The Art (Science) Of Pelleting: Part 2

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.