a useful tool to enhance
Steam addition gelatinization in high-fat formulas.
Steam should be added to the preconditioning
Processing variables
chamber from the bottom of the chamber to ensure
contact between the steam and the raw material Preconditioning processing variables include dry
particles as shown in Figure 2-4. Steam pressures recipe flow rate; water injection flow rate; steam
should not be higher than 200 kPa (30 psig) to injection flow rate; additive(s) injection flow rate;
prevent materials from being blown out of the preconditioner configuration; preconditioner speed;
preconditioner. Good engineering practice should average retention time; and degree of mixing. This
be followed in the design and installation of the list of variables may be limited by some
steam plumbing system to ensure that only steam preconditioner installations and enhanced by others
free of condensate is introduced into the due to the particular options included. Two of the
preconditioning chamber. Adequate water most important processing parameters are average
separation methods and steam traps should be retention time and degree of mixing. These
employed to remove condensate. variables are those which really determine how
effective the preconditioning process is at meeting
the objectives of hydration, heating and mixing.
Retention time throughput for a given fill factor will cause a

Retention time is required for particles to decrease in average retention time and vice versa. In
completely hydrate and to become uniform in addition, increasing the fill factor by adjusting the
temperature and moisture. The average retention beater configuration at a given capacity will result
time is best measured by first operating the in increased average retention time. Longer
preconditioner at steady state with all mass flows retention times during conditioning can also be
being added. Second, determine the total throughput achieved by reducing agitator speed, increasing the
of the materials through the preconditioner by volume of the conditioning cylinder or decreasing
measurement or calculation. Third, stop all flows the production rate. These have their disadvantages
and the preconditioner simultaneously. Fourth, due to reduced mixing abilities and large, bulky
empty and weigh the material retained in the equipment.
preconditioner, and finally, calculate the average
retention time using the equation: Retention Time = Retention time controlled preconditioners
Mass in Conditioner/Throughput. The average
retention time is affected by the dry mash flow rate Another method for controlling the retention time is
and the actual paddle configuration. Their effect can the retention time controlled (RTC) preconditioner.
best be explained by looking at an example of a This system allows the operator to control and
preconditioner average retention time “map.” This adjust the preconditioner retention time on-line.
map shows the average retention times obtained This system gives the operator the following
from combinations of flow rate and fill factor as benefits:
shown in Figure 2-5. The fill factor is the • Continuous control of conditioning cylinder
percentage of the volume inside the preconditioning retention time.
chamber which is actually filled with product. • Simplified start-up sequence and reduced off-spec
product during start-up.
• Constant discharge rate of feed during shutdown
Figure 2-5. Example average retention time map or product changeover.
for preconditioners. • Increased retention time on current conditioning
cylinders.
• Time and temperature documentation for process
verification records.
This system requires two key components to be

added to a conventional preconditioner. First, a
metering device must be mounted at the discharge
of the conditioning cylinder. It acts as a “choke”
point enabling the conditioning cylinder to be filled
to a much higher level. This allows the operator to
make use of a greater percentage of the
conditioner’s free volume. Figure 2-6 shows an
illustration depicting the control schematic for this
system. The operator enters the retention time and
The fill factor will vary from 5% to 50% depending the desired production rate. Raw materials are
on the beater configuration, shaft speed and metered into the conditioning cylinder by a feeding
throughput. It is intuitive that configuring additional device. To maintain proper control, this feeding
beaters to convey in a reverse direction will cause device must operate in a gravimetric mode. The
additional fill in the preconditioner. Conversely, other critical component is that the preconditioner
configuring so that they convey in a forward must be mounted on load cells which measure the
direction will result in the opposite effect. From weight of feed held in the cylinder.
Figure 2-5, we can see that increasing the
Figure 2-6. RTC preconditioner. temperature and moisture content. During

shutdowns or product changeovers, the discharge
feeding device continues to deliver the conditioned
mash at the specified rate. Thus, the extruder
continues operating at its optimum capacity until
the conditioning cylinder is virtually empty. In
traditional systems, the extrusion rate slowly
decreases once the raw mash is no longer metered
into the conditioning cylinder. This feature reduces
the amount of waste material and the amount of off-
spec product produced.
Retrofitting a current conditioning cylinder with the

loss-in-weight controls can increase the retention
time, and possibly allow an increase in production
capacity. Since the discharge device acts as a
restriction and allows the cylinder to be filled to a
higher level, a current conditioning cylinder that
operates with a fill level of 40% may be able to
reach a fill level of 60% or even 70%. This would
greatly increase the amount of retention time. Also,
Based on the retention time and feed rate set points, by more fully utilizing the conditioning cylinder’s
the loss-in-weight (L-I-W) controller sets the volume, an increase in production capacity may be
discharge feeder speed to deliver the appropriate realized if there are not downstream process
rate to the extrusion system. This feature allows the restrictions. Finally, this system allows process
process retention time to be adjusted depending on documentation of the times and temperatures the
the product characteristics. Some formulations, such mash was subjected to during processing. This is
as high-fat diets, may require additional retention especially useful for those concerned with pathogen
time to allow for complete hydration of the mash. destruction and food safety. Since the retention time
The retention time is adjusted during the process is one of the user inputs for the control system, the
without the operator needing to shut down and operator can document with certainty that the mash
make any hardware adjustments to the beater or was held at a given temperature for a specified
paddle configuration. In addition, this feature period of time.
simplifies the start-up, shutdown and product
changeover sequence. It allows better utilization of Although a loss-in-weight conditioning cylinder
raw materials and reduces cross-contamination offers many benefits, it is not required for all
between recipes and products. During start-up, the situations. In situations where long production runs
raw material is metered into the conditioning on a single product occur, the additional cost for
cylinder and is mixed with steam and/or water to this system may not yield sufficient economic
begin the hydration and cooking process. The benefits to offset the additional capital costs.
discharge feeder remains off until the mash within However, for those systems in which frequent
the conditioning cylinder has been held for the product changeovers occur, or wide variation in raw
desired retention time. Then the discharge feeder materials exists, the additional capital investment
begins delivering the conditioned mash to the could quickly be recouped from the reduction in
extruder. This dramatically reduces the material product waste, increased product quality and
wasted during start-up procedures for standard increased product capacity.
conditioning cylinders by reducing the amount of
mash that must be discarded while waiting for the Mixing
conditioning cylinder to reach the desired operating Adequate mixing is essential to the preconditioning
process. This is especially true for those processes mixing efficiency of the preconditioner, as shown in
in which slurries such as fresh meat are added. If Figure 2-7. In certain instances, operators often
effective mixing is not present, individual particles reduce the preconditioner shaft speed in an effort to
may tend to agglomerate, and thus increase the increase the degree of fill, and therefore gain
effective particle size. This increases the resistance retention time for their process. However in doing
to energy and moisture transfer into the raw this, one should also be aware that decreasing the
material particles. The particles end up with a preconditioner shaft speed can significantly
wetted surface and a dry center which leading to an decrease the mixing efficiency.
inferior product and an increase in extruder wear. In
cases where slurries are added and poorly mixed in
the preconditioner, clumps of wet product will be Results of proper preconditioning
evident at the discharge and can plug the inlet of the When the three essential objectives (hydration,
extruder. heating and mixing) of preconditioning prior to
extrusion are adequately satisfied, several results
Figure 2-7. Mixing efficiency for a should be expected. First, in the area of machine
preconditioner. life, preconditioning will increase the life of wear
components in the extruder barrel by several times.
Second, in the area of extruder capacity,
preconditioning has proven to increase the
throughput of the extrusion system. Third, in the
area of product quality, preconditioning assists in
altering product textures and functionality. Finally,
adding preconditioning to the extrusion process
enhances product flavor.
Un-preconditioned raw materials are generally

crystalline or glassy, amorphous materials. These
materials are very abrasive until they are plasticized
by heat and moisture within the extruder barrel.
Preconditioning prior to extrusion will plasticize
these materials with heat and moisture by the
The mixing mechanism in the preconditioner is addition of water and steam prior to their entry into
complex and not well understood (Levine, 1995). It the extruder barrel. This reduces their abrasiveness
is also difficult to obtain a physical measurement of and results in a longer useful life for the extruder
mixing efficiency in a continuous mixing device barrel and screw components. Extruder capacity can
such as a preconditioner. However, a quantitative be limited by many things, including energy input
measure of mixing can be calculated to help capabilities, retention time and volumetric
understand the process, as well as to give a conveying capacity. While preconditioning cannot
comparison between preconditioners of differing overcome the extruder’s limitations in volumetric
design. The degree of mixing can be expressed as conveying capacity, it contributes to energy input
the number of times the beaters in the and retention time. Retention time in the extruder
preconditioner contact the material while it is in the barrel can vary from as little as 5 seconds to as
preconditioning chamber. The beater contact much as two minutes, depending on the extruder
frequency is essentially controlled by shaft speed configuration. Average retention time in the
and the number of mixing elements on those shafts. preconditioner can be as long as 5 minutes. For
This measure of beater contacts per residence time some processes, the energy added by steam in the
is affected then by both shaft speed and average preconditioner can be as much as 60% of the total
retention time. A map can be drawn that indicates energy required by the process.
the effect of degree fill and throughput have on the
The raw material particles should be thoroughly increased starch gelatinization, and that the first 120
hydrated and heated to eliminate the dry core seconds of retention time are the most important.
present in the center of raw material particles prior Increasing the amount of total steam injection also
to entering the extruder barrel. This leads to more increased starch gelatinization, but at sufficient
efficient cooking of starch and protein. This results retention time, additional steam injection above
in more complete starch gelatinization and protein 10% had little additional effect on starch
denaturation. Theoretical principles of heat and gelatinization.
mass transfer indicate that hydration usually takes
2-8 times longer than does heat penetration.
Pathogen and toxin destruction
A measure of the effectiveness of a preconditioning An additional area in which preconditioning is
process is to examine its effect on key constituents becoming important, is in producing pathogen-free
of the recipe being preconditioned. One key feed. Research has proven that proper conditioning
constituent is the level of starch gelatinization as of the feed prior to final processing can eliminate
measured by the susceptibility of the starch to pathogens such as E. coli, Salmonella and Listeria.
enzymatic conversion to glucose (Mason and If the discharge temperature of the mash exiting the
Rokey, 1992). It has been well documented that conditioning cylinder reaches 72ºC, Salmonella can
starch gelatinization requires three basic elements: be destroyed (Fung and Hoffman, 1993). This work
Elevated temperature, moisture content and time. is further supported by the data in Figure 2-9.
Because the amount of gelatinized starch has a These curves, if extrapolated, indicate that all three
proportional relationship with the amount of heat pathogens would be destroyed if a temperature of at
exposure, it can be used as an indicator of the final least 80ºC is reached.
pellet quality.
Figure 2-9. Thermal death curves for E. Coli,
Figure 2-8. Effect of steam addition and total Salmonella and Listeria.
retention time on cook.
Table 2-3 also illustrates the ability of the DDC

