Effect of Ingredients and Processing Parameters on Pellet Quality J. L. BRIGGS,* D. E. MAIER,*,1 B. A. WATKINS,† and K. C. BEHNKE‡ *Department of Agricultural and Biological Engineering and †Department of Food Science, Lipid Chemistry and Metabolism Laboratory, Purdue University, West Lafayette, Indiana 47907-1146 and ‡Department of Grain Science and Industry, Kansas State University, Manhattan, Kansas 66506-2201 ABSTRACT Rations containing varying ratios of corn, high-oil corn, soybean meal, and mechanically expelled soybean meal were pelleted. The effects of ingredients, conditioning steam pressure, and mixing paddle configuration inside the conditioner on pellet quality were investigated. Ration ingredients strongly affected pellet quality. Increasing the protein content increased the pellet durability, whereas increasing the oil content above 7.5% greatly decreased pellet durability. High-oil corn and me-
chanically expelled soybean meal produced acceptable pellets when combined with soybean meal and regular corn, respectively. However, poor pellet quality resulted when rations containing high-oil corn and mechanically expelled soybean meal were processed. Increasing the residence time in the conditioner by changing mixing paddle pitch resulted in an average 4.5-point increase in pellet durability indices among 65:35 (wt) corn:soybean meal and 65:35 high-oil corn:soybean meal rations.
(Key words: pelleting, feed manufacutring, high-oil corn, expelled soybean meal) 1999 Poultry Science 78:1464–1471
INTRODUCTION Pelleted feeds are fed nearly exclusively to broilers and turkeys (Behnke, 1998). To the integrator, the benefits of pelleting include enhanced handling characteristics of feeds and improved animal performance. Pelleting increases bulk density and flowability and decreases spillage and wind loss. Improved weight gain:feed ratios from feeding pellets as compared with mash have been documented (Hussar and Robblee, 1962; Hull et al., 1968; Runnels et al., 1976; Proudfoot and Sefton, 1978; Choi et al., 1986). Reasons for the enhanced performance may be due to increased digestibility, decreased ingredient segregation, reduction of energy during prehension, and increased palatability (Behnke, 1998). Feeding pelleted rations is not enough to ensure enhanced performance of poultry. The quality of pellets must be taken into account also. Research has shown that feeding poor quality pellets diminishes the benefits of pelleting. Feed conversion increased 2.4% when broilers were fed a combination of 75% pellets and 25% fines as compared with 25% pellets and 75% fines (Scheideler, 1991). Similar performance trends were found in turkeys (Proudfoot and Hulan, 1982; Salmon, 1985). Considering the immense gains of feeding quality pellets, limited published research is available that focuses
on understanding and optimizing the pelleting process. Skoch et al. (1981) determined quantitatively the importance of steam conditioning. Rations were pelleted with steam conditioning and without. Steam increased production rates by up to 64% and increased pellet durability indices (PDI) by up to 26%. The feed temperature across the die decreased by 5 C when rations were conditioned with steam prior to pelleting, compared with an increase of up to 42 C when rations were pelleted dry. These results indicate that steam acted as a lubricant. The effect of steam operating pressure has often been a source for controversy in the feed industry (Maier and Gardecki, 1992). Stevens (1987) investigated the effect of steam pressure (20 and 80 psig) on pellet quality. Steam pressure showed no significant effect on pellet durability or production rate. Stevens (1987) also evaluated the effect of particle size. No significant differences were found in PDI from pellets made with coarse (1,023 µ), medium (794 µ), or fine (551 µ) particles of a primarily corn (72.4%) and soybean (20.0%) ration. Pellet durability increased with medium and fine particles of a mainly wheat (72.4%) and soybean (20.0%) ration. For both rations, pellet mill efficiency decreased with decreasing particle size by up to 12%. Ingredients strongly influence pellet quality. Wood (1987) investigated the impact of raw vs denatured protein and raw vs pregelatinized corn starch on pellet durability and hardness. Rations of soy protein isolates and starch were pelleted with and without steam conditioning. Re-
Received for publication September 25, 1998. Accepted for publication June 18, 1999. 1 To whom correspondence should be addressed:
[email protected]
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Abbreviation Key: PDI = pellet durability index.
