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Introduction

The advantages of feeding a pelleted feed over mash have been documented in the industry for many years. The advantages include the following:

• Increased bulk density
• Less bridging/hang-up in bins
• Less dust
• Reduced ingredient segregation
• Less feed waste
• Increased nutrient density
• Improved palatability
• Increased nutrient availability
• Decreased microbiological activity

In the year 2000, U.S. commercial feed mills averaged 20.3 tons per hour per pellet mill line. The average pellet durability index (PDI) of the final pelleted products was 92.7 and average processing cost per ton was $9.03. These figures do not include data from integrated operations where the average PDI is 25 to 40 percent and processing costs per ton may be as low as $3 per ton. Integrators usually operate the mills at full capacity and outputs rather than trying to obtain the optimum pellet quality. Quality of feed (measured in PDI) and production costs (dollars per ton) have shown gradual improvements and this is believed due to improved processing techniques – especially in the preconditioning phases¹. The reported benefits of enhanced preconditioning are as follows:

• Increased PDI
• Pasteurization of feed
• Destruction of insect eggs
• Improved FCR
• Decreased processing costs
• Increased starch gelatinization

Operational Principals

Conditioners “precondition” the mash before pelleting by accomplishing three unit operations: heating, hydrating, and mixing. Mill operators focus on the heating factor as it is easy to monitor and control. Thermal energy is occasionally added through “dry heating” methods by jacketing or plating equipment with insulation or heating elements. Exhaust streams of hot air or steam from other processes have also been used to provide heat.

Heating is accomplished most commonly and efficiently through direct steam injection, and this is the principle energy source in Wenger systems. Conditioning devices often include a sensor and control system to maintain set-point temperatures at the discharge of the preconditioner. The addition of 4 to 5 percent steam is usually sufficient to achieve desired discharge preconditioner temperatures of 93°C (200°F). Research has proven that with the excellent mixing imparted by Wenger DDC systems, product temperature at the discharge of the DDC need to only reach 75°C (165°F) to achieve pasteurization.

Moisture addition and hydration are very critical in pelleting operations, since most pellet mills can handle a maximum of 18 percent moisture in the preconditioned mash. This low moisture requirement means that little or no water can be added as steam usually provides sufficient moisture. However, this is very dependent on the raw mash moisture and the industry is awaking to the fact that water addition can greatly improve pellet quality as long as the maximum level is not exceeded. Moisture transfer into feed is much slower than heat transfer. However, because the operating parameters dictate low moistures, preconditioning occurs in a “moisture-starved” environment. As long as mixing is sufficient to get uniform moisture distribution, actual retention times in DDC/pellet mills may not be as critical as extrusion processes where moisture levels are usually in the 25 to 28 percent range.

Mixing is a very critical parameter in preconditioning. Much of the new conditioning technology that blossomed in the last few years has included long retention times (as much as 3 to 5 minutes). The Wenger DDC technology is able to meet or exceed the performance of these systems because of excellent mixing. Typical retention times have been two minutes or less with some mills running as short as 30 seconds. Throughputs and installation area are the major criteria for selecting the proper size.


Process Flow

Recipes are blended in batch mixers and then conveyed to the pelleting process. Often the grains are ground while many of the ingredients in meal or powder form are not ground. However, improved feed conversion has caused the feed millers to look at post grinding systems where the complete recipes are ground to an average of 600 microns. Grinding systems are fitted with screens and magnets to remove tramp metal and other foreign objects. The ground mash is transported to a bin/feeding system which serves to meter the mash into the preconditioner. Steam, water, and other additives can be added in the preconditioner. The conditioned mash gravity-flows out of the preconditioner into the rolls area of the pellet mill (Figure 1).



As an option, Wenger has developed a constant discharge/self-emptying preconditioner by designing the DDC to act as a loss-in-weight system. By mounting the DDC on load cells and attaching a “choke” feeder at the discharge of the preconditioner, several benefits are realized:

• Instant control of retention time to produce desired pellet quality
• Constant rate discharge of feed upon shutdown
• Simplified startup sequence
• Reduce off-spec product during startup

This feature (Figure 2) may not be necessary for many operations but provides excellent control of the preconditioning process when these requirements exist.




Pasteurization

The DDC preconditioner has been shown to pasteurize pelleted feed products. As previously mentioned the DDC is capable of holding the feed for up to two minutes and can reach temperatures of 90º to 95ºC. This combination of temperature and retention time will destroy many microbial populations. Table 1 illustrates the ability of the DDC preconditioner to destroy some of these microbes.




Product Improvement

Throughout the feed industry feed conversion is used to judge animal performance. Typically, it is just pounds of feed given to the animal per pound of animal weight. For example, if a flock of birds is given 1000 pounds of feed throughout the growing cycle, and 400 pounds of live weight is recorded, the feed conversion for that flock would be 2.50 (1000/400). Scheideler (1995) noted the effect of pellet quality on the feed onversion
ratio (FCR) for several poultry species including broilers, roasters, and turkeys. Poultry fed pellets with a PDI of 70 percent required less feed to reach market weight than those fed pellets with a PDI of 25 percent (Table 2).




In a recent series of trials the Wenger DDC was compared to a single shafted cylinder to determine its advantage. Starch gelatinization (percent cook) and pellet durability index (PDI) were measured and compared. The percent cook was measured by determining the susceptibility of starch to enzymatic digestion. PDI was determined by tumbling the pellets for five minutes then determining the amount of product that stayed on a number eight sieve. The results are shown in Table 3.



The DDC showed an advantage of 4.4 points of increased cook and 12.4 points of increased pellet durability. Feeding trials showed that the FCR for the broilers fed pellets produced using the Wenger DDC feed was lower than the broilers fed pellets produced on the single shaft preconditioner. As shown in Table 4, the FCR for the feed produced on the DDC conditioner was 0.025 (or 2.5 points) better than the feed produced on the single shaft preconditioner.



If we assume that one feed conversion point is worth 60 cents per ton of feed produced, the DDC conditioner could pay for itself very quickly. For example a feed mill producing 80 tons of feed per hour, operating 5 days per week, 24 hours per day would see a benefit of $5760 per week or about $300,000 per year if they saw an increase of 2.5 feed conversion points.

The Wenger DDC’s ability to provide longer retention times and more efficient mixing allows a higher quality pellet to be produced. This can lead to many advantages for feed mills through lower feed to gain ratios and increased PDI.




Prepared by:

Wenger Livestock Feed Process Team
Galen Rokey, Manager, Wenger Technical Center
Rob Strathman, Director Technical Service
Brian Plattner, Process Engineer


¹ Lobo, Phillip. 2001. How mills are managed: Percent capacity decreases, while production efficiency increases in 2000. Feed Management. 52:1. 13-16.

² Rokey. G. 2001. Pelleting, Conditioning, and Steam Addition. 2001 Feed Management Seminar. US Egg and Poultry Association. Nashville TN.

³ Scheideler, S. E. 1995. Is Pelleting Cost Effective? Feed Management. 46:1. 21-26.

⁴ Wenger 2001. Wenger Laboratory Analysis, Reference Number L20010103-001.

⁵ Rokey, G. J. 2001. Pelleting, Conditioning, and Steam Addition. US Egg and Poultry Association – 2001 Feed Mill Management Seminar. Nashville, TN.

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