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collaboration from AFMA (Animal Feed Manufacturers Association) www.afma.co.za
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Developing a full understanding of pellet quality and the factors that
influence it is still fertile ground for research and idea development. As new
ingredients become available and equipment and technological advances occur,
a thorough understanding of factors affecting pellet quality will be mandatory.
A literature review.
(This paper was presented at AFMA Forum 2001 in February 2001 in South Africa.
It was then extended and edited and published in FeedTech 2001 Vol 5 Nr 4)
Pelleting was introduced into Europe about 1920 and into the U.S. feed industry
in the late 1920's (Schoeff, 1994). Its popularity has grown steadily until
about 80% of all feed in the U.S. are currently pelleted. Today, the process
is widely used because of both the physical and the nutritional benefits it
provides. The physical benefits include improved ease of handling, reduced ingredient
segregation, less feed wastage, and increased bulk density. Nutritional benefits
have been measured through animal feeding trials (Falk, 1985).
As a rule, feeding pelleted feed improves animal performance and feed conversion
compared with feeding a meal form of a diet. The improvements in performance
have been attributed to (Behnke, 1994):
- Decreased feed wastage
- Reduced selective feeding
- Decreased ingredient segregation
- Less time and energy expended for prehension
- Destruction of pathogenic organisms
- Thermal modification of starch and protein
- Improved palatability
Research has concentrated primarily on the benefits of feeding pellets versus
meal. Pellet quality has become more important in the swine and poultry industries
as integrators continue to expand and recognise the value of feeding quality
Effects in pigs
Research studies conducted in Europe and the United States have shown pelleted
nursery diets will increase ADG and G/F by 9 to 10%. Pelleted grow-finish diets
result in a 3 to 5% increase in ADG and 7 to 10% in G/F (Table 1). Research
on the effect of pellet fines is limited and is generally confined to observations
of poor quality pellets failing to result in improved animal performance. Gill
and Oldfield (1965) and Tribble et al. (1979) reported poor animal performance
when the feed contained significant levels of fines. However, when pellet quality
was improved by changing the pelleting operation (i.e. thicker dies), animal
performance was improved. Hanrahan (1984) reported no difference in finishing
pig performance between pigs restricted fed a 69% or 62% PDI (Pellet Durability
Index) pellet. Stark (1994) conducted swine feeding experiments demonstrating
that feed containing a high quality pellet (no fines) resulted in greater efficiency
of gain than feed containing 30% fines.
Improvement in poultry
Pelleted broiler diets improve growth performance and feed conversion. Hussar
and Robblee (1962) reported reground pellets did not affect early bird performance.
However, as the birds matured, birds fed whole pellets had better growth and
feed conversion. Hull et al. (1968) reported birds fed pelleted diets had a
5% better feed conversion, but regrinding the pellets resulted in lower feed
conversion than the meal diet. A field study conducted by Scheideler (1991)
indicated birds fed 75% whole pellets as compared to 25% whole pellets had better
feed conversion (F/G 2.08 vs 2.13). This result was likely due to selective
feeding on the part of the broilers.
Turkeys appear to be more sensitive to pellet quality and fines than broilers.
Several studies indicate pellet fines decrease turkey performance. Proudfoot
and Hulan (1982) reported pelleted diets improved feed conversions. However,
as pellet fines increased from 0% to 60%, performance decreased. Moran (1989)
showed a decrease in growth and performance when re-ground pellets were fed.
Salmon (1985) reported no difference in bird performance when high quality pellets
were fed. This may explain why feed manufacturers place pellet quality as a
high priority. It is also an area in which the industry is continually trying
to make improvements.
Pellet quality testing
If one assumes that pellet quality has some influence on animal performance,
then an accurate, precise, and objective assay is necessary to document that
influence. Pellet quality can be measured using several methods. Indirect methods
such as the Stoke's® Tablet Hardness Tester (Britsol, PA) (developed for
the tablet industry) was one of the first tests used in the feed industry (McCormick
and Shellenberger, 1960).
Indirect testing methods allow feed manufacturers to make predictions of pellet
quality immediately after the pellet mill and therefore make adjustments accordingly.
