Density of pelleted feed

INTRODUCTION

Wenger's involvement in the Livestock Feed and Pelleting industry dates backto 1935 when Wengerdeveloped and manufactured mixers, pelleting machinery, and other basic feedmilling equipment for thelivestock industry. Since this time Wenger has installed its pelleting technologyworldwide for use withinthe livestock feed industry. The UP/C® (Universal Pellet Cooker) is Wenger'slatest technology forproduction of pellets and pasteurization of mash.

In livestock feed processing plants, pelleting has become a primary processingstep to enhance thequality of feed. It is a time-proven method of improving feed efficiency andfeed quality, which explains itslongevity in the field. Pelleting, the agglomeration of ingredients into densepellets, produces many traitsdesired by livestock producers. These include:

In the past, livestock producers have come to demand these qualities, but as research reveals the benefits of high temperature processing, feed manufactures are expected to add to this list of desired traits. High temperature/short time processing techniques, such as extrusion and expansion, have addednew qualities to livestock feed that producers are requesting such as:

The UP/C® (Figure 1) helps meet or exceed these expectations. It is significantly more effective, efficient, and versatile than the traditional systems, such as the expander plus pelleting press (Figure 2) that are currently used. Since the UP/C® utilizes fewer pieces of equipment, it is easier to justify because it reduces capital, formulation,operating, and maintenance costs.




RAW MATERIAL SPECIFICATIONS

Every feed production facility manufactures abroad range of products. These can include several different diets for a singlespecies (integrators) orseveral different diets for many species (commercial mills). Broad product assortmentsrequire a vastnumber of available ingredients to meet the nutritional requirements of eachspecific diet. Since thenumber of possible ingredient combinations is endless and selection is normallybased on least-costformulations, demographics, or nutritional value, the formulations may changefrequently. Therefore,proper attention must be taken to ensure high quality pellets are consistentlyproduced. Ingredient grind (mean particle size) and formulation play a majorrole in producing high quality pellets. These factorssimilarly affect the UP/C® as they do other pelletizers.


GRINDING CONSIDERATIONS

Many researchers have studied the importance and effect of particle size reductionon animalperformance. They have tried to determine the“optimum” particle sizeto achieve maximum growth rates. The optimum size varies for each species,age group, and selection of ingredients.Researchers have found that the common thread inparticle size reduction is that a smaller meanparticle size will improve animal performance due toan increased surface area available for enzymaticattack. However, there are limitations to how fineone can grind feed before health of the animalsbecomes a concern.

Not only is particle size reduction important foranimal performance, but it is also very crucial forpelleting. Coarse grinds create voids and fracturesin pellets making them sensitive to handling andpresumably to end up as fines at the feeder.Evaluating particle size is commonplace in mostfeed mills. Particle size is usually determined byperforming a sieve analysis. The feed particles areseparated by size, weighed, and the mean particlesize is calculated based upon a log-normaldistribution. Table 1 shows an example sieveanalysis.




If the maximum particle size or foreign matter in thefeed is larger than the die opening, it is possible thatthe opening can be plugged or partially blockedresulting in a change of appearance of the pellets.In cases of severe blockage, the pelleting die willneed to be cleaned before normal operation canproceed. As a rule of thumb, when the desired pellet diameter is 4 mm or less,the suggested maximumparticle size should be one-third the diameter of the opening (i.e. maximum particlesize of 1333 micronsfor 4 mm pellets).

BINDER CONCERNS

The UP/C® system, which utilizes the naturalbinding qualities of the ingredient formulations totheir fullest extent, does not depend on the use ofnonnutritive binding agents to produce a durable,high-quality pellet. These natural bindingelements of the raw material are starch, protein,and fiber. Starch portions of the mix hold thegreatest binding capability. In most formulationsenough starch is present to produce the desired pellet durability without giving much considerationto the other two elements.