preconditioner to destroy the pathogenic organisms.
However, notice that the feed was not sterilized as
A well-designed and properly-operated the total plate count (TPC) was not completely
preconditioner is capable of routinely cooking from eliminated. While many processing technologies
30-40% of the starch present in a given formulation, result in an agglomerated feed, only a few have
and under certain situations the cook level can sufficient energy inputs to ensure food safety. Food
approach 70%. A study was conducted in which the safety is a major factor in choosing extrusion-based
retention time and steam addition level were varied methods over traditional pelleting methods.
and percent starch gelatinization was measured as a
response. The results of the study (Figure 2-8)
indicate that increased retention time results in
Table 2-3. Effect of preconditioning on Forming extrusion is usually a low temperature

microbial populations (Rokey, 2001). (often called cold-forming) process that increases
Microbe Raw Recipe After DDC product bulk density and cools the feed, resulting in
TPC, CFU/g 240,000 9,300 a feed bulk density that is equal to or greater than
Coliform 22,600 < 10 the bulk density of the starting raw materials.
Mold count 54,540 < 10
Clostridium 16,000 < 10 Ingredients
Listeria Positive Negative
Raw material preparation for extrusion and related
processes is very similar to that required for pellet
Extrusion is a hydrothermal process where the mill installations. Individual whole grains are pre-
critical process parameters of retention time, ground to reduce particle size and then mixed with
moisture and thermal and mechanical energy inputs the balance of the recipe. Different from many
can be varied over a wide range. Although extrusion pelleting systems, in extrusion processes the mix is
does not completely eliminate toxins and other anti- then passed through a final post-grind step to
nutritional or anti-growth factors, in many cases achieve the desired particle size and distribution.
these substances or their activity are reduced to The effect of a smaller, raw material particle size on
permit some level of incorporation into the recipe. product appearance is clearly evident in the
Research at the University of Nebraska indicates extruded samples in Figure 2-11.
that certain temperatures of extrusion are sufficient
to reduce fumonisin levels (see Figure 2-10; Katta, The correct particle size is important for many
et al., 1999). Studies have also indicated that reasons:
viruses, molds and other pathogenic organisms can • Improved product appearance;
be destroyed by the operating parameters employed • Reduced incidences of die orifices plugging;
during extrusion. However, very little published • Greater ease of cooking; and
data are available on this subject, and there is a need • Improved retention of liquid coatings due to
for carefully-designed studies to investigate the smaller cell size in the final product matrix.
effects of the extrusion process.
Figure 2-11. Photograph of extruded products.
Figure 2-10. Effect of extrusion temperature on Sample on the left was ground through a 0.8 mm
fumonisin levels. hammermill screen opening prior to extrusion. The
sample on the right was ground through a 1.2 mm
hammermill screen opening prior to extrusion.
Extrusion
The extrusion process can generally be divided into
two basic categories: Cooking extrusion and A guideline to follow in grinding recipes prior to
forming extrusion. Both processes affect the feed as extrusion is to select a hammermill screen with
the name indicates. Cooking extrusion elevates feed holes being one-third the size of the extruder’s final
temperature to a level that often results in an die orifice. Adhering to this guideline will ensure
expanded product (final feed bulk density less than that all recipe particles will easily pass through the
the bulk density of the starting raw materials). extruder die orifice without danger of plugging or
partially plugging the orifice. A sifting device is pellet mills is usually drastically reduced if the fat
often inserted into the process flow between the levels in the recipe exceed 5%, while extruded
grinder and the extruder to remove all foreign products have been processed with internal fat
material and particles that are larger than the die levels as high as 25%.
orifice. It is critical in the extrusion process to avoid
plugging of die orifices, as product is actively The pelleting process depends largely on starch and
flowing through all orifices simultaneously. In a other binding agents to give durability to the final
pellet mill, active product flow occurs only where product. Mild operating parameters in the pelleting
the rolls are forcing or “pressing” the recipe through process yield low levels of starch gelatinization.
the die ring. Several openings can plug in a pellet Gelatinization increases the binding properties of
mill die and little capacity is lost. The total die open the recipe starch. The extrusion process gelatinizes
area in an extrusion application is typically much more of the starch present and thus binding is
less than the pellet mill process and any reduction in increased. This often reduces the level of starch
die open area directly impacts throughput and required in an extruded feed compared to the levels
product quality. required in pelleted feeds for product binding and
structural strength. Extrusion provides flexibility in
The grinding step for an extruder follows the formulating for product characteristics such as
guidelines discussed above, and usually precedes a pellet quality. An increasing number of requests
sifting operation to remove foreign material and come from various industries to process material
large particles. A magnet is usually installed prior to currently classified as waste streams. The intent in
the grinding step in all feed mill process flows to many scenarios is to utilize these materials as a feed
remove tramp metal. It is recommended to also or feed ingredient.
include a magnet just prior to the extrusion or
pelleting process to prevent accidental metal from
the grinding operation from damaging the Hardware components
equipment components. An extrusion system includes a live bin/feeder,
preconditioner, extrusion cooker and die/knife
A major difference in process flows occurs after the assembly as shown in Figure 2-12.
extrusion or pelleting steps. Extruded products
usually contain more moisture than pelleted Figure 2-12. Extrusion system.
products. This moisture must be removed in a
drying step if moisture is greater than 12-15% by
weight of the extruded product. The higher moisture
levels required for most extrusion processes can
lend versatility to the process and expand the feed
manufacturers’ product possibilities. Ingredient
flexibility is an important tool for feed millers in
that it allows the opportunity to take advantage of a
wide variety of ingredient sources. The more
positive conveyance features of an extrusion system
permit the use of wet, sticky ingredients. The high-
temperature/short-time extrusion cooking process is
able to accommodate a wide range of raw materials Each component is designed to accomplish a
that might otherwise be discarded as unqualified specific function in the process of cooking and
material. Pellet mills are limited to 15-18% process forming feed products. The operating conditions
moisture to avoid plugging of the roll and die can be adjusted to vary the characteristics of the
components. Wet byproducts and other high- finished product.
moisture ingredients can be utilized in the extrusion
process at levels up to 60%. The functionality of
The live bin/feeder provides a means of uniformly cooking extruded feeds. This additional energy
metering the raw materials into the preconditioner input results in capacity increases, more tolerance
and subsequently into the extruder. This flow of raw for high-fat levels in the formulations and reduced
material must be uninterrupted and the rate requirements for large drive motors. Moisture
controlled. The live bin/feeder controls the product addition in the form of water or steam and a
rate or throughput of the entire system. Variable- properly-configured extruder barrel could result in a
speed augers or screw conveyors can be used to final pressure of the extrudate prior to the extruder
volumetrically meter ingredients into the system. die of 34-37 atmospheres, a temperature of 125-
These same devices can be designed and 150°C and a moisture content of 23-28%.The three
manufactured to act as loss-in-weight (gravimetric) types of extruders most common in the feed
feed systems by mounting the bin/feeder assembly industry are the single-screw, co-rotating twin-
on load cells and continuously monitoring its screw and conical co-rotating twin-screw extruders.
weight. Preconditioning hardware, an important and
necessary step in extrusion of feeds, is discussed in Figure 2-13. Effect of screw speed on specific
depth earlier in this chapter. mechanical energy.
As the material leaves the preconditioner, it enters

the extruder barrel. Here the major transformation
of the preconditioned material occurs which
ultimately determines the final product
characteristics. The initial section of the extruder
barrel is designed to act as a feeding or metering
zone to convey the preconditioned material away
from the inlet zone of the barrel and into the
extruder. The material then enters a processing zone
where the amorphous, free-flowing material is
worked into dough. The compression ratio of the
screw profile is increased in this stage to assist in
blending water or steam with the raw material. The
temperature of the moist dough is rapidly elevated
in the final few seconds of dwell time within the Single screw extruder
extruder barrel. Most of the temperature rise in the
The single-screw cooking extruder (see Figure 2-
extruder barrel is from mechanical energy
14) has been the “heartbeat” of dry-expanded
dissipated through the rotating screw. It may be
petfood and other feed industries for over 40 years.
assisted by the direct injection of steam or from
The screw and barrel configurations represent many
external thermal energy sources. The screw profile
years of analytical design, research and
may be altered by choosing screw elements of
comprehensive testing.
different pitch or with interrupted flights, or by
adding mixing lobes configured to convey either in
Figure 2-14. Single-screw extruder.
a reverse or forward direction. All of these factors
affect the conveying of plasticized material down
the screw channel and therefore the amount of
mechanical energy added via the screw.
As shown in Figure 2-13, the extruder screw speed
is also an influential variable for controlling
mechanical energy input. This influence of extruder
speed indicates the advantage of installing a
variable-speed drive on an extruder. Steam injection
into the extruder is also a contributing factor to
A better understanding of the interaction between products).

