Pellet Quality
For purposes of this paper, pellet quality will be equated to the ability of pellets to withstand repeated
handling without excessive breakage or fines generation. There are many factors that affect pellet
quality, but the following will be discussed in some detail:
· formulation
· ingredient particle size
· mash conditioning
· feed rate
· die speed
· die specifications (design)
· other factors
Formulation
There are feedstuff materials that pellet well and produce a durable pellet and others that will not.
MacBain (1966) developed a pelletability chart in which he ranked feed ingredients in their pelletability
and degree or abrasiveness. Bartikoski (1962) experimented with applying numerical value to each
major (feed) ingredient to indicate its "stickiness" or its ability to help form a tough, durable pellet.
He called that value a "stick factor" and fed that factor into the computer along with the various
nutritive values of each ingredient to provide formulas that meet all nutritional specifications as well
as supplying a formula that will produce a quality pellet at least cost.
Those early workers led others to experiment with the effects of various ingredients - grains, milled
grain by-products, fats, pellet binders, minerals, etc. - on pellet quality or durability. They also led to
the development of a standard method for testing pellet durability perfected in the 1960's by Dr. Harry
B. Pfost at Kansas State University and accepted as a standard by the American Association of
Agricultural Engineer - ASAE S-269.3 (ASAE, 2003). That method is generally known as the K-State,
or tumbling can, durability test; and it provided a means of quantifying the toughness of pellets or
their ability to withstand the downstream handling that is typical in feed plants and feed delivery
systems. That was a major breakthrough in the technology of pelleting and has served the industry
for all these years.
Minerals
Pellet mill performance can be significantly affected by the physical and chemical forms of the calcium
and phosphorus sources used in the formula. Sutton (1979) investigated the effect of deflourinated
phosphate (two particle sizes) and dicalcium phosphate (18.5%) on pellet mill performance with a
broiler grower formula. He found the production rate for the diet containing regular grind deflourinated
phosphate to be 68.9% greater than for the diet containing an equal amount of dicalcium phosphate.
The finely ground deflourinated phosphate had a 52.5% advantage over dicalcium phosphate.
In a similar study (Behnke, 1981) we also studied the effect of mineral sources on pellet mill performance
and pellet quality. Two deflourinated phosphate sources, a fine grind (DPF) and a regular grind (DPR), and an 18.5% dicalcium phosphate (DCP) were used. A practical layer diet was used in which each
test mineral source was evaluated at both high (2.5%) and low (1.5%) levels in the diet.
At both levels tested, the production rate for the deflourinated phosphate sources significantly
outperformed dicalcium phosphate; while the DCP had a slightly, but not significantly, higher pellet
durability index. That would indicate that a physical change - thicker die or reduced feed rate - could
be made to improve pellet quality without a substantial loss of system throughput. Behnke (ibid),
Verner (1988), and McEllhiney and Zarr (1983) reported similar results comparing phosphorus sources
in a variety of pelleted feeds produced under many conditions.
Those studies are cited, not to encourage or discourage the use of any mineral source or any other
ingredient - that's the nutritionist's decision - but to indicate that those sources and ingredients can
affect pellet quality and production rate and should be considered in the quest for improved pellet quality.