Starch possesses a unique ability to lose itscrystalline structure and become a viscous gel during processing. This allowsit to disperse through andaround structures of other origins. This loss of crystallinity is known as gelatinization.Upon exiting the UP/C® and cooling, the starch returns to a crystalline state,resulting in a durable structure. Between 50to 80 percent of the starch fraction in most diets can be gelatinized duringprocessing.Protein, like starch, can also function as a binder.Protein denaturation is the modification of aprotein’s three-dimensional structure whenexposed to high temperatures. This threedimensionalstructure is modified when theproteins are subjected to mechanical and thermalenergy. The re-association, which aligns theprotein molecules, occurs during laminar flow andforms a rigid structure. However, not all sources of protein are good binders. Those sources withlow amounts of pre-processing, such as sometypes of blood plasma meals, contain “functional”protein which has a greater binding ability thanheavily processed sources, such as meat andbone meal. Functional proteins are those that arenot already denatured.

Fiber strengthens pellets by “melting”. Thereassociation of the lignin present in fiber givesbinding power to the pellet. It takes much higherprocessing temperatures to melt lignin than it doesto gelatinization starch or denature protein.Therefore, its influence is often only low tomoderate in binding ability, yet high fiber diets willtypically form very durable pellets.


HARDWARE REQUIREMENTS

Processing principles of the UP/C® are differentfrom the expander and pelleting press. Onemachine is designed to do the job of theconventional two. A rotor and stator cook the feed similarly to an expander;however, the feed is formedinto dense pellets rather than expanded chunks. With fewer pieces of equipmentrequired and less spaceneeded, the process flow is simplified.




PRECONDITIONING

The UP/C® system utilizes an initial cooking zoneso that the system depends less on mechanicalenergy and more on thermal energy. This initialcooking zone, known as preconditioning, is aprerequisite for the production of quality pellets.Preconditioning initiates the heating process bythe addition of steam into the feed. With Wenger’spatented preconditioner (Figure 5), retention timesof up to 2 minutes are achievable. The Wengerpreconditioner exposes raw materials to steamand water for longer periods of time than othersimilar types of preconditioners. This allows thesteam to fully penetrate the feed particles.



Retention time and temperature of the exiting feedare the two most important processing variables ofa preconditioner. These variables, which affectFigure 3: Expander-Pellet Mill Flow DiagramFigure 4: UP/C® Flow Diagram© 2001 Wenger Mfg, Inc. Sabetha, KS USA66534 Page 5 of 11 the final product quality, must be monitored properly. Forexample, when the feed throughput increasesboth the retention time and the exit temperature will decrease, and fines inthe final product can result.
Thus to improve pellet quality, additional steam would be required to elevatethe exit temperature and toprovide an adequate level of cook.

Cook is the percentage of starch that has beengelatinized during processing. Because gelatinizedstarch has a proportional relationship with the amountof heat exposure, it can be used as an indicator of thefinal pellet quality. The Wenger preconditioner iscapable of cooking from 30 to 40 percent of the starchpresent in a given formulation.


ROTOR AND STATOR

The rotor and stator are designed to convey feedthrough a restricting plate, build pressure, andincrease the product temperature. The increasedtemperature is the result of mechanical energy input or shear. This aids in thecooking of raw materials.

The rotor consists of a segmented-flighted shaft designed to increase the internalproduct temperaturevery quickly. Each segment of the rotor can be removed and replaced accordingto wear of that particularpart. Since the whole rotor does not needreplacement, the wear cost is lowered considerably

The stator also consists of segmented parts. Eachstator segment has a wear sleeve that requiresreplacement as needed. It is uniquely designed to aidin the forward conveying of raw material. Shear boltsor stop bolts, which are common in expanders andneed frequent replacement and maintenance, are notrequired for the UP/C®

PELLETING DIE

A pelleting die is required to restrict the flow ofmaterial and provide the cylindrical shape of the pellet.The number of orifices in the die is determined basedon the desired capacity, raw material formulation, and final product specifications. Change-over time ofvarious dies is kept to a minimum due to their comparative light weight.

When a raw material formulation contains significant amounts of lipids, modifyingthe pelleting die canincrease the pellet durability. Figure 6 shows how a die spacer can be installedbetween the stator andthe die. This additional length increases the retention time of the raw materialinside the stator, in turnincreases the amount of shear on the product and thus creates a more durablepellet.



PELLET SIZING

A variable speed rotary cutter controls the pellet length. For example, by increasingthe cutter speedshort pellets and crumbles are produced, and by reducing cutter speed longerpellet lengths areproduced. This flexibility eliminates the need for crumbling rolls to producea crumbled feed.