the machine and materials has led to the • Co-extruded products (complex petfood treats).
development of screw and barrel geometries for
single-screw extruders that are more efficient in As fat levels in a formulation are increased above
converting mechanical energy to heat through 15%, it becomes increasingly difficult in a single
friction. barrel extruder to transmit mechanical energy from
the screws into the product. Fat actually provides
These screws have increased volumetric capacity, lubricity and reduces friction within the extruder
permitting higher levels of steam injection into the barrel. However, through more positive transport
heads. For both single-screw and twin-screw provided by the two intermeshing screws, the co-
extruders, screw elements of single or multiple- rotating twin-screw permits internal fat levels
flight geometries may be used. Single-flight approaching 25% while maintaining a cooked
elements generally yield products of higher bulk product. While it is true that single-screw extruders
densities compared to double-flight screws when process formulations containing up to 20% fat,
operating with the same extrusion parameters. The product consistency is more easily maintained in the
barrel segments may also be ribbed to alter the twin-screw system. The positive conveyance factor
function of each specific extruder segment. maintains die pressure, product expansion and
textural development (Rokey, 2004).
Twin-screw extruder
Twin-screw cooking extruders (see Figure 2-15) C2TX extruder
have typically found limited utility in the
production of feeds. The major drawback of these The C2TX (conical co-rotating twin-screw extruder)
extruders is their high capital investment and their is the most recent extrusion system introduced to
higher relative costs of maintenance and operation. the feed industry (see Figure 2-16).
The capital equipment cost of a co-rotating twin-
screw extruder is 1.5 to 2 times the cost of a state- Figure 2-16. Conical co-rotating twin-screw
of-the-art single-screw extruder with comparable extruder.
hourly production capacity. Because of the
increased costs, only those feed products with
strong value-added potential are processed via the
twin-screw extruder.
Figure 2-15. Twin-screw extruder.
Specific product characteristics or processing The C2TX’s conical design allows for positive
requirements where twin-screw extrusion systems compression in the barrel and reduces possibility of
have found applications are as follows: back feeding. Positive compression yields an
• Ultra-high fat feeds (above 17% internal fat). efficient manner of imparting mechanical energy
• Products which have high levels of fresh meat or into the extrudate. The conical design of the C2TX
other high moisture slurries (above 35%). causes the material to be kneaded and sheared along
• Uniform shape/size product (portioned foods). the screw profile. In traditional twin-screw
• Ultra-small products (0.6 to 2.0 mm diameter extruders, the melt is kneaded and sheared by shear
locks, mixing lobes or cut-flight screw elements. Other design advancements in die configurations
The “profile kneading” present in the C2TX design have resulted in “rapid change multiple dies,” where
eliminates the need for such special screws and dies can be changed without stopping the extrusion
locks to provide the appropriate cooking. Therefore, system. This design reduces set-up time by up to
the extruder shafts and screws can be machined 50%, resulting in smaller lot sizes, easier
from a single piece of steel. The result is a lower scheduling, reduced inventory, increased plant
manufacturing cost of the screws and reduced efficiency and increased profitability (Rokey and
maintenance and downtime, since a screw profile Aberle, 2001). A face cutter is used in conjunction
change is not needed for each different product. The with the die, which involves cutting knives
C2TX design provides the possibility for a feed revolving in a plane parallel to the face of the die.
manufacturer to more economically process those The relative speed of the knives and the linear speed
feeds requiring twin-screw extrusion attributes. of the extrudate result in the desired product length.
The blades of the knife run in very close proximity
to the die face, and in the case of spring-loaded
Die/knife design blades, may actually ride on the surface of the die.
The extrusion chamber is capped with a final die Knife blade metallurgy, design, positioning relative
which serves two major functions. The die provides to die face, speed and extrudate abrasiveness
restriction to product flow, causing the extruder to determine their life.
develop the required pressure and shear. The final
die also shapes the extrudate as the product exits the Many feed extrusion applications require changing
extruder. Die design and its effect on expansion, or re-sharpening blades every six to eight hours.
uniformity and appearance of the final product are This is especially critical with intricate shapes. Dull
often overlooked. The amount of expansion desired blades distort the product shape and increase the
in the final product can be controlled by formula number of “tails” or appendages on the product
manipulation and open area in the die. Unexpanded, which later are broken off in drying and handling,
but fully-cooked feeds generally require 550 to 600 resulting in fines. Final product characteristics can
square millimeters of open area per metric tonne of be controlled by the extruder or die configuration
throughput. Highly-expanded feeds require 200 to selected for processing feeds. However, feed millers
250 square millimeters of open area per metric prefer not to lose production time by having to
tonne throughput. change the extruder configuration to modify
specific product characteristics such as final product
Final dies may be as simple as single plates with a bulk density. There are other hardware tools that
pre-determined number of sized round openings, or can be used to control product bulk density. Four
they may consist of two or more plate elements. The tools that are available to the industry include the
first plate element of a two-piece die serves to following:
increase the resistance to flow and to aid in • Vented extruder barrel with or without vacuum
imparting shear to the extrudate. The second die assist;
plate in a two-piece die is used to size and shape the • Separate cooking and forming extruders where the
extrudate by forcing it to flow through a number of product is vented between the two units;
orifices. Very high shear rates are experienced by • Restriction device at the discharge end of the
the extrudate as it flows in a radial direction extruder; and
between the two die plates. Typical products made • Pressure chamber at the extruder die.
on two-piece dies are light-density snacks or treats
for pets and are not applicable to most feed
products. Spacers may be added between the Vented extruder barrel
extruder barrel and the final die plate to even out the
flow from the extruder screw to the final die plate The extruder barrel is normally closed to the
and give additional retention time for cooking. atmosphere and the extrudate is subjected to an
environment of increasing pressures until it exits the
die orifice. The high process pressures (0 to 40 bar) Figure 2-18. Vented extruder barrel with
result in significant expansion ratios and product vacuum assist.
densities low enough to produce feeds such as
floating aquafeeds. Expansion can be further
enhanced by injection of steam into the extruder
barrel, which increases thermal energy inputs. Feeds
with high bulk densities are preferred for several
reasons such as:
• Reduced transportation costs;
• Aquatic feeds that are sinking in fresh and sea
water; and
• Increased product bin capacity within a feed mill.
Where higher product densities are required for
certain feeds, the extruder barrel can be configured
to include a vent which releases process pressure Separate cooking and forming extruders
and reduces product temperature through
evaporative cooling (see Figure 2-17). Another hardware tool utilized by the feed
manufacturers to control product bulk density is a
dual extrusion process (see Figure 2-19). In this
Figure 2-17. Extruder with vented barrel.
process, the first extruder is used in solo for the
production of expanded feeds, or it can be used as a
cooking extruder for the two-stage cooking/forming
process. The second, forming extruder (product
densification unit, or PDU) is used only when
processing very dense feeds, such as fast-sinking
aquafeeds.
Figure 2-19. Dual extrusion process for cooking

and densification.
A vacuum assist can be added to the vented barrel
(see Figure 2-18) to increase the product density
even further by more evaporative cooling and de-
aeration of the extrudate. Vacuum assist (up to 0.7
bar) will improve pellet durability, increase piece
density and reduce extrudate moisture.
Disadvantages of a vacuum-assisted, vented
extruder barrel include the following:
• Increased investment for hardware;
• Potential capacity of extruder reduced 25-50%;
• Disposal of product fines from vent and water
from vacuum pump (waste streams can recycle
back into the system as shown in Figure 2-18);
This processing system has the advantage of being
• Control of SME (specific mechanical energy)
able to operate both extruders at their maximum rate
inputs are reduced.
potential. When only one extruder is used to
produce a very dense pellet, the extruder may have
to be operated at lower throughputs to prevent
expansion. Adding the second forming extruder
(PDU) can allow a feed manufacturer to produce a
wide range of feed densities from highly-expanded Table 2-4. Controlling feed density with the
feeds with one extruder, or very dense feeds with BPV.
the cooking extruder and PDU. Final Product
Oil
Back pressure valve Back Uncoated After
Pressure Extruder Product Vacuum
Final product characteristics such as density can be
Valve, Speed Density, Infusion,
controlled by extruder die restriction. One device
% Closed Index g/L %
commonly used by feed manufacturers is termed a
45 1.0 654 16.2
“back pressure valve” (BPV) which is used to adjust
55 1.0 628 19.5
die restriction while the extrusion system is in
65 1.0 530 23.8
operation. By changing the restriction at the
65 1.3 504 28.4
discharge of the extruder during operation, the
70 1.2 420 37.8
product density can be varied by up to 25% without
70 1.3 392 40.5
changing the screw configuration or the final die.
The BPV mounts on the end of the extruder prior to
The extrusion process for feeds is reported to be
the final die assembly (see Figure 2-20).
more stable with a BPV, and
preconditioning/extrusion process temperature
Figure 2-20. Back Pressure Valve requirements are lower, resulting in improved
nutrient retention. The BPV eliminates the need for
altering extruder configurations between different
product families. An integral part of the BPV is a
bypass feature to divert product from the die/knife
assembly and product conveyor for service and
start-up/shutdown procedures, which improves
sanitation in this area.
Post-extrusion pressure chamber