Young (1970) developed the tumbling box test, which has become an industry standard
for measuring pellet quality.
The pellet durability index (PDI) (ASAE S269.3) was developed as a predictor
of pellet fines produced during mechanical handling. Young (abid) reported a
correlation of R=0.967 and 0.949 for hot pellets and pellets cooled for 24 hr.
respectively, using the tumbling can as a predictor of pellet fines. Methods
which measure individual pellets resulted in the lowest correlation (Stokes,
R=0.78; Shear test R=0.72).
The Holman Pellet Tester (Holman Chemical Ltd, United Kingdom) is a pneumatic,
rather than mechanical, method of measuring the durability of pellets. Pellets
are transferred through tubes with high velocity air to model the handling process
(MacMahon and Payne, 1981). McEllhiney (1988) reported the Holman Pellet Tester
gave consistent results, however, pellet durability results were lower than
the values obtained from the tumbling can method (ASAE, 1987). The use of indirect
methods for predicting pellet quality may be useful at the feed plant for adjusting
equipment. However, livestock producers are concerned with the direct measurement
of fines at the feeders. Fines in feeders can result in feed wastage, animal
refusals, and increased feeder management.
Adhesion in pellets
Adhesion is the process by which materials are held together by a physical to
chemical interaction of the material. This is accomplished by joining the surfaces
of the material by melting the materials together or by applying an adhesive
between them. An adhesive is defined as a material which, when applied to surfaces,
can join them together and resist separation (Wake, 1976). It should be obvious
that in pelleting, we seldom "apply" an adhesive; however, we do try,
through temperature and moisture control, to activate the natural adhesives
that are typically found in the feed ingredients.
Several theories on the mechanism of adhesion at the interface between particles
have been proposed. The theories with application in the pelleting process include:
+ Mechanical interlocking + Diffusion + Adsorption Kinlock (1987) described
the basic concepts of each theory and the mechanisms by which adhesion occurs.
Mechanical interlocking is based on the fact that adhesives flow into rough
surfaces, become rigid, and hold the materials together. The theory also suggests
that rough surfaces will improve the contact area and thus improve bond strength.
The diffusion theory is based on the diffusion of polymers at the interface
between material surfaces. Diffusion occurs when materials are heated and allowed
to diffuse across the interface between materials.
This phenomenon can occur only when the temperature of the polymer is above
the glass transition temperature of the polymer. Adsorption adhesion occurs
due to interatomic and intermolecular forces established between atoms and/or
molecules at the surface of the adhesive and the substrate. The attractive forces
are ionic, covalent, hydrogen bonding dipole interactions, and Van der Waal
forces. The bond energies of the forces and effective ranges have been summarized
by Allen (1990).
Rheological characteristics of feed ingredients
The rheological and functional characteristics of feed ingredients vary depending
on their physical structure (crystalline vs amorphous) and chemical composition.
Materials that are heated go through either a first or second order glass transition
or a combination of first and second order transitions. First order transitions
involve the melting of crystals, whereas second order transitions are a relaxation
of polymers. Crystalline materials (e.g. sugar) go only through a first order
transition. Partially crystalline materials (e.g. starch) go through a second
order transition prior to the first order transition. Amorphous materials (e.g.
cellulose, lignin) exhibit only a second order transition. The temperature at
which the amorphous regions of a polymer begin to relax or become mobile is
defined as the glass transition temperature.
Glass transition temperatures have been reported for starch (Zeleznak and Hoseney,
1987); wheat gluten (Slade, 1984; Hoseney et al., 1986), and for corn gluten
(Lawton, 1992). Glass transition temperature is inversely related to moisture
content. As the moisture in the system is increased, the temperature at which
the material becomes mobile decreases. Feed ingredients have glass transition
temperatures below the temperatures normally associated with the conditioning
process (70-90° C) when the moisture content is between 15 and 18%. This
suggests that feed ingredients begin to flow during the conditioning and pelleting
process, and the amount and location of material flow depends on the temperature
and location of the water (surface or intra-particle).
The level of total pellet starch gelatinisation and starch damage has been reported
to be negatively correlated with pellet quality (Stevens, 1987; Lopez, 1993).
