By Ron Turner
For many years, pelleting was considered an art, a process involving imprecise measurement, uncertain results, and that undefinable quality of feel. This so-called "art" of pelleting came about in an environment lacking the understanding of the effects occurring when the differing characteristics of feed ingredients are subjected to the pelleting processes of heat, moisture, and pressure. The science of grain processing has now reached a point that, by knowing the characteristics of the feed and using machinery with accurate measurements, the guesswork has been removed from pelleting.
Feed and ingredient characteristics are determined by the following factors: protein; density; fat; fiber; texture; starch or carbohydrates and moisture. Each feed or ingredient has a unique characteristic depending upon the variance of one or more of these factors. Each factor has a certain latitude of variance before the characteristic of a feed or ingredient is changed. For example, change the protein of 48% soybean meal by 0.2% and it is still considered 48% soybean meal. But, if the protein was analyzed as 46%, or 2% less, the ingredient would not be considered 48% soybean meal.
While ingredient suppliers are, at times, striving to keep these ingredient factors constant so that their quality will not vary, they have also, through modern technology, been able to reduce the amount of starches, fats and oils from these products. While these improvements normally enhance the supplier’s primary products, we in the feed industry find it increasingly difficult to pellet these ever-changing products.
The quality of pelleted feed will vary as the factors of ingredients vary. In order to keep pellet quality constant, ingredient characteristics or quality must be consistent, or compensating adjustments made.
Figure 1, the Pelletabilty Chart, lists common ingredients used in feed. The chart can be used to help predict the rate that each ingredient will pellet. The column headed "abrasiveness" will give an indication on how this ingredient will affect die life. Each feed formula can be analyzed using this chart to give an indication of its pelletability.
Many times pelleting rates and pellet quality are reduced, because it is not always possible to control the ingredients that make up a formula or make compensating adjustments. While some least cost formulation programs do set standards for pellet quality specifications, ingredient costs continue to be an overriding factor when formulating the final product.
PROTEIN AND DESITY
Normally, the production rate or pellet quality of a formula can be predicted when the factors of protein and density are considered. Ingredients with high natural protein will plasticize under heat, causing good quality pellets. Ingredients or feeds with high density have high production rates.
If an ingredient or feed is high in both natural protein and density, then high production rates and good pellet quality can be expected. When low protein and high density are the factors, high production rates and poor pellet quality can be expected. Ground corn is a good example of this type of ingredient.
At the other end of the spectrum, an ingredient or feed that has low protein and low density should produce a good quality pellet with a poor production rate. Some examples of this type of ingredient are alfalfa meal, ground corn cobs, cottonseed hulls and corn gluten feed.
The bulk density of a formula is an important factor in determining the rates a pellet mill will produce. You will get less production with a light fibrous material, such as alfalfa meal, which has a bulk density of 17 pounds per cubic foot, as opposed to cottonseed meal which has a bulk density of 40 pounds per cubic foot off the same 100 horsepower pellet mill.
Small particles increase the bulk density of light, fibrous materials. They also reduce work in the pellet mill to "densify" lighter materials and permit faster running (Figure 2.)
If you can reduce your particle size for pelleting, the small particles, or "fine grind," will result in:
• Improved quality
• Increased production rate
• Increased bulk density
• Greater horsepower efficiency
• Longer die life
A better quality and higher production rate results from particle size reduction because the steam penetrates the smaller particles through to the core, making them soft and pliable. However, due to the retention time in the conditioner, the steam is unable to penetrate the larger particles completely, leaving the center dry (Figure 3).
Large particles of your formulation will cause cracks and fractures in pellets (Figure 4). It is recommended that the hammermill screens for grinding do not exceed 1/8-inch diameter. Better results can be achieved from use of a 7/64-inch screen.
The smaller particles will increase your horsepower efficiency by increasing the throughput of material
through the die holes with less
horsepower. The finer grind also extends the die life as it decreases the "grinding" or milling of material on the solid
land between the holes on the die face (Figure 5).
The fat content of an ingredient or feed can indicate either natural fat or added fat. Both aid in increasing production rates. The pellet quality could be seriously affected with the addition of too much fat, normally more than 2%. The added fat can be of two kinds, animal fat or vegetable fat. Animal fats are presently the most commonly used in commercial feeds.
High fiber creates production rate problems, as fiber is hard to compress into a pellet. Because of the natural binders inherent to fiber, it does result in good pellet quality.
When referring to texture, three general types need to be considered: coarse, medium and fine. Fine- or medium-ground materials provide greater surface area for absorption of moisture from steam, resulting in better lubrication and increased production rates. Also, more particles are exposed to steam, resulting in possible chemical changes that may be needed for quality. The starting density can be increased when it includes a mix of medium and fine ground material.
Very coarse grinds or large particle sizes provide natural breaking points in pellets, creating more fines.
High starch formulations or ingredients are difficult to produce into a tough, durable pellet. Effective gelatinization can be activated only with high temperatures and moisture. The gelatinized material acts as a binder to produce the desired pellet quality.
If, for some reason, the natural starches in a feed have been gelatinized before pelleting, poor pellet quality can be predicted. An example would be the drying of corn at high temperatures, which causes pre-gelatinization.
Sufficient inbound moisture added prior to pelleting can be desirable in reaching good pellet durability. As stated earlier, many of today’s ingredients, such as brewer’s grains, are of a very dry nature. Added moisture will benefit the pellet mill stream if it has an opportunity to soak in prior to reaching the pellet conditioning chamber.
Too much moisture, as present in high-moisture corn, can have an adverse affect on pellet quality, as it will restrict the amount of steam conditioning required at the conditioner, and thus not allow the high temperatures necessary to gelatinize the starch and produce good pellet quality.
A binder may be added to the feed if adequate pellet quality is not obtained through proper steam conditioning and die selection. The artificial binders add to the cost of a feed and are only used when all other means are exhausted.
---Ron Turner is Manager of Latin America for California Pellet Mill (CPM).