Another device available in the industry is an
enclosed chamber which surrounds the die/knife
assembly and permits control of pressure external to
the extruder and die (often referred to as an external
density management system, or EDMS). Desired
Specific mechanical energy (SME) and extrusion pressures are maintained in the die and knife
pressure are process parameters controlled by the enclosure by a special airlock through which the
valve positioning. The BPV provides internal product discharges. Compressed air or steam can be
control of shear stress and SME for regulation of used to generate the required pressure in the
important product properties: chamber. As pressure increases, the water vapor
• Bulk density (see Table 2-4); point increases and reduces product “flash-off
• Size and uniformity of cell structure; expansion,” and thus increases density (see Table
• Starch gelatinization; 2-5).
• Shape definition; and
• Water and fat absorption (see Table 2-4). Expanded or partially-expanded products which
normally exit the extruder die at a bulk density that
is lower than desired, can be “densified” with this
post-extrusion pressure chamber (EDMS) around
the die/knife assembly. One particular challenge in
the aquatic feed industry is to produce a fully-
cooked feed of sufficient bulk density to sink The combined impact of a pressure chamber and a
rapidly and still absorb the required oil during the BPV is illustrated in Figure 2-22. The BPV can be
coating step. used as an independent tool to alter product density
and other critical properties, or can be used in
Table 2-5. Effect of increasing pressure in conjunction with a pressure chamber to further alter
die/knife chamber. product density over a wide range. The hardware
Over- Expected tool of choice to manage product bulk density
Pressure in Increase depends on the process application. Each tool has
Chamber, Boiling Point in Product advantages and disadvantages, and these must be
Bar of Water, °C Density, % evaluated in light of the process requirements. For
0 100 0 example, for very small diameter pellets (<3mm)
0.5 112 10.0 that contain high levels of starch, such as a high-
1.0 121 18.3 carbohydrate shrimp feed, the processing system of
1.5 128 25.0 choice may be the combination of a cooking
2.0 134 28.3 extruder followed by a forming extruder (PDU).
The level of product density increase expected from Figure 2-22. Effect of chamber pressure and
over-pressure in the EDMS depends on several BPV closure on bulk density of 8 mm feed
factors. For example, as the feed pellet size pellets.
(diameter and mass) decreases, a given pressure in
the chamber results in a lower density increase, as
illustrated in Figure 2-21.
Figure 2-21. Density increase for various pellet

sizes at constant chamber pressure.
Because the pellet diameter is small the EDMS

system may increase the density only 8-10%, and
because the starch level is high the vented head
approach and EDMS may have more difficulties
with the sticky nature of the recipe. Another
The pressure chamber can be coupled with a BPV advantage of the PDU system in this scenario is the
to provide additional process control such as the higher capacity potential. Disadvantages of the
ability to adjust SME on-line for control of critical PDU system are the total, initial capital costs for
product properties, divert off-spec product during this system, which are partially off-set by the
start-up from the pressure chamber, accurately greater capacity potential. A further disadvantage is
control of product density external to the extruder the reduced stability of the pellet in water, which
and die and no extruder configuration changes may or may not be a critical requirement for other
required to make expanded or dense feeds, or applications.
increase extruder capacity over vented
configurations by 25-50%.
Process parameters 24% soybean meal and 16% wheat was extruded at
Extrusion and similar agglomeration techniques six internal fat levels. The internal fat level was
have been utilized to process various feedstuffs for adjusted by continuously injecting fish oil into the
many years. Extrusion cooking is universally preconditioning phase of a single-screw extrusion
recognized as a high-temperature/short-time system at 0%, 0.9%, 1.8%, 3.6%, 7.2% and 14.4%.
process. The higher temperatures employed during As the added internal fat level during extrusion
the extrusion process present an interesting increased, the bulk density of the final product
challenge in the assessment of nutrient retention. increased significantly (see Figure 2-23).
During extrusion, the recipe and its constituents are
subjected to a succession of almost instantaneous As internal fat levels increase, durability decreases.
treatments or unit operations. There is a remarkable decrease in durability when
the total fat level of the extrudate exceeds 12%. Fat
These variables include moisture and temperature added in the extruder has a lubricating effect and
profiles, extruder configuration, extruder speed and reduces mechanical heat dissipation and starch
preconditioning of the material prior to extrusion. gelatinization. Fat also weakens the product matrix
The critical process parameters could be and thus reduces the pellet strength. However,
summarized into four areas—specific mechanical extrusion processes have been used to produce
energy, specific thermal energy, retention time and feeds of up to 22% internal fat, while pelleting
product moisture. processes are limited to 4-5% fat. Energy
management is essential in controlling bulk density
The following process parameters are utilized to of feeds. As energy inputs increase during
control product characteristics such as bulk density: extrusion, the bulk density decreases. Figure 2-24
• Internal and/or external fat levels; indicates the correlation between specific
• Specific mechanical and thermal energy inputs; mechanical energy inputs and the final bulk density
and of the extruded product.
• Extrusion moisture.
Figure 2-24. Effect of specific mechanical energy
Figure 2-23. Effect of internal fat on product on extrudate bulk density.
density.
Extrusion moisture is also an important process

variable for controlling final product density. Low
Nutritional requirements dictate the levels of total
extrusion moistures yield products with high
fat required in most feeds. The total fat levels can
densities. As extrusion moisture increases, the
vary from 3% to greater than 40% in some aquatic
product density trends lower. The higher moisture
feed diets. As the fat level increases, there is an
levels facilitate starch gelatinization, resulting in
expected increase in the bulk density of the feed. In
product expansion. As the extrusion moisture
one study conducted at the Wenger Technical
continues to increase past a critical level, the
Center, an aquatic diet containing 60% fishmeal,
product will begin to increase in density. Ultra-high Large electrical motors are used to drive expanders,
moisture levels decrease the viscosity of the and up to12 kWh per metric tonne of product is
material in the extruder barrel and make it more required for the expander process alone. Reported
difficult to expand the product. The improvements to pellet quality by coupling an
moisture/density curve (see Figure 2-25) is specific expander to a pellet mill have been inconsistent.
for each product. This may be due to recipe characteristics such as
high internal fat levels, but much of this is due to
Figure 2-25. Effect of extrusion moisture on the low moisture levels employed during
product bulk density. processing. Extruders and expanders have general
similarities in design and function, but they are not
the same. Even within the extruder family, there are
many not-so-subtle differences that have a major
impact on the characteristics of the end product.
Extruders can be broadly classified as dry or moist
and as single- or twin-screw.
Dry extrusion usually implies process moistures of