Starch damage was found to be greater at the outer surface of the pellet at
lower conditioning temperatures. However, starch damage decreased as the conditioning
temperature increased, indicating that the damage was primarily due to mechanical
shear between the die surface and the starch and not due to hydrothermal elevation
Woods (1987) examined the functional role of starch and protein in the pelleting
process. The addition of raw soybean flakes increased pellet quality as compared
to heat treated denatured soybean meal. In addition, pre-gelatinised starch
improved pellet quality compared to native starch. Woods concluded that protein
had a greater influence on pellet quality than starch. This finding has been
recently confirmed by Briggs et al. (1999).
The data suggests that the level of starch gelatinisation may not be as important
as the location of the gelatinised starch. It is apparent that the gelatinisation
at the surface of the feed particles is critical to the formation of intra-particle
bonds necessary for the formation of strong, durable pellets. Starch gelatinisation
at the particle interface in conjunction with protein plasticisation would result
in polymer diffusion between starch granules and protein molecules, resulting
in adhesion to the particles.
Effect of formulation
Least-cost formulation is designed to meet the nutritional parameters required
by the target animal. However, the effect of formulation on processing, specifically
pelleting, is seldom considered by most nutritionists. Ingredients currently
used by the feed industry have been used as adhesives for over 100 years. The
addition of fat to the mash pre-pellet usually results in decreased pellet quality
(Richardson and Day, 1976; Headly and Kershner, 1968). However, the addition
of protein and fibrous materials increase pellet quality. Fahrenholz (1989)
reported an increase in the pellet durability of swine diet pellets and the
level of wheat middlings increased from 0 to 45%. McKee (1988) increased pellet
quality and water stability of catfish diets by increasing the level of wheat
gluten from 0% to 10%. Lopez (1993) also reported the addition of vital wheat
gluten resulted in a positive affect on pellet quality and water stability,
but the addition of cassava meal had a negative effect. Lawton (1989) reported
a linear increase in tensile strength as the amount of protein in a tablet increased
at the expense of starch.
Effect of particle size
Decreasing the particle size of ingredients results in a greater surface area
to volume ratio. Smaller particles will have a greater number of contact points
within a pellet matrix as compared to larger particles. Anand (1970) demonstrated
contact points between polystyrene beads increased 3 to 4 to 7 as bead size
decreased to allow 3, 4, or 7 particles per unit area, respectively.
Penetration of heat and moisture to the core of a particle can be achieved in
a shorter amount of time with small particles and a large surface area per unit
of weight. Stevens (1987) reported no difference in pellet quality when the
mean particle size of corn and wheat was reduced from 1023 to 551 microns (Ý)
and from 802 to 365 Ý, respectively. Martin (1983) reported similar results
using corn and grain sorghum. However, Wondra et al (1995) reported an increase
in pellet durability as particle size was reduced from 1000 to 400 Ý.
The aquaculture feed industry will typical grind ingredients to less the 250
Ý for greater pellet water stability. The combination of small particle
size and long term, high temperature conditioning produces pellets that have
the greatest water stability.
The importance of steam conditioning was quantified by Skoch et al (1981) in
an experiment comparing dry pelleting with pelleting using steam conditioning.
The results of this study indicated that steam conditioning improved pellet
durability and production rates and decreased the amount of fines generated
and energy consumption. From this, it was concluded that steam acted as a lubricant
to reduce friction during pelleting.
Mash entering the conditioner may be comprised of a wide variety of ingredients
that make up the diet formulation. The nutritional, as well as physical properties
of this mash have an effect on conditioning and eventual pellet quality. According
to Reimer (1992), pellet quality is proportionally dependent on the following
factors: 40% diet formulation, 20% particle size, 20% conditioning, 15% die
specifications, and 5% cooling and drying (Figure 1). If this is correct, 60%
of pellet quality is determined before the mash enters the conditioner. This
increases to 80% after conditioning, but before mash has even entered the die
chamber of a pellet mill.