COOLING/DRYING

Because heat and moisture are added during processing, extra equipment is requiredto lower thetemperature, remove moisture, prevent mold growth, and prolong storage life.The heat and moisture areremoved from the pellets by drying and cooling them after the UP/C®.

There are two types of coolers: vertical (counter current) and horizontal (Figure7). Horizontal belt coolerstypically have a higher capacity than the vertical coolers. They convey feedon perforated conveyingbelts through the dryer. As the product moves throughthe dryer, air flows through the bed of pellets. Thistype of cooler is usually fitted with one or twoconveyors (single or double pass). The double pass ismore efficient than the single, since it requires lessairflow per ton of finished feed.



Vertical coolers (counter current or bin) allow pellets todescend opposite the direction of the airflow. Thisallows the coolest air to pass through the coolestpellets and warmest air to pass through the warmestpellets. This type of cooler requires less floor spacethan horizontal coolers. Vertical coolers are typicallyconfigured with one or two cooling decks dependingon the capacity requirements.

The UP/C® generally operates within the samemoisture constraints as other pelletizers. Exitmoistures reach a maximum of 18 percent. Thisrequires a cooler capable of driving off at least three tosix percent moisture to achieve a final moisture of 12percent or less and cool the pellet to within 10°C ofambient temperature. In situations where aconventional cooler will not provide adequate moistureremoval a dryer will be required.

EXTERNAL COATING

The advantages of topically coating feeds can include:decreased dust, increased palatability, and increasedfeed intake. Pellets can be coated with nutritiveingredients such as fat, molasses, lactose, vitamins,enzymes, or a combination of these and otheringredients.

Coating equipment consists of an applicating reel, liquid tank, and a pump (Figure8). For fat application,the reel can be fitted with steam coils and a shroud toprevent build up of congealed fat and fines.



SOFTWARE

To this point, both thermal and mechanical energyhave been loosely defined, but it is important tounderstand how these process variables affect theUP/C® process. Production of quality livestock feeddepends on many processing variables.Pasteurization and production of durable pelletsrequires the addition of steam and/or water in thepreconditioner to increase product moisture from 14 to18 percent and a temperature of 70° to 90°C. Theshear provided by the rotor, stator, and the pelletingdie can elevate the product temperature to 115° to170°C depending on the die configuration andingredient formulation.

PASTEURIZATION

The UP/C® system offers two opportunities to pasteurize pelleted feed products.The first stage is theDDC preconditioner. As previously mentioned the DDC is capable of holding thefeed for up to twominutes and can reach temperatures of 90º to 95ºC. This combinationof temperature and retention timewill destroy many microbial populations. Table 2 illustrates the ability of theDDC to destroy some ofthese microbes.




The second opportunity to destroymicrobes is in the UP/C® rotor and stator.The technological concept behind theUP/C® differs somewhat from thecurrently used methods of heat treatmentprocesses. Other methods depend onhigh temperature/short time (HT/ST™)processing, meaning the feed spends arelatively short amount of time (i.e., 20 to30 seconds in an extruder and 15 to 25 seconds in an expander) at conditionsof high temperature andhigh pressure. However, the UP/C® utilizes High Temperature/Micro Time (HT/MT™)processing,meaning feed spends a much shorteramount of time under these conditions,usually three to four seconds and stillreach temperatures of 125° to 170°C.This ability to cook feed quickly, ensuresthat heat sensitive nutrients such asvitamins and amino acids, are handledmore delicately to prevent degradation.However, harmful microorganisms, suchas salmonella, are destroyed.

Table 3 shows various heat sensitivenutrient retention and microorganismdestruction in feed produced on theUP/C®. In each case none of thenutrients were degraded, but thedetrimental microorganisms weredestroyed. Table 4, shows the results of expanding plus pelleting on vitaminretention. This data showsthat the expander does partially destroy some vitamins.






PELLET DURABILITY

The ability for the UP/C® to produce an extremely durable and dense pelletis illustrated in Figure 9. Thisgraph shows how the raw material viscosity changes inside the preconditionerand stator as energy andmoisture are added. When energy inputs are sufficient and the product temperaturemoves above theglass transition temperature (T_g), major components of the raw material, suchas protein and starch,transform from a highly viscous, glassy state into a rubbery dough. This changebegins to occur in thepreconditioner.