18% or less, while moist extrusion generally
processes recipes at levels above this moisture level.
Dry extrusion does not employ preconditioners, and
is therefore limited in its ability to process a wide
range of raw materials. The similarities between dry
The adjustment of process parameters, as described extruders and expanders are very striking, and close
previously, can be used to control bulk density, but examination of the principles involved and effects
may unfavorably impact other process parameters on final products reveal only subtle differences.
such as system capacity. However, various Expanders are usually quoted at much higher
hardware tools described earlier are available to capacities than dry extruders. This is made possible
process feeds to the desired bulk density while by imparting less energy into the product per unit of
allowing optimum process parameters, such as throughput. This fact is reflected in the typical cook
extrusion moisture, to be employed. A summary of values of cereal grains processed through each
operating parameters of various pelleting, extruded system. Extrusion imparts more thorough
and agglomerating processes is necessary to processing of feeds compared to pellet mills or a
evaluate and compare the processes. The addition of pellet mill/expanders combination. The major
expanders to pellet mills was intended to further difference is due to the use of more steam and
improve pellet quality. Expanders represent higher levels of moisture (moist extrusion) in the
additional energy inputs to pelleting by increasing extrusion process. Moist heat is generally regarded
product temperature. Three to five percent steam is as more effective in gelatinization of starch,
usually injected into an expander, in addition to the denaturation of protein and pasteurization of
mechanical energy generated by the main drive products. Differentiation between dry and moist
motor, to achieve 90 to 130ºC product temperature extrusion is summarized in Table 2-6 (Hancock,
inside the expander. Product discharging the 1992).
expander is usually 70 to 80ºC and contains 16-18%
moisture (Heindreich and Eberhard, 1994). Retention time in the extruder barrel can be as low
as 12 seconds, and it is this principle of high-
To heat product sufficiently to gelatinize the starch temperature/short-time processing that has made
and to reach pasteurization temperatures requires a extrusion an effective processor of individual
given amount of energy, depending on the ingredients and complete diets in feed
efficiency of the processor and the ability of the manufacturing.
process to transfer energy inputs to the product.
Table 2-6. Variations in complexity and conveying of products from the extruder to the
capacity of extruder types. dryer inlet reduces product moisture content 1-2%.
Extruder Type Output Versatility Pneumatic systems help separate sticky products
Single-screw that tend to clump with belt conveyors and improve
Low Low sanitation around the extruder die.
(dry extrusion)
Single-screw The two types of dryers used for most feed products
High Moderate are conveyor and vertical style dryers (Plattner,
(moist extrusion)
Twin-screw 2001).
High High
(moist extrusion)
Final product applications
Post-extrusion processing Many of the advantages claimed for agglomerated
or pelleted feeds are really due to the form in which
For most “dry” feeds, the final moisture content
the feed is presented to the animal, and the fact that
needs to be less than 12% to prevent mold and
the feed has been subjected to a heat treatment. The
bacterial growth. Final products with moistures
relatively dry treatment employed during pelleting
above 12% are sometimes referred to as semi-moist
followed by a pressing step yields a final pellet
products. This group of products may have moisture
matrix that may deteriorate during transportation
levels greater than 30% and represent a category of
and handling. The process flexibility and the
products that cannot be processed on pellet mills.
processors’ philosophy toward total quality
When considering a soft-moist product, one needs
management are the greatest factors in pellet
to determine the water activity of the product.
quality.
Water activity is the critical factor in determining
the lower limit of available water for microbial
Low-moisture or dry extrusion has been utilized in
growth. In general, if the water activity of a product
the feed industry for many years. Although
is less than 0.65, no microbial growth can occur.
applications have usually been limited to extrusion
of dietary ingredients such as full-fat soy, extrusions
of complete diets without a pellet mill have been
Drying and cooling employed. Extrusion has been used to process the
The primary purpose of drying is to reduce the level following feeds:
of moisture in an extrusion cooked product. Many • Full-fat soybeans and other high-oil ingredients;
extruded products exit the extruder die at moisture • Piglet feed and calf starters;
levels above 18%, which necessitates product • Hygienic feeds for poultry;
drying for shelf stability. In some cases, the drying • Protein bypass feeds for ruminants;
process can involve additional heat treatment of the • Aquatic feeds;
product. One example of this is the drying at • Petfoods; and
elevated temperatures to impart a “baked” or • Feeds containing high levels of wet byproducts.
“toasted” flavor and appearance to the product. As
mentioned earlier, many feeds are best processed at
Full-fat soy
extrusion moistures between 23-28%. Some of the
moisture is lost due to flash evaporation as the Full-fat soybeans are thermally processed to destroy
superheated product exits the die and expands. anti-nutritional factors and to increase oil
Further moisture will be lost through evaporative availability, while preserving the nutritional quality
cooling, as the product cools during conveying or of the protein. The major anti-nutritional factor of
when a cooling step is employed. Pellet coolers will concern in raw soybeans is a trypsin inhibitor.
generally result only in a reduction in moisture Trypsin inhibitor is a protease that is harmful to
levels of about 3% and further reductions in most animals and humans, and nutritionists have
moisture levels require a drying step. Pneumatic documented this effect conclusively. This protease
enzyme can be inactivated by heat treatment. A
reduction of at least 85% of the trypsin inhibitor been the subject of many studies. However, reports
units is considered necessary by feed technologists indicate moist extrusion of high-fat dairy feeds
to avoid nutritional problems (Schumacher). Pellet increased palatability and milk production by over
mills are unable to process full-fat soya due to the 2.0 kg per day when compared to the same diets
high levels of fat (18-20%) indigenous to soybeans. being pelleted (Castaldo, 1995). Extruded feed
Expanders are capable of destroying 70% of the processing costs were higher, but still netted a 2:1
trypsin inhibitor by processing at 120ºC. In return on the added feed cost. Several patents exist
comparison, moist extrusion destroys up to 95% of for extrusion processing of feedstuffs to increase the
the trypsin inhibitor through heat treatment. Higher protected or bypass protein contents. Soybean meal
moisture during heat processing results in more is extruded in combination with rapeseed meal or
efficient destruction of the trypsin inhibitor and cottonseed meal under specific process parameters
urease activity (McNaugton and Reece, 1980). to yield a protein supplement. The moist heat
Additionally, full-fat soybeans can be moist treatment denatures protein, which escapes large-
extruded to destroy over 95% of the trypsin scale degradation in the rumen and thus serves as a
inhibitor without damaging lysine (Mustakas, et al., protein source for digestion in the abomasum
1964). Evidence indicates that “dry” heat (ruminant stomach).
processing is not as successful as extrusion in
enhancing subsequent nutritional value of raw full- Complete diets
fat soybeans. It may be concluded that any
processing treatment involving moisture tends to Complete diets for livestock, such as piglet feeds,
have a beneficial effect. Thus steam (moist) calf starters and grower and finishing diets for
extrusion tends to result in higher subsequent poultry and swine have been successfully extruded.
nutritive value than dry extrusion and dry roasting Moist, extruded swine finishing diets were
(Wiseman, 1990). compared to mash diets in a Texas A&M University
study. The studies indicated a 13% improvement in
feed efficiency with the moist extruded diet
Pasteurization and Salmonella control (Herbster, 1991). The extruded diets did not
The feed industry is acutely aware of the increase the incidence of ulcers or hyperkeratotic
possibilities of food-borne illnesses related to activity in slaughtered animals. Moist extrusion and
microbial contamination—which can occur at any similar agglomeration technologies have been used
point along the food chain. This is an especially to produce aquatic feeds for many years. Feeds for
important consideration for animal foods/feeds shrimp and other aquatic species are among the
entering the home, such as petfoods. As early as most expensive feeds on the market today. These
1965, widespread testing indicated that moist diets usually contain high-quality ingredients that
extrusion was much more effective than pelleting in are highly digestible and of a high nutrient density.
Salmonella control (see Table 2-7). Moist extrusion using single- or twin-screw designs
are the most common method of processing aquatic
Table 2-7. Effect of extrusion and pelleting on feeds. Feeds are processed to various bulk densities
Salmonella destruction. depending on the species being cultured:
Extruded Pelleted • Floating (carp, tilapia, catfish);
Number of Samples 775 35 • Slow-sinking (trout, salmon yellowtail); and
Process Temperature, °C 95-120 60-85 • Sinking (shrimp, river crab, cod).
Process Moisture, % 25-35 11-19
Salmonella positive, % 0 60 Extrusion permits sinking and floating diets via
density control that is not possible with pellet mills.
Factors that affect product density include the
Ruminant feeds following:
Processing the concentrate portion of beef and dairy • Starch and soluble protein contents of the recipe;
rations through expanders and extruders has not • Thermal and mechanical energy inputs during
preconditioning and extrusion; and through a 1.2 mm or smaller hammermill screen