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PELLET QUALITY
gardless of the conditioning scheme (with or without steam), rations containing raw protein produced dramatically stronger pellets than those containing denatured protein. Starch (raw or pregelatinized) had a minimal effect on pellet quality when pelleted with raw protein. The average PDI for rations containing 40% raw starch:60% raw protein was 85, and, for rations containing 40% pregelatinized starch:60% raw protein, the average PDI was 94. The effect of pregelatinized starch was more prominent when pelleted with denatured protein. The average PDI for rations containing 40% raw starch:60% denatured protein was 19, and, for rations containing 40% pregelatinized starch:60% denatured protein, the average PDI was 70. The strong influence of protein is supported further through work by Winowiski (1988) and Stevens (1987). Increasing the quantity of wheat in rations, which increased the overall protein content, resulted in an increase in the PDI from 32 to 73 when the wheat content increased from 0 to 60%, respectively (Winowiski, 1988). In the study by Stevens (1987), rations containing 72.4% wheat as compared with 72.4% corn produced pellets with PDI an average of seven points higher. These studies clearly reveal the important role of protein in pellet quality. However, a majority of feed industry representatives appear to be primarily concerned with the effect of starch gelatinization and its role in pellet durability. Research investigating starch gelatinization caused by steam conditioning and pelleting is conflicting. Stevens (1987) investigated the extent of starch gelatinization in 100% corn rations. Results showed that, in the outer portion of the pellet, 58.3% of the starch gelatinized when the ration was pelleted dry and 25.9% when the ration was steam conditioned to 80 C prior to pelleting. It was proposed that mechanical shearing in the pellet die, which causes frictional heating, was the cause for the extensive gelatinization. The lubricating effect of steam conditioning decreased friction through the die, causing a decrease of starch gelatinization. Heffner and Pfost (1973) and Skoch et al. (1981) estimated that the extent of gelatinization caused by steam conditioning was 10 to 12 and 0%, respectively. Both studies used a maltose equivalency method, which from a cereal science viewpoint is questionable. Stevens (1987) did not investigate the amount of starch gelatinization caused by conditioning. With the emergence of high-oil corn as a relatively new feed ingredient, less supplemental fat may need to be added to feed rations. The oil contents of regular dent
corn and high-oil corn are 3.5% and 6 to 8% (dry basis), respectively. Observed benefits of feeding high-oil corn in poultry and livestock include greater feed and gain efficiencies (Bartov and Bar-Zur, 1995; Adeola and Bajjalieh, 1997). Richardson and Day (1976) found that the addition of fat above 2% in a corn-soy broiler finisher diet produced unacceptably high levels of fines and low PDI. Considering the detrimental effects of oil on pellet quality, research is needed to investigate the effect of high-oil corn on pelletability. Mechanically expelled soybean meal is another emerging feed ingredient containing a relatively high amount of oil. Compared with solvent-extracted soybean meal, it contains about 5% more oil. The production of mechanically expelled soybean meal has provided opportunities for implementing rural value-added processing operations. No data are available in the literature on the pelleting behavior of rations with expelled soybean meal. Furthermore, a knowledge of the pelleting characteristics of expelled soybean meal would be of value to exporters, who would like to increase the bulk density of meal shipments. The objective of this research was to evaluate the pelleting characteristics of rations containing high-oil corn, regular dent corn, mechanically expelled soybean meal, and chemically extracted soybean meal. In addition, the effects of steam pressure and blade pitch in the conditioner on pellet quality and milling efficiency were investigated.
MATERIALS AND METHODS Experimental Method Rations investigated are listed in Table 1. Ration 1 included 3% soybean oil in the corn ration. The added soybean oil acted as a lubricant in the pellet die to prevent excessive wear on the pellet mill motor and die. The primary purpose of pelleting 100% corn was to evaluate the extent of gelatinization occurring inside the conditioner using differential scanning calorimetry without interference from soybean proteins in future studies. Varying levels of soybean meal blended with ground corn were steam-conditioned and pelleted to evaluate the effect of protein on pellet quality. Rations containing highoil corn and expelled soybean meal were processed to
TABLE 1. Ingredients in pelleted rations
Ration
Corn
High-oil corn
1 2 3 4 5 6
94 73 63 — 63 —
— — — 63 — 63
Soybean meal — 24 34 34 — —
Expelled soybean meal
Soybean oil
Vitamin premix
— — — — 34 34
3 — — — — —
3 3 3 3 3 3
(%)
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BRIGGS ET AL. TABLE 2. Unbalanced experimental block design (X represents repeated experiments) Standard pitch
Ration 1 2 3 4 5 6
552 kPa X X XXX XXX X X
138 kPa X X XXX XXX X X
TABLE 3. Proximate analysis results of the rations based on 12% moisture
Parallel pitch 552 kPa X X XXX XXX X X
Manhattan, KS 66506-2201. Crawfordsville, IN 47933.
3
Protein
1 2 3 4 5 6
8.7 16.3 21.0 21.2 20.3 20.3
138 kPa X X XXX XXX X X
determine the pelleting characteristics of these emerging feed ingredients. Experiments were conducted at the Kansas State University2 pilot feed mill. Regular dent corn and high-oil corn were ground through a 3.2-mm hammermill screen. Then, the ground corn was mixed with the remaining ingredients in a 450-kg horizontal ribbon mixer. The mash was conditioned to a temperature near 77 C. A Master Model California Pellet Mill3 