The following is a snapshot of what experiments and experiences have shown regarding factors that affect pellet quality, such as ingredient characteristics, grinding, process uniformity and pellet mill parameters.
By CHARLES STARK, LELAND MCKINNEY and ADAM FAHRENHOLZ*
*Dr. Charles Stark is the Jim & Carol Brown associate professor in feed technology at Kansas State University. Dr. Leland McKinney is with DFS Inc. in Johnston, Iowa. Dr. Adam Fahrenholz is assistant professor in the Prestage department of poultry science at North Carolina State University.
RAISE your hand if you've heard this before: "Pelleting is an art, not a science." As feed manufacturers, we might agree; formulas that ran fine one week but resulted in challenges the following week just because (so we think) the sun went down or the corn moisture changed slightly are enough to make anyone crazy.
Although no one has all of the answers about pelleting, there are experiments and experiences that can shed light on the process. Here, we aim to share observations and research results that contribute to industry knowledge. As always, we welcome questions and comments in hopes of learning together.
One could compare pelleting to adjusting a stock car throughout the course of a race. As conditions change, small adjustments help maximize performance. For example, keeping rolls adjusted and the die scoured can mean significant differences in production rate and pellet quality.
That being said, just as in the racing analogy, there is also a science to what goes on behind the scenes. Collecting in-depth information can be complicated, but with new technologies, we gain the tools to further understand and manipulate the pelleting process.
We can start with the assumption that the process can be modeled, i.e., there is enough consistency that we can use previously collected data to predict what will happen in the future. In pilot situations, this assumption has proved to be true but requires extreme diligence over time.
However, not all pelleting lines will behave exactly the same. Just ask anyone running two identical systems side by side if they both manufacture every formula with the same efficiency and pellet quality. So, assuming that a specific line can be modeled, we must also accept that the model can — and probably will — be different from system to system.
The next step is to define the terms "pellet quality" and "pellet durability."
Pellet quality is the ratio of pellets to fines in the finished feed, with the most relevant location of interest being the feeder directly in front of the animal.
Pellet durability is a measurement used to predict pellet quality. This may be determined using the Kansas State University tumble box, a Holmen pellet tester or some other in-house method. The two most important items in testing durability are consistency and generating numbers that make sense. Given their importance, these qualifications will be further discussed in another installment of this series. For now, just keep in mind that durability is the tool, and quality is the final characteristic of concern.
Taking all of the above into account, the purpose here is to discuss factors that affect pellet quality. The following is a snapshot of what relevant experiments and experiences have shown regarding factors such as ingredient characteristics, grinding, process uniformity and pellet mill parameters.
It is well known that formulation plays a significant role in pellet quality. Much of this has to do with ingredient composition, with starch, protein and fat being the main items of concern.
Starch is a natural adhesive when heat and moisture are applied, even without large amounts of gelatinization, which occurs mostly due to friction at the die and is not necessarily correlated with pellet quality.
The impact of protein can be positive and significant, although it seems to be dependent on the source (zein protein in corn versus gluten protein in wheat) and the other ingredients in the diet. Protein that has been heat treated also has less of an effect on pellet quality than native protein.
Adding fat to the mixer will typically lead to decreased pellet quality at amounts greater than 1% inclusion. However, the source can have an impact, with some fats (e.g., choice white grease and tallow) leading to slightly better pellets.
Pre-pellet mill processing is the next area for consideration. Particle size and distribution of ground grains can affect pellet quality, but the effects depend on the range considered. Data have shown differences of approximately 200 microns to have no effect, and anecdotal observations suggest that the number is likely higher.
It stands to reason that smaller particles are easier to hydrate and that greater uniformity leads to more consistent conditioning. However, it has been shown that it's possible to include some coarse particles without significantly degrading pellet quality.
Mix uniformity, which is typically viewed as a nutritional requirement, is also important in pelleting, since a non-uniform mix will lead to inconsistent pellet mill operation and poor pellet quality.
Once we reach the pellet mill, the first variable of concern is throughput. The bad news: Running a pellet mill hard will decrease pellet quality. The good news: Research suggests that once a threshold is reached (somewhere near "rated" capacity), pushing the pellet mill harder will further degrade quality less and less.
The next step is steam conditioning, which will typically have the single greatest process impact on pellet quality. Regarding the steam itself, consistent quality is likely the biggest key. Steam pressure is less of a factor. The range of pressures typically seen (20-40 lb./sq. in. gage) has very little difference in energy content per pound of moisture. Adequate retention time and conditioner fill complete the required environment.
Finally, we consider the pellet die. Because of the expense, die specifications are not manipulated as simply as other factors, but if you are looking for a big hammer, this is it. Can't pellet something? Put on a thinner die. Absolutely have to improve quality? Put on a thicker die.
However, when it comes to die manipulation, the pellet mill must have enough horsepower to handle a thicker die. Typically, the motor on the pellet mill is sized for a set throughput and die specification when it is purchased, which could result in lower throughput as the die thickness is increased.
Perhaps the most important realization is that all of these factors will interact with each other, and this can be positive or negative.
Maybe, in your system, a certain particle size and specific set point on the conditioner will yield a near-ideal conditioning situation and the best possible pellets. Conversely, you might be able to put a little extra fat in the mixer or use more of a certain byproduct in the formula without affecting quality too greatly, but combine the two, and quality suffers more than the sum of the parts.
This is where the "art" comes back into play. It's the ability to think critically and come up with ways to experiment with what works in your plant(s) and what doesn't. The "science," then, is the ability to collect, document and examine the data and make useful observations.
So, is pelleting an art or a science? Maybe it's both.
Cavalcanti, W.B., and K.C. Behnke. 2005a. Effect of composition of feed model systems on pellet quality: A mixture experimental approach. I. Cereal Chem. 82(4): 455-461.
Cavalcanti, W.B., and K.C. Behnke. 2005b. Effect of composition of feed model systems on pellet quality: A mixture experimental approach. II. Cereal Chem. 82(4): 462-467.
Fahrenholz, A.C. 2012. Evaluating factors affecting pellet durability and energy consumption in a pilot feed mill and comparing methods for evaluating pellet durability. Ph.D. dissertation. Kansas State University, Manhattan, Kan.
Gilpin, A.S., T.J. Herrman, K.C. Behnke and F.J. Fairchild. 2002. Feed moisture, retention time and steam as quality and energy utilization determinants in the pelleting process. App. Eng. in Ag. 18(3): 331-338.
Moritz, J.S., K.J. Wilson, K.R. Cramer, R.S. Beyer, L.J. McKinney, W.B. Cavalcanti and X. Mo. 2002. Effect of formulation density, moisture and surfactant on feed manufacturing, pellet quality, broiler performance. J. Appl. Poult. Res. 11:155-163.
Skoch, E.R., K.C. Behnke, C.W. Deyoe and S.F. Binder. 1981. The effect of steam conditioning rate on the pelleting process. Anim. Feed Sci. Technol. 6:83.
Stark, C.R. 1994. Pellet quality and its effect on swine performance; functional characteristics of ingredients in the formation of quality pellets. Ph.D. dissertation. Kansas State University, Manhattan, Kan.
Stark, C.R. 2009. Effect of die thickness and pellet mill throughput on pellet quality. Abstr. T89. Southern Poultry Science Society Meeting.
Stevens, C.A. 1987. Starch gelatinization and the influence of particle size, steam pressure and die speed on the pelleting process. Ph.D. dissertation. Kansas State University, Manhattan, Kan.