• Extrusion moisture and retention time. and fed into the extruder for final transformation
into a cooked chunk of whatever size or shape that
Aquafeeds are extruded in a wide range of pellet may be desired. Typical extrusion processes will
diameters ranging from 0.5 to 60 mm. Single-screw incorporate some means of conditioning the
extruders can produce pellets as small as 1.2 mm, material with both steam and water as it passes
while twin-screw designs can extrude pellets as through the extrusion system. In addition, the
small as 0.7 mm. Extrusion is now preferred over material will become gelatinized by means of
pelleting as the processing method of choice for friction, shear, temperature and pressure within the
aquatic feeds because extruded products retain their extruder barrel chamber.
shape longer in water, exhibit less leaching of Upon exiting the die orifices located on the
nutrients in water and have fewer fines resulting discharge end of the extruder barrel, the now visco-
from transportation and handling. There is a strong amorphic mass will expand upon being subjected to
connection between feed management and the atmospheric pressure, will be shaped and sized by
environment with intensively-raised species. Poor the orifice of each die opening and will be cut to a
quality feeds that are not stable in water can have desired length by means of a rotating external knife-
detrimental effects on water quality and this often cutter device. The injected steam and water
results in poor performance, disease and high (moisture) that have been added to the product must
mortality rates. be removed. Typical extrusion moistures of dry-
expanded petfood products will range from 22-28%,
The greatest majority of petfoods are processed via and that moisture level must be reduced to final
extrusion cooking. Petfood categories include dry- moisture of 8-10% prior to packaging or storage.
expanded, semi-moist, soft-expanded and pet treats. That process is usually accomplished by means of
Petfoods are extruded to: some type of continuous dryer with a separate
• Render the starch components digestible by cooler or a dryer/cooler combination. It should also
cooking; be noted that typical dry-expanded products will
• Satisfy the physical requirements of density, size possess a wet bulk density of 352-400 grams per
and shape; liter prior to drying and 320-352 grams per liter
• Pasteurize the recipe components; and after drying.
• Impart desirable textures, flavors and colors.
Semi Moist Products
Dry Expanded Products The second petfood category is semi-moist
The dominant position of dry-expanded petfood in products. Semi-moist products are typically
the market is evident in the fact that it comprises the extrusion cooked through an extrusion process
largest share (over 60%) of sales volume. These similar to those utilized with dry-expanded
petfoods usually contain 8-10% moisture and are products. There are distinct processing differences
processed from cereal grains, cereal byproducts or and variations in formulation that differentiate semi-
their derivatives, soybean products, animal moist from dry-expanded products. Semi-moist
products, milk products, fats and oils, minerals and products involve many of the same basic
vitamin supplements. Dry dog and cat foods usually ingredients as dry-expanded products. In addition to
contain 5-13% and 8-12% crude fat on a dry basis, the dry grain mixtures, some sort of meat or meat
respectively. Palatability is improved by the higher byproduct liquid slurry is often blended with the dry
fat levels and is usually achieved by spraying ingredients prior to extrusion. Ratios of dry-to-wet
liquefied fat and/or flavor enhancers on the surface ingredients will vary from one manufacturer to
of the final products. Crude protein contents (dry another, and exact proportions are generally
basis) of dry-expanded dog foods are usually 18- considered to be proprietary information.
30% and, for dry-expanded cat foods, 30-36%.
Unlike extrusion-cooked dry-expanded products, it
The ingredients are blended and ground to pass is not the intention to “expand” semi-moist products
at the extruder die but, rather, to “form” a strand or must take place prior to extrusion, and the end
a shape that is similar in size and shape to the die product is expanded at the die. However, the
orifice. The intention is to gain as much cooking as ingredient characteristics are similar to those of
possible during extrusion. Generally, it is not semi-moist products and the final product, although
possible to expand the product a great deal due to expanded, is soft and pliable, much like real meat.
the higher levels of fats and oils associated with the
meat portion of the mix, but when the extruder Semi-moist and soft, expanded dog and cat foods
barrel is properly configured, it is possible to fully contain moderate levels of moisture (25-32% on a
cook the mass within the extrusion chamber. wet basis). Due to the elevated moisture contents,
semi-moist and soft, expanded petfoods are
Another major difference between semi-moist and stabilized and protected from spoilage without
dry-expanded petfood products involves extrusion refrigeration. Preservation systems are built into the
moistures and final processing to handle those formulation to adjust the final product water activity
moisture levels. Typical semi-moist products are (Aw) to a level (0.60 to 0.8) where the growth of
extruded in the range of 20-30% moisture. microorganisms is prevented or greatly reduced.
Preservatives are included in the ingredients to The Aw is lowered by humectants (sugars, syrups,
provide shelf stability, and since it is most desirable salts and polyhydric alcohols such as propylene
for the final product to be soft (similar to meat), the glycol). These petfoods are further stabilized by
moisture in the product is not removed following adjusting the pH to levels (4.0 to 5.5) that are too
extrusion or prior to storage. Bulk densities of both low to support many microorganisms. The recipes
the wet stages of semi-moist products as well as the also prevent mold growth by the inclusion of an
packaged final stages differ greatly from typical antimycotic agent such as potassium sorbate.
dry-expanded products. Wet densities of semi-moist
products will range from 480-560 grams per liter, Common ingredients in this category of petfoods
with final densities very much in the same range, include animal products, milk products, fats and
since moisture removal is not required. oils, soybean products, cereal grains and their
byproducts, marine products, minerals and vitamin
Soft expanded products supplements. Semi-moist petfoods are heavier in
bulk density and usually contain fresh animal
The third petfood product category is soft-expanded products while soft, dry products usually contain
products. This category represents an innovative dehydrated animal products and possess bulk
product type that is similar to semi-moist products. densities similar to dry-expanded petfoods.
Both products often contain a relatively high Formulations usually reflect dogs’ preference for
percentage of meat or meat byproducts and are sweetness and cats’ preference for acidic flavors.
typically higher in fats and oils than dry-expanded
products. Meat-type ingredients may be introduced Snacks and Treats
into the extruder by either of the means previously The final market category involves those products
mentioned under the semi-moist category. They often referred to as snacks or treats for pets. These
differ from semi-moist products in that they take on products usually take the shape and appearance of
the expanded appearance associated with dry- real bones; however, there are other snack-type
expanded products after they are extruded. products for pets that come in a biscuit or variety of
Alterations to equipment required to convert a dry- other shapes. In recent years, more and more
expanded petfood system to semi-moist production producers of those types of pet snacks, as well as
are also required in order to produce soft, expanded- would-be producers of those types of petfood
type products. products, are investigating the potential of extrusion
cooking. The primary reason for the interest is the
With soft, expanded-type products, the basic potential cost savings that may be realized from the
extrusion process is similar to that of dry, expanded short-time/high-temperature of the extrusion
products in that conditioning with steam and water cooking process, the high thermal efficiency of
extruders, the floor space saved by the process and performance is enhanced by this high-
reduced labor costs may make production of such temperature/short-time process.
products more profitable. A typical extrusion snack
petfood system would utilize the same basic
principles and equipment arrangements of a typical Raw material specifications
semi-moist extrusion system. In some instances, a Every feed production facility manufactures a broad
dry-expanded system can be applied to the range of products. These can include several
production of pet treats or snacks. The primary different diets for a single species (integrators) or
differences would include the final die and cutting several different diets for many species (commercial
apparatus designed for producing a relatively large mills). Broad product assortments require a vast
piece, such as a bone, biscuit or wafer. number of available ingredients to meet the
nutritional requirements of each specific diet. Since
the number of possible ingredient combinations is
Universal Pellet Cooker (UPC) endless, and selection is normally based on least-
cost formulations, demographics or nutritional
A patented UPC cooking system and process (see value, the formulations may change frequently.
Figure 2-26) is an extrusion-based pelleting system Therefore, proper attention must be taken to ensure
(Wenger, 1997; Wenger, 1999). It was designed high-quality pellets are consistently produced.
specifically for the production of livestock feeds; Ingredient grind (mean particle size) and
but because of its design, can also be used to make formulation play a major role in producing high-
other extruded products such as aquatic feeds and quality pellets. These factors similarly affect the
some petfoods. It appears to be more effective, UPC as they do other pelletizers.
efficient and versatile than the traditional pelleting
systems, such as the expander plus pelleting press Many researchers have studied the importance and
that are currently used. The UPC also allows the effect of particle size reduction on animal
processor to utilize many raw materials that do not performance. They have tried to determine the
process well in a conventional pellet mill, such as “optimum” particle size to achieve maximum
those which contain high fiber or high levels of growth rates. The optimum size varies for each
sugar. species, age group and selection of ingredients.
Researchers have found that the common thread in
Figure 2-26. Wenger Universal Pellet Cooker®. particle size reduction is that a smaller mean
particle size will improve animal performance due
to an increased surface area available for enzymatic
attack. However, there are limitations to how fine
one can grind feed before health of the animals
becomes a concern.
Not only is particle size reduction important for

animal performance, but it is also very crucial for
However, the UPC needs to be considered as an pelleting. Coarse grinds create voids and fractures
alternative to rather than a replacement of the in pellets, making them sensitive to handling and
conventional pelleting process since, from an presumably to end up as fines at the feeder.
economic standpoint, it will probably never directly Evaluating particle size is commonplace in most
compete in the production of large-volume, low- feed mills. Particle size is usually determined by
margin pelleted feeds. Its unique design gives performing a sieve analysis. The feed particles are
special processing advantages to producers who use separated by size, weighed and the mean particle
raw materials that vary significantly, have high size is calculated based upon a log-normal
levels of internal oil or high lactose levels. The UPC distribution. Table 2-8 shows an example sieve
should also be considered in cases where the animal analysis.
If the maximum particle size or foreign matter in dimensional structure when exposed to high
the feed is larger than the die opening, it is possible temperatures. This three-dimensional structure is
that the opening can be plugged or partially modified when the proteins are subjected to
blocked, resulting in a change of appearance of the mechanical and thermal energy. The re-association,
pellets. In cases of severe blockage, the pelleting die which aligns the protein molecules, occurs during
will need to be cleaned before normal operation can laminar flow and forms a rigid structure. However,
proceed. As a rule of thumb, when the desired pellet not all sources of protein are good binders. Those
diameter is 4 mm or less, the suggested maximum sources with low amounts of pre-processing, such
particle size should be one-third the diameter of the as some types of blood plasma meals, contain
opening. For larger diameter pellets, the grind size “functional” protein, which has a greater binding
should be less than one-half of the die opening size. ability than heavily processed sources such as meat
and bone meal. Functional proteins are those that
Table 2-8. Example particle size are not already denatured.
distribution. Three mm grind;
sorghum-based ration. Figure 2-27. Expander-pellet mill flow diagram.
US Sieve Weight, g Weight, %
12 0.03 0.02
16 1.64 1.20
20 27.21 19.88
25 43.29 31.64
30 40.33 29.47
40 17.09 12.49
Pan 7.25 5.30
Total 136.84 100.00
The UPC system, which utilizes the natural binding

qualities of the ingredient formulations to their
fullest extent, does not depend on the use of non-
nutritive binding agents to produce a durable, high-
quality pellet. These natural binding elements of the
raw material are starch, protein and fiber. Starch
portions of the mix hold the greatest binding
capability. In most formulations enough starch is
present to produce the desired pellet durability
without giving much consideration to the other two
elements.
Starch possesses a unique ability to lose its Most sources of fiber strengthen pellets by
crystalline structure and becomes a viscous gel “melting.” The re-association of the lignin present
during processing. This allows it to disperse through in fiber gives binding power to the pellet. It takes
and around structures of other origins. This loss of much higher processing temperatures to melt lignin
crystallinity is known as gelatinization. Upon than it does to gelatinize starch or denature protein.
exiting the UPC and cooling, the starch returns to a Therefore, its influence is often only low to
crystalline state, resulting in a durable structure. moderate in binding ability; yet high-fiber diets will
Between 50-80% of the starch fraction in most diets typically form very durable pellets.
can be gelatinized during processing. Protein, like
starch, can also function as a binder. Protein
denaturation is the modification of a protein’s three-
Hardware requirements temperature very quickly. Each segment of the rotor

Processing principles of the UPC are different from can be removed and replaced according to wear of
the expander and pelleting press. One machine is that particular part. Since the whole rotor does not
designed to do the job of the conventional two. A need replacement, the wear cost is lowered
rotor and stator cook the feed similarly to an considerably. The stator also consists of segmented
expander; however, the feed is formed into dense parts. Each stator segment has a wear sleeve that
pellets rather than expanded chunks. With fewer requires replacement as needed. It is uniquely
pieces of equipment required and less space needed, designed to aid in the forward conveying of raw
the process flow is simplified. Figure 2-27, as material. Shear bolts or stop bolts, which are
compared to Figure 2-28, shows how the UPC can common in expanders and need frequent
easily adapt to existing manufacturing facilities replacement and maintenance, are not required for
without costly modifications. the UPC. These components are designed quite
similar to the barrel and screws of an extruder.
Figure 2-28. UPC flow diagram.