As the temperature continues to rise inside the stator, the product reaches itsmelt transition temperature(T_m). When a product is heated above its T_m the rubbery mass’s viscositydeclines quickly and becomesa fluid.⁴ The reduction of viscosity allows the raw material to pass throughthe orifices of the die withrelative ease at low moisture and pressure (i.e., 200 to 900 psi).

Upon exiting the pelleting die, the pellet’s temperature declines and somemoisture flashes from thesurface of the pellet. The pellet returns to aglassy structure. This reassociation andhardening of the melt can be witnessedwhen examining hot pellets exiting thepelleting die. At this point the pellets seemfragile, but after cooling they become verystrong and durable. Since each feed mixhas a different T_g and T_m, each feed formulation will process differently.To further clarify this, consider the feed mixas a mass of wax. At room temperature it isin a crystalline state, but when heated thewax becomes pliable. The temperature atwhich the wax shows a considerableamount of flexibility, could be considered asits T_g. Continuing to heat the wax willeventually convert it into to a fluid, so thetemperature at which it fluidizes can beconsidered its T_m.

Figure 10 shows photos of a pelleted feed made using a conventional expanderplus pellet mill processand one from the UP/C® system, magnified with a scanning electron microscope.Notice the laminarstructure that develops with the UP/C® process. It provides superior strengthover the expander pluspelleted product, which does not have this same structure





FINAL PRODUCT CHARACTERISTICS

Every livestock producer has different ideas for what the appearance and qualitycharacteristics of feedshould be. These specifications include: pellet size, bulk density, durability,fines content, moisture, andother various considerations.

These product specifications can becontrolled by the independentprocessing variables of the UP/C®,which include the following:

Pellet size can be easily controlled. The possible pellet diameters range from 2 to 18 mm and adjustments are made by a quick and easy replacement of the pelleting head. The pellet length can be varied to any size or even into crumbles when desired by adjusting a variable speed cutter and/or varying the numberof knife blades.

Bulk density can also be controlled and varied during operation. However, pelletdiameter and length dohave a significant effect on the density range. As the diameter and length increase,the bulk densitydecreases. Typically the bulk density of UP/C® pelleted feeds is about 550to 650 grams per liter.

The raw material affects the finished product density to the greatest extent.High fiber diets tend to havethe lowest raw material densities; therefore, one can not expect to achieve thesame finished productdensity as a feed high in protein or starch.

Durability is probably the most important characteristic of pelleted feed. Consumersexpect the mostdurable pellets possible. Poor pellet durability results in the generation offines. Durability can bepredicted by determining the Pellet Durability Index (PDI)7 (Appendix A), whichgives reference to howwell pellets hold their integrity during packaging and handling. The U P/C®,however, typically producespellets with a PDI of over 95 percent.

Studies with swine have shown that pelleted feeds with 10 to 15 percent finescan negatively influenceanimal performance. The findings show that as the fines content increases feedwastage, low palatability,and lower feed conversion ratios are noted. Fines create waste at the feederand are not as palatable aswhole pellets.

Several factors influence the ability of the UP/C® to prevent the productionof fines. Mean particle size,diet formulation, and starch gelatinization all affect the production of fines.Large feed particles candisconnect from the pellets as the cutter shears them to length at the pelletinghead. Low levels of cooklead to poor pellet durability and inevitably lead to the breakdown of pellets.Also, high fiber diets tend toproduce more fines than high starch diets, since these ingredients have differentbinding abilities.


BENEFITS OF THE UP/C®

The UP/C® has shown advantages over pellet mills and expanders in severalfeeding trials with poultry,swine, and dairy cattle. Table 5 shows the advantage of a UP/C® for poultry.Those animals feed pelletsproduced on the UP/C® reached grown weight more quickly and needed less feedto reach the targetbody weight.



Other than the mainstream production of compound feed, the UP/C® can alsoproduce types of feeds thatare all but impossible for pellet mills and expanders to produce. Full fat soy(FFS), soft-moist pellets, andfeeds high in by-pass protein and by-pass fat are the most notable.

