Pelleting feed provides many benefits from improved palatability and flowability, to decreased feed wastage, reduced ingredient segregation and destruction of pathogens. While pelleting feed comes at an additional cost, the benefits can offset these costs through improved animal performance.
When pelleting diets, feed mills must determine what is their target pellet quality. Ingredient composition, equipment design, and manufacturing parameters influence pellet quality. For swine and poultry feeds, reducing the percentage of fines at the feeder is the objective of producing good quality pellets. However, each production system is different, and a variety of factors can influence the handling stress pellets undergo and the amount of fines generated. Although the amount of fines is the major concern, feed mills require a methodology to estimate pellet quality for quality assurance purposes. Therefore, cooled pellet samples are collected at the feed mill to determine pellet durability index to estimate pellet quality.
Feed mash conditioning
Pelleting properties of mash feed can be influenced by a range of variables, some better understood than others. The pelleting process begins by feeding dry mash feed into a conditioning chamber where heat and moisture is applied via steam. As the feed exits the conditioner, it is compressed through a die to form pellets and then cooled before being stored prior to shipment.
For decades, researchers have explored the relationship of raw ingredient characteristics, feed conditioning and die specifications on optimized pellet quality. Proper mash feed conditioning is the first major step of the pelleting process and is necessary to achieve optimum production rates, pellet quality, energy usage, and die and roll life. Increasing the amount of steam used in the conditioning process will in turn result in increased conditioning temperature and moisture added to the feed.
Generally, this response to the addition of heat and moisture has been attributed to altered physico-chemical properties of the feed, typically leading to improved binding properties between. In addition, conditioning the feed is necessary to help lubricate the feed and push it through the die. However, different amounts of heat and moisture are required to adequately pellet different types of feed.
Traditionally, it has been determined that conditioned mash moisture should approach 17% or 18% and the temperature should reach approximately 82.2°C when pelleting corn and soybean meal-based diets. This recently was confirmed in research conducted at Kansas State University that demonstrated conditioning the diet to 89.9°C via added steam acted as a lubricant to help push the feed through the die with reduced friction, while conditioning the particles to bind together when extruded through the die.
This lubrication effect was confirmed by the decrease in change in temperature across the die as the conditioning temperature was increased to 89.9°C. In addition, increasing the feed temperature from 37.4°C to 89.9°C, via steam addition, increased conditioned mash moisture content by 4.2%. Feed conditioned to 89.9°C had a moisture content of 17.3%, which is within the target range of 17% to 18% moisture. These increases in feed temperature and moisture content resulted in improved PDI from 3.3% to 91.1%. Therefore, an improvement in PDI with increasing conditioning temperature is likely due to the increase in moisture, which acts as a binding agent, plasticizing the soluble fractions of the diet and increasing the agglomeration of dietary components.
In addition to increasing conditioning temperature, pellet quality can potentially be improved by increasing the amount of time feed particles are exposed to the conditioning process, which would allow moisture more time to penetrate the feed particles.
Pellet die options
After the conditioning process, conditioned mash feed is compressed through a die to form pellets. Various types of dies are available for use in the feed manufacturing industry. The type of feed being pelleted, target production rate and desired pellet quality will be influenced by die thickness (length) and hole size (diameter) of the die. Previous research has demonstrated that increasing die L:D from 5.6 to 8.0 improves pellet durability but increases pellet mill energy consumption. Keith Behnke, emeritus professor in KSU’s Department of Grain Science and Industry, in 2001 described the same positive correlation between die L:D and pellet durability and attributed this to the increased pressure and resistance generated by a larger die L:D.
When die hole diameter remains constant, a pellet die with a larger L:D is thicker than a die with a smaller L:D. Thus, feed retention within the die is longer with a thicker die and is a primary factor in determining pellet durability.
In addition to pellet mill die size, pellet mill die speed can vary from mill to mill. Pellet mill die speed is a product of the main drive speed, whether gear or belt driven, and any subsequent gear or belt reducers. In general, increased rotational speed not only maximizes throughput, but also reduces the thickness of the feed pad in front of the die rolls. A peripheral die speed of 610 meters per minute has been suggested as the optimum speed for pellets ranging from 3.2 to 6.4 millimeters in diameter, while reduced speeds between 366 to 396 meters per minute are preferred with pellet diameters exceeding 16 millimeters. Researchers have determined that when conditioning diets at lower temperatures (74°C), decreasing die speed improved pellet durability, while high conditioning temperatures (85°C) yielded greater durability regardless of die speed. However, reducing die speed resulted in increased specific energy consumption.
Ingredients and nutrient composition
In addition to processing parameters, ingredients and nutrient composition can also influence pellet quality. Including fat at the mixer can act as a lubricant for the pellet die and help to increase production rate. However, excessive fat addition levels can lead to poor pellet quality as a result of over-lubrication and reduced inter-particle bonding. Therefore, it is important to acknowledge differences in fat concentrations when formulating to an energy requirement when using a formulation software.
Previous research at Kansas State University demonstrated that when diets were balanced by energy for every 1% added fat, PDI was reduced by 10% to 14%. However, it is important to keep in mind that this response will depend on pelleting parameters such as conditioning temperature and time, die specifications, production rate, etc.
In addition to added fat, proteins and starch are altered during the conditioning and pelleting process and can potentially lead to changes in binding properties. Diets containing high protein can denature and plasticize under pelleting conditions and form a harder pellet, increasing pellet durability. The influence of protein and starch on pellet quality is dependent on their functional properties, with raw protein and pre-gelatinized starch improving pellet quality compared to denatured protein and native starch.
Pelleted diets containing high amylase corn that led to increased gelatinized starch compared to pelleted conventional corn diets at similar processing parameters, but there was no evidence for improvement in PDI with the increase in starch gelatinization. Therefore, the level of starch gelatinization that occurs during pelleting may not be enough to influence pellet quality. This leads to the hypothesis that proteins or the protein starch interactions may play a larger role in influencing pellet quality.
This agrees with previous research that demonstrated an improvement in PDI when crude protein content was increased by 3.9% or 4.7%.
Pelleting feeds plays an important role in optimizing animal performance and improving feed handling characteristics like increased flowability and bulk density with reduced ingredient segregation. Improving pellet quality has a positive influence on the observed response on swine and poultry performance.
When pelleting feed, there are many factors that can influence subsequent pellet quality. These factors must be considered when trying to optimize pellet quality. However, actions taken to improve pellet quality come at a cost via increased ingredient cost, increased energy consumption, or reduced production rate.
Therefore, it is important to calculate the overall economic impact when determining the optimal pellet quality and how to achieve it.
Chad Paulk is an assistant professor of feed science and management in the Department of Grain Science and Industry at Kansas State University. He may be reached at firstname.lastname@example.org.