Pelleting die
A pelleting die is required to restrict the flow of
material and provide the cylindrical shape of the
pellet. The number of orifices in the die is
determined based on the desired capacity, raw
material formulation and final product
specifications. Changeover time of various dies is
kept to a minimum due to their comparative light
weight. When a raw material formulation contains
significant amounts of lipids, modifying the
pelleting die can increase the pellet durability.
Figure 2-29 shows how a die spacer can be
installed between the stator and the die. This
additional length increases the retention time of the
Preconditioning raw material inside the stator; in turn increases the
amount of shear on the product; and thus creates a
The UPC system utilizes an initial cooking zone so more durable pellet.
that the system depends less on mechanical energy
and more on thermal energy. This initial cooking Figure 2-29. UPC pelleting head assembly.
zone, known as preconditioning, is a prerequisite
for the production of quality pellets. A previous
section of this chapter covers the importance of
preconditioning.
Rotor and stator

The rotor and stator are designed to convey feed
through a restricting plate, build pressure and
increase the product temperature. The increased
temperature is the result of mechanical energy input
or shear. This aids in the cooking of raw materials.
As the pellet leaves the die, a variable-speed rotary
The rotor consists of a segmented-flighted shaft
cutter controls the pellet length. For example, by
designed to increase the internal product
increasing the cutter speed, short pellets and
crumbles are produced; and by reducing cutter This combination of temperature and retention time
speed, longer pellet lengths are produced. This will destroy many microbial populations.
flexibility eliminates the need for crumbling rolls to
produce a crumbled feed. The second opportunity to destroy microbes is in
the rotor and stator. The technological concept
behind the UPC differs somewhat from the
Cooling/drying currently-used methods of heat treatment processes.
Other methods depend on high-temperature/short-
Because heat and moisture are added during
time (HT/ST™) processing, meaning the feed
processing, extra equipment is required to lower the
spends a relatively short amount of time (i.e., 20-30
temperature, remove moisture, prevent mold growth
seconds in an extruder and 15-25 seconds in an
and prolong storage life. This is one of the most
expander) at conditions of high temperature and
significant differences between the UPC process
high pressure. However, the UPC utilizes high-
and a traditional pelleting process. The UPC
temperature/micro-time (HT/MT™) processing.
generally operates within the same moisture
This means that the feed spends a much shorter
constraints as other pelletizers. Exit moistures reach
amount of time under these conditions, usually 3-4
a maximum of 18%. This requires a cooler capable
seconds and still reaches temperatures of 115-
of driving off at least 3-6% moisture to achieve
150°C. This ability to cook feed quickly ensures
final moistures of 12% or less. The pellets must also
that heat-sensitive nutrients such as vitamins and
be cooled within 10°C of ambient temperature. In
amino acids are handled more delicately to prevent
situations where a conventional cooler will not
degradation. However, harmful microorganisms,
provide adequate moisture removal, a dryer will be
such as Salmonella, can be completely destroyed.
required. A more complete discussion of the drying
and cooling requirements appear later in this book.
Table 2-9. Nutrient retention and
microorganism destruction. Source: Wenger
Technical Center Test Data. (1996).
Process Impacts Vitamin Lysine, Mold
To this point, both thermal and mechanical energy A, % count,
have been loosely defined, but it is important to Sample IU/kg CFU/g
understand how these process variables affect the Raw Material 1 8,580 0.70 300,000
UPC process. Production of quality livestock feed Processed 12,320 0.71 < 10
depends on many processing variables. Sample 1 Lot 1
Pasteurization and production of durable pellets Processed 13,046 0.72 < 10
require the addition of steam and/or water in the Sample 1 Lot 2
preconditioner to increase product moisture from Raw Material 2 9,042 0.70 300,000
14-18% and a temperature of 70-90ºC. The shear Processed 14,278 0.71 < 10
provided by the rotor, stator and the pelleting die Sample 2 Lot 1
can elevate the product temperature to 110-150ºC Processed 14,190 0.72 < 10
depending on the die configuration and ingredient Sample 2 Lot 2
formulation. Raw Material 3 n/a 1.36 500,000
Processed n/a 1.41 40
Sample 3 Lot 1
Pasteurization
The UPC system offers two opportunities to Table 2-9 shows retention of various heat-sensitive
pasteurize pelleted feed products. The first stage is nutrients and destruction of microorganisms in feed
the DDC preconditioner. As previously mentioned, produced on the UPC. In each case, none of the
the DDC is capable of holding the feed for up to nutrients were degraded, but the detrimental
two minutes and can reach temperatures of 90-95ºC. microorganisms were destroyed. Table 2-14
indicates the results of expanding plus pelleting on
vitamin retention. This data show that the expander feed mix has a different Tg and Tm, each feed
does partially destroy some vitamins. formulation will process somewhat differently.
Figure 2-30. State diagram of the UPC process To help understand the Tg phenomena, consider the
(Strahm and Plattner, 2001; Strahm and feed mix as a mass of wax. At room temperature it
Plattner, 2000). is in a crystalline state and breaks when one tries to
bend it. As the wax is heated it becomes pliable.
The temperature at which the wax begins to show a
considerable amount of flexibility could be
considered as its Tg. Continuing to heat the wax will
eventually convert it into a fluid, so the temperature
at which it fluidizes can be considered its Tm.
Figure 2-31 shows photos of a pelleted feed made
using a conventional expander plus pellet mill
process and one from the UPC system, magnified
with a scanning electron microscope. Notice the
laminar structure that develops with the UPC
process. This structure provides superior strength
over the expander plus pelleted product.
Pellet Durability Figure 2-31. Scanning electron micrographs of
The ability for the UPC to produce an extremely pelleted feeds.
durable and dense pellet is illustrated in Figure 2-
30. This graph shows how the raw material
viscosity changes inside the preconditioner and
stator as energy and moisture are added. When
energy inputs are sufficient and the product
temperature moves above the glass transition
temperature (Tg), major components of the raw
material, such as protein and starch, transform from
a highly viscous, glassy state into a rubbery dough.
This change begins to occur in the preconditioner.
As the temperature continues to rise inside the

stator, the product reaches its melt transition
temperature (Tm). When a product is heated above
its Tm, the rubbery mass’s viscosity declines quickly
and becomes a fluid (Strahm, Plattner, Huber and
Rokey, 2000). The reduction of viscosity allows the Final product characteristics
raw material to pass through the orifices of the die Every livestock producer has different ideas for
with relative ease at low moisture and pressure (i.e., what the appearance and quality characteristics of
200-900 psi). Upon exiting the pelleting die, the feed should be. These specifications include: Pellet
pellet’s temperature declines and some moisture size, bulk density, durability, fines content, moisture
flashes from the surface of the pellet. The pellet and other various considerations. These product
returns to a glassy structure. This re-association and specifications can be controlled by the independent
hardening of the melt can be witnessed when processing variables of the UPC, which include the
examining hot pellets exiting the pelleting die. At following:
this point, the pellets seem fragile but after cooling • Feed delivery rate;
they become very strong and durable. Since each • Knife speed;
• Steam; waste at the feeder and are not as palatable as whole

• Water; pellets. Several factors influence the ability of the
• Pellet die configuration; and UPC to prevent the production of fines. Mean
• Recipe formulation. particle size, diet formulation and starch
gelatinization all affect the production of fines.
Pellet size can be easily controlled. The possible
pellet diameters range from 2-18 mm and Table 2-10. Poultry feeding trial (40 days). Wenger
adjustments are made by a quick and easy Technical Center Test Data (2000). Feeding trials
replacement of the pelleting head. The pellet length by independent third party.
can be varied to any size or even into crumbles Feed
when desired by adjusting a variable-speed cutter Needed to
and/or varying the number of knife blades. Bulk Reach
density can also be controlled and varied during Body Target
operation. However, pellet diameter and length do weight, Feed Weight,
have a significant effect on the density range. As the kg Conversion kg
diameter and length increase, the bulk density Universal
decreases. Typically the bulk density of UPC Pellet 1.95 1.53 2.7
pelleted feeds is about 550-650 grams per liter. The Cooker
raw material affects the finished product density to Pellet Mill 1.91 1.65 3.0
the greatest extent. High-fiber diets tend to have the Expander +
lowest raw material densities; therefore, one cannot 1.84 1.75 3.2
Pellet Mill
expect to achieve the same finished product density
as a feed high in protein or starch. Durability is Large feed particles can disconnect from the pellets
probably the most important characteristic of as the cutter shears them to length at the pelleting
pelleted feed. Consumers expect the most durable head. Low levels of cook lead to poor pellet
pellets possible. Poor pellet durability results in the durability, and inevitably lead to the breakdown of
generation of fines. Durability can be predicted by pellets. Also, high-fiber diets tend to produce more
determining the pellet durability index (PDI), which fines than high-starch diets, since these ingredients
gives reference to how well pellets hold their have different binding abilities. Other than the
integrity during packaging and handling mainstream production of compound feed, the UPC
(McEllhiney, 1994). The UPC, however, typically can also produce types of feeds that are all but
produces pellets with a PDI of over 95%. impossible for pellet mills and expanders to
produce. Full-fat soy (FFS), soft-moist pellets and
feeds high in bypass protein and bypass fat are the
Benefits of the UPC most notable.
The UPC has shown advantages over pellet mills
and expanders in several feeding trials with poultry, FFS production has been limited to HT/ST
swine and dairy cattle. Table 2-10 shows the extrusion systems due to the high energy input
advantage of a UPC for poultry. Those animals fed requirements needed to destroy the anti-nutritional
pellets produced on the UPC reached grown weight factors that exist in raw soybeans. However, the
more quickly and needed less feed to reach the UPC has shown to be capable of producing
target body weight. equivalent quality FFS. Figure 2-32 shows the
results of four tests run at different specific
Studies with swine have shown that pelleted feeds mechanical energy levels (SME). At the higher
with 10-15% fines can negatively influence animal SME inputs acceptable, product can be produced.
performance. The findings show that as the fines Destruction levels between 80-90% are found to be
content increases, feed wastage, low palatability and sufficient for trypsin inhibitor in most livestock
lower feed conversion ratios are noted. Fines create feeds.
Figure 2-32. Effect of SME on trypsin barrel will plug die orifices. Production runs are
destruction (Wenger Technical Center Test often less than one hour due to plugging of the die
Data, 1996). orifices.
Figure 2-33. Starter aquatic feeds.
Some producers have resorted to first making a

