The Science of Pelleting

Pelleting converts a finely ground blend of ingredients into dense, free flowing agglomerates (pellets). It is a fairly expensive process in terms of both capital and variable costs, but the expense is usually justified in improved plant profit as well as animal performance.
The article will further discuss the pelleting process in terms of operations and describe how the success or failure of the operation can impact profits as well as animal performance.
pThe Process
The formation of the pellet actually occurs at the “nip” between the rolls and the die. All other activities associated with the operation such as conditioning, cooling, etc. really support and augment the action at that point in the system. In order to understand the process and be in a position to make intelligent decision to improve throughput, quality or appearance, one must have a thorough understanding of what happens at the nip point. Figure 1 is a representation of the die-roll assembly to understand the process.
Depending upon the physical characteristics of the feed, a lesser or greater proportion of the work done by the pellet mill is used for compression. For example, if the formula contains a high level of fibrous ingredients such as bagasse, bran, or ground alfalfa, the mill will expend a large amount of energy simply compressing the mash to the density of the subsequent pellet. Conversely, for a relatively dense feed such as high grain and soy meal, the mill will expend a lesser amount of energy for compression and a greater amount for throughput.
The primary purpose of the roll is to provide a force on the mash to densify the feed and cause it to flow toward the die. The gap between the roll and the die, the roll surface characteristics and the physical properties of the mash determine how great this potential force might be.
The die provides, not only the final diameter of the pellet, but the resistance force on the feed and has a direct influence on throughput rate and pellet quality. These two forces (roll and die) are opposite each other, but must work together to provide quality pellets at an acceptable production rate. The force generated by the roll must be greater than the resistive force provided by the die; if not, throughput is zero.
Pellet Quality
Pellet quality is equated to the ability of pellets to withstand repeated handling without excessive breakage or fines generation. There are many factors that affect pellet quality, following are few discussed in some detail

  • formulation
  • ingredient particle size
  • mash conditioning

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 experiment led to the development of a standard method for testing pellet durability which 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.
Particle Size
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.
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. 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.
Mash Conditioning
Many researchers have proven 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 counterproductive 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. 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.
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.
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.
Producing economically viable best pellet quality possible pellets is becoming apparent as the animal performance can be affected by poor quality pellets. 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. It can be concluded pelleting is a complex issue that deserves a good deal of thought and investigation, which one learns only through day to day experience.
Source: Kansas State University

by Keith C. Behnke, Professor, Kansas University