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.
The results of the gelatinization measured in the samples taken immediately after the die are shown in Table 1. There was a negative relationship between the conditioned meal temperature and degree of gelatinization. As the temperature of the conditioned mash was increased, the degree of gelatinization decreased.
The high degree of gelatinization that occurred in the outer portion of the pellet at a 23 c conditioning temperature indicated that heat and mechanical shear next to the surface of the die hole caused a substantial portion of the gelatinization at all temperatures but, especially, when there were greater temperature differentials between the conditioned meal and the pellet. There is a relationship between that temperature difference and the degree of gelatinization observed. As the temperature differential decreased, the degree of gelatinization decreased.
Stevens (ibid) suggested that the conditioning temperature of 80 c was adequate to gelatinize corn starch; however, the length of time in the pellet mill conditioner at that temperature was probably not adequate for a substantial amount of gelatinization. It would appear, from that research, that most starch gelatinization occurred as the feed material passed through the die.
The temperature of conditioning mash has long been a pelleting criterion and an indication of thorough conditioning, that may, or may not, be a totally viable indicator since time at a given mash temperature will affect the conditioning, may affect the degree of gelatinization, and will certainly affect the pelletability of the mash.
Behnke (1990) studied the effect of effective die thickness, on length (L), on pellet durability in his experiments reported earlier in this paper. His results indicate, clearly, that durabilities were significantly enhanced with the use of the thicker die; however, production rates were as significantly reduced.
There is no magic. Almost anything that is done to improve pellet quality (durability) will either increase the cost of the ration or reduce the capacity of the pelleting system, or both. Adding to the effective thickness of the die is a perfect example of the sort of trade off that can be expected, and must be recognized, in the search for improved pellet quality.
One of the primary objectives of all commercial feed manufacturers is to economically produce the best pellet quality possible. This is not only important from a customer satisfaction standpoint, but it is becoming apparent that animal performance can be affected by poor quality pellets. Dairy cattle used to consuming pellets, readily reject fines. Even the U.S. broiler integrators are recognizing that poor pellet quality can reduce bird performance.
There are numerous factors that affect pellet quality and many are inter-related. It takes a great deal of effort to determine what changes to make and how other aspects of the system or operation might be affected.
Factors not addressed in this paper but that are currently being investigated include: double pelleting, optimum cooling, automation of the pelleting system, gentler handling of pellets and new binders. This paper has not dealt with issues of water stability of pellet aquatic diets, but that topic is gaining great importance around the world.
As can be seen, pelleting is a very complex issue and one that deserves a good deal of thought and investigation.
This article was originally published in feedmachinery.com.