standard extruded pellet of substantial size,
Production of soft-moist pellets is an available normally greater than 4 mm in diameter. The pellets
option with the UPC, giving feed producers even are dried and cooled and then they are crumbled
more flexibility to satisfy consumers and open new using a roller mill. Then the particles are sifted and
markets. With the proper ingredients included in the classified into size ranges. With this method, typical
formula, final moisture and mold growth will not be on-size products are usually in the range of 50% or
a concern. The final moisture can vary from 15- less, with the balance being smaller and larger
20% when humectants and mold inhibitors are particles. These particles often have very ragged
included in the ingredient mix to control water and sharp edges, which some believe can cause an
activity. increased mortality rate in the fry. An alternate
Feed manufacturers have been bombarded recently process for producing these starter feeds is the SAS
with technological advances in the compound feed process. This system is much different than the
processing industry. As with any technology, typical extrusion cooking process. The SAS process
however, continuous development brings about is designed to produce more uniform and
major improvements. The UPC is a direct result of nutritionally homogeneous particles than a
the rapid increase in demand for processing traditional crumbling system. A uniformly mixed
equipment required to heat treat and pelletize and pulverized formulation is passed through a low-
livestock feeds. The UPC is another option to shear, low-temperature extrusion process where it is
provide high-quality feed with the ease and conditioned with water as well as other possible
simplicity of using one machine. The flexibility liquid additives and then compressed through a
provided allows producers to gain greater customer special die to form extruded strands. These strands
satisfaction by developing new characteristics into are then transported to the Sphere-izer™. This
existing feed lines at lower cost. machine, by cyclonic motion, sizes and shapes the
strands into pellets with lengths about the size of the
strand in diameter. The SAS™ will produce
Sphere-Izer Agglomeration System (SAS) finished feeds in a size range of 0.3-1.2 mm
Producing starter feeds (see Figure 2-33) for diameter with >85% “on-size” product. The low
aquaculture has long been a challenge. Traditional processing temperatures required are suitable for
single-screw extruders are often limited to pellet minimizing nutrient damage, production of
sizes of 1.2 mm, while a twin-screw extruder can medicated feeds and utilization of other
consistently produce feeds as small as 0.7 mm in temperature-sensitive ingredients.
diameter. These systems are often difficult to run
since any contamination in the feed or hardened,
overcooked material that builds up in the extruder
Ingredient preparation a die plate that forms and separates the dough into
The raw ingredients used in the process can be individual, continuous strands which have a
much different than the requirements for diameter of the desired product. The agglomerated
conventional extrusion. In typical extrusion, starch strands are then fed into the Sphere-izer for sizing
is required to bind the ingredients together in order and forming to the desired size. The Sphere-izer is a
to form a durable pellet, and contents of 15-20% spinning disc with a series of radially-symmetrical
must be present in the formulation. With the SAS grooves or corrugations. As the ropes drop onto the
process, low-starch formulations can be disc they are broken into small pieces, which
successfully used since the starch required for eventually turn into spheres as the pieces roll over
traditional extruded products does little to bind the the corrugations. Continuous drying of the small
product together because the temperature of the diameter starter feeds presents problems when
process is low, usually in the 40-50°C range. The considering the traditional horizontal continuous
particles are bound by using natural binders from bed dryers and vertical dryers. The small diameter
fishmeal, fish soluble, gluten and other organic products cannot be handled in a static bed or
ingredients. moving bed perforated tray dryer because of several
factors. The SAS products pack together so tightly
Preparation of the formulation mixture includes that they create a bed of product that does not allow
micro-pulverizing to a specified particle size range air to pass through. Therefore, the product will not
to prevent die plugging during processing. After dry completely. To eliminate this problem, a
grinding, the formulation is passed through a rotary vibrating bed/fluid bed dryer is utilized for drying
sieve to remove any particle larger than specified and cooling these products. The fluid bed dryer
for the finished product size. The same particle size forces air through perforations in the vibrating bed
rules apply for the SAS system as in a conventional with enough velocity to suspend the particles in the
extrusion system. The raw materials should have a airflow and keep the product moving and exposed
maximum particle size less than 1/3 of the die to the heated air stream. The cooling portion of this
opening. If this criterion is not met, it becomes very unit does the same only using ambient temperature
difficult to successfully produce a product for any air. A complete flow diagram for the SAS is shown
length of time. in Figure 2-34.
Process equipment and parameters Figure 2-34. SAS process flow.

The product agglomeration is accomplished by
using a forming extrusion system. The material is
metered into a preconditioner where it is moistened
with water. Other liquids such as oil or medications
can be added to the preconditioner as well. It is
essential that all of the liquids be strained before
injection and that a proper sized spray nozzle is
used to give uniform distribution of the liquid. Even
water lines need to contain fine strainers or screens
to prevent any particle, such as a hard water deposit
which could plug the die, from entering the system.
After leaving the preconditioner, the wetted Advantages and disadvantages of the SAS
material enters the extruder barrel. The extruder is
Since the SAS is a low-temperature process, it
designed and operated to prevent substantial
allows the feeds to be medicated. This allows the
cooking of the raw material; but instead kneads and
producers to more easily control disease in their
forms it into a semi-elastic dough. The extruders
aquatic species. It also produces pellets with a
typically turn slowly and the length-to-diameter
smooth surface; as compared to a crumbing process,
ratio is relatively short. The extruder is capped with
which produces pellets with ragged edges and sharp
corners. One of the main disadvantages of the SAS

process is that the processed feed is not pasteurized.
The processors must be careful in cleaning the
system and choosing ingredients that are not
contaminated. The final products produced by the
SAS also tend to have poor water durability. Since
relatively low heat is applied, the starches are not
cooked, and thus the pellets do not have anything to
hold them together. The other option is to use pre-
gelled starches, which would give the products
much better durability; however, they greatly
increase the cost of the operation. The other major
disadvantage with this system is that it is only
capable of producing sinking feeds. Conventional
extruders allow the processor to produce partially-
floating feeds down to 600 microns.
The SAS process is designed to produce more

uniform and nutritionally-homogeneous particles
than a traditional crumbling system. A uniformly
mixed and pulverized formulation passes through a
low-shear, low-temperature extrusion process where
it is conditioned and compressed to form
agglomerated strands.
Mr. Galen J. Rokey is the Manager of the Pet Food

Process Group at Wenger Manufacturing Inc. He
has more than 30 years of laboratory, extrusion
process and research experience with Wenger.
Email:grokey@wenger.com. Mr. Brian Plattner is
Processing Engineering Manager at Wenger. He
joined the company in 1998 after receiving his
Bachelor’s degree in Agricultural Engineering from
Kansas State University. Email:
brianp@wenger.com
This content was edited and reviewed by Dr. Adam

Fahrenholz, Assistant Professor of Feed Milling at
North Carolina State University, Dr. Charles Stark,
Jim and Carol Brown Associate Professor of Feed
Technology at Kansas State University, and Dr.
Cassandra Jones, Assistant Professor of Feed
Technology at Kansas State University.

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Extrusion and other terminal

preconditioning step. The initial portion of this

Figure 2-1. The objective of preconditioning is to Double Conditioner

Differential Diameter Conditioner

Preconditioning hardware and operation

When analyzing the various preconditioner designs,

When adding water-based slurries, it is especially

Retention time throughput for a given fill factor will cause a

This system requires two key components to be

Figure 2-6. RTC preconditioner. temperature and moisture content. During

Retrofitting a current conditioning cylinder with the

Un-preconditioned raw materials are generally

Table 2-3 also illustrates the ability of the DDC

Table 2-3. Effect of preconditioning on Forming extrusion is usually a low temperature

As the material leaves the preconditioner, it enters

A better understanding of the interaction between products).

Figure 2-15. Twin-screw extruder.

Figure 2-19. Dual extrusion process for cooking

Post-extrusion pressure chamber

Figure 2-21. Density increase for various pellet

Because the pellet diameter is small the EDMS

Extrusion moisture is also an important process

Dry extrusion usually implies process moistures of

preconditioning and extrusion; and through a 1.2 mm or smaller hammermill screen

Not only is particle size reduction important for

The UPC system, which utilizes the natural binding

Hardware requirements temperature very quickly. Each segment of the rotor

Figure 2-28. UPC flow diagram.

Rotor and stator

As the temperature continues to rise inside the

• Steam; waste at the feeder and are not as palatable as whole

Figure 2-33. Starter aquatic feeds.

Some producers have resorted to first making a

Process equipment and parameters Figure 2-34. SAS process flow.

corners. One of the main disadvantages of the SAS

The SAS process is designed to produce more

Mr. Galen J. Rokey is the Manager of the Pet Food

This content was edited and reviewed by Dr. Adam

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