There has been some research conducted looking at the effects of the first two
of these variables, diet formulation and particle size, on pellet quality. Studies
by Stevens (1987) and Winowiski (1998) have compared the pellet durability of
diets containing corn with those where some or all of the corn was replaced
with wheat. In both instances, pellet durability was higher for the diets containing
wheat. It can be reasoned that this is due to the higher crude protein content
of wheat (at 13%) as compared to corn (at 9%). This finding is in agreement
with a study conducted by Briggs et al. (1999) which found that increasing the
protein content in a poultry diet from 16.3% to 21% increased the average pellet
durability from 75.8 to 88.8%.
Particle size is the second factor that Reimer (1992) proposed would dictate
about 20% of pellet quality. Decreasing particle size from a coarse to a fine
grind exposes more surface area per unit volume for absorption of condensing
steam and increases the surface area available for bonding. MacBain (1966) indicated
that a variation in particle size produces a better pellet than a homogeneous
particle size. Work by Stevens (1987) when pelleting corn or wheat based diets,
however, found that particle size had no effect on pellet durability index (PDI)
as determined by the tumbling can method.
After reviewing the body of published scientific literature as well as reports
from field studies and skilled practitioners of pelleting, it is easy to conclude
that there is still a great deal of art in the science of pelleting. There is
truly a great deal that we don't understand or, perhaps, that we misunderstand
about pelleting. Reimer (1992) indicated that the factors that affect pellet
quality can be identified as: formulation, particle size, conditioning, die
specification, and cooling and drying. If his hypothesis is true, these allocations
provide a useful roadmap to solving many of the quality problems associated
Dr. Keith C. Behnke, Department of Grain Science and Industry, Kansas State University, Manhattan, Kansas, USA
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Anand, J.N. 1970. Adhesion and the formulation of adhesives. Ed. Wake, W.C.
Applied Science Publishers Ltd. London.
Behnke, K.C. 1994. Maryland Nutrition Conference. Dept. of Poultry Science and
Animal Science, College of Agriculture, University of Maryland, College Park.
Briggs, J.L., et al. 1999. Poultry Sci. 78:1464-1471.
Fahrenholz, C.F. 1989. PhD. Dissertation, Kansas State University, Manhattan.
Falk, D. 1985. Feed Manufacturing Technology III. Ed. R.R. McEllhiney. American
Feed Industry Assn. Arlington, VA.
Gill, D.R. and J.E. Oldfield. 1965. J. Anim. Sci. 24:599 (Abstr.).
Hanrahan, T.J. 1984. Anim. Feed Sci. Technol. 10:277.
Headly, V. and R. Kershner. 1968. Feedstuffs 40(30).
Hoseney, R.C., K. Zeleznak, and C.S. Lai. 1986. Cereal Chem. 63:285.
Hull, S.J., et al. 1968. Poultry Sci. 47:1115.
Hussar, N. and A.R. Robblee. 1962. Poultry Sci. 41:1489.
Kinlock, A.J. 1987. Adhesion and Adhesives Science and Technology. Chapmand
and Hall. New York, NY.
Lawton, J.W. Jr. 1990. Ph.D. Dissertation. Kansas State University, Manhattan.
Lawton, J.W. 1992. Cereal Chem. 69:351.
Lopez, J.G. 1993. Ph.D. Dissertation. Kansas State University, Manhattan.
MacBain, R. 1966. American Feed Manufacturers Association. Arlington, VA., pp.
MacMahon, M.J. and J.D. Payne. 1981. The Holmen Pelleting Handbook. Bradley
and Sons Ltd. Berkshire, England.
Martin, S.A. 1983. First International Symposium on Particle Size Reduction
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Salmon, R.E. 1985. Anim. Feed Sci. Tech. 12:223.
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|Gill and Oldfield(1965)
||Poor quality pellets did notimprove performance
|Jensen and Becker(1965)
|NCR 42 (1969) Swinenutrition
|Hanke et al. (1972)
|Tribble et al. (1979)
|Tribble et al. (1980)
|Harris et al. (1980)
||Quality pellets, 8%^ADG, 15%^G/F
||Poor quality pellets8%^ADG
|Tribble et al. (1980)
||Pellets, 6%^ADG, 17%^G/F
||Pellets plus binder
|Skoch et al. (1983)
||Pellet durability, 69%
||Pellet durability, 62%
|Walker et al. (1989)
|Wondra et al. (1994)