FFS production has beenlimited to HT/ST™ extrusionsystems due to the high energyinput requirements needed todestroy the anti-nutritionalfactors that exist in rawsoybeans. However, the UP/C®has shown to be very capableof producing equivalent quality FFS. Figure 11 shows the results of four testsrun at different specificmechanical energy levels (SME). At the higher SME inputs acceptable product canbe produced.Destruction levels between 80 to 90 percent are found to be sufficient for trypsininhibitor in most livestockfeeds.



Production of soft-moist pellets are an available option with the UP/C®,giving feed producers even moreflexibility to satisfy consumers and open new markets. With the proper ingredientsincluded in theformula, final moisture and mold growth will not be a concern. The final moisturecan vary from 15 to 20percent when humectants and mold inhibitors are included in the ingredient mixto control water activity.

By-pass protein and by-pass fat are characteristics of heat-treated feeds thatproducers of ruminantanimals desire. By-pass protein is the result of denatured protein. The protein’sreduction in solubility,allows the protein to “by-pass” or escape the rumen and be digestedin the small intestine. By-pass protein can be measured by determining the NitrogenSolubility Index (NSI) (Appendix B) of theprocessed feed. The NSI value represents the amount of protein that is soluble;therefore, the remainingprotein is considered insoluble or by-pass protein (i.e., NSI value = 20 percent,by-pass protein = 80percent).

Lipids included in feeds specified for ruminant animals can interfere with fiberdigestion and even destroynecessary microorganisms thataid with fermentation in therumen. However, by-pass fatescapes the rumen withoutinterfering with the fermentationprocess and is allowed to bedigested downstream. By-passfat is the result of the formation ofa complex between fat and starchor protein that occurs during hightemperature processing. It canbe quantified by determining thedifference between the AcidHydrolysis method (AH) and theEther Extract method (EE) of fatanalysis (i.e., AH - EE = by-passfat). The ether extract methodcannot measure fat that hascomplexed with starch or protein,therefore, it will be the lesser ofthe two values.

CONCLUSION

Feed manufacturers have been bombarded recently with technological advances inthe compound feedprocessing industry. As with any technology, however, continuous developmentbrings about majorimprovements. The UP/C® is a direct result of the rapid increase in demandfor processing equipmentrequired to heat treat and pelletize livestock feeds.

The UP/C® enables feed producers to provide high quality feed with the easeand simplicity of using onemachine. The flexibility provided allows producers to gain greater customer satisfactionby developingnew characteristics into existing feed lines at lower cost. The UP/C® isthe machine of choice forproducers looking toward the next generation in pelleting technology.


Prepared by:

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



¹ Rokey. G. 2001. Pelleting, Conditioning, and Steam Addition. 2001Feed Management Seminar. USEgg and Poultry Association. Nashville TN.

² Wenger Technical Center Test Data. 1996

³ Coelho 1994. Vitamin Stability in Expanders. Feed Management. 45(8). 10-15.

⁴ Strahm B., B. Plattner, G. Huber, and G. Rokey. 2000. Applicationof Food Polymer Science andCapillary Rheometry In Evaluating Complex Extruded Products. Cereal Foods World.45:7. 300-302.

⁵ Strahm B. and B. Plattner. 2001. Put the right tools in your toolboxto ease aquafeed extrusion. FeedManagement. 52:3. 19-22.

⁶ Strahm B. and B. Plattner. 2000. Thermal Processing: Predictingprocessing characteristics of feedmaterials. Feed International. 21:4. 26-29.

⁷ Pellet Durability Index - Laboratory Procedure. 1994. Feed ManufacturingTechnology IV. R.McEllhiney, ed. American Feed Industry Association, Arlington, VA.

⁸ Wenger Technical Center Test Data. 2000. Feeding trials by independent thirdparty.

⁹ Wenger Technical Center Test Data. 1996.

APPENDIX A

Laboratory Procedure - Pellet Durability Index

The following procedure is for measuring the durability of feeds to indicatetheir ability to withstandhandling.

Equipment

Source: Feed Manufacturing Technology IV., 1994. R. McEllhiney, ed. American Feed IndustryAssociation, Arlington, VA.