Art of Pelleting: Particle Size, Mash Conditioning and Die Specifications (Part 3)

Optimum particle size for best pelleting results has been a matter of controversy for almost as long as feeds have been pelleted. Young (1960) found no significant differences in pellet durability when he experimented with feed rations containing 40, 60, and 70 percent ground corn or grain sorghum when the grain portions were ground coarse, medium, and fine.

Martin (1984) compared pelleting efficiencies and durabilities using a hammermill and a roller mill to grind the corn portion (59.5%) of a pelleted feed. He did not find any differences (P<.05) among the various treatments. The average particle size of the hammer milled corn (3.2mm and 6.4mm screens) ranged from 595 to 876 microns, and the roller milled corn (fine and coarse) ranged from 916 to 1460 microns. Stevens (1987) conducted similar experiments in which No. 2 yellow corn was used as the grain portion of the typical swine formula. The corn was ground with a hammermill through three screen sizes: 1/16"(1.6mm) (fine); 1/8"(3.2mm) (medium) and 1/4"(6.4mm) (coarse). He then measured the effect of the ground grain particle size on the pelleting production rate, electrical efficiency, and pellet durability. There were no significant (P<.05) differences in the pelleting production rate or PDI values from different particle sizes of corn mixed into the swine ration; although, the total electricity required to grind the corn and pellet the mash was significantly greater for the fine ground corn. When ground wheat was used as the grain portion of the swine ration, pellet production rates and PDIs improved as the grain was ground finer; but the finer ground wheat also required substantially more electrical energy.

While the research cited may seem to provide conflicting results, there is overwhelming evidence that the average particle size of the ground grain portion of a ration or of the total ration (mash) affects the pelleting process - throughput and/or pellet quality. The effects, simply, are not the same under all conditions or for all rations. The operators must conduct their own research under their own operating conditions and on the feeds that they produce.

It is common that some portion of a plant's product mix is often in mash or meal form and that grinding the grains more finely in a pre-grind system or the whole mix in a post-grind system causes handling problems in those mash feeds. There are two solutions to that dilemma - either provide two ground grain bins over the mixing system or find a grind (particle size) in the middle somewhere that will produce the better quality pellet and still provide the flowability or angle of repose that is needed for mash feeds. The first option is, of course, the better one but may not be possible, or too expensive, in a given grinding/mixing system situation.

Grind as fine as you must for best pellet quality in your operation and with your operation and with your feed rations, but don't over grind. That is wasteful of energy, reduces production rates, adds to manufacturing costs, and may do more harm than good to the consuming animal.

This is a subject unto itself and will not be addressed in total detail in this paper. Many researchers and practitioners have proven over and over again that pellet durability and pelleting efficiency can be substantially improved by the proper steam conditioning of mash. Steam brings to the surface of pellet mash particles the natural oils which are common to most grains and provides lubrication of the pellet die reducing wear on the die and roller assembly and increasing production rates (Behnke, 1990). In some instances, thorough conditioning may be counter productive from the standpoint of pellet durability. If the material slips through the die too easily, dwell time in the die hole is reduced causing the pellet to be less durable, and the starch gelatinization caused by the heat and friction in the die may be reduced.

Stevens (1987) conducted extensive research into the phenomenon of starch gelatinization during the feed pelleting process by pelleting corn that was hammermill ground through a 1/8" screen. He used a Perkin-Elmer DSC-23 (Differential Scanning Calorimeter) for gelatinization analysis. Ground corn before pelleting was used as the control. Samples of the pellets were prepared for analysis in the DSC by grinding them in a Braun coffee grinder, then regrinding in the UDY mill. A 2 mm thick outer portion of pellets was scraped with a razor blade from selected samples and ground in the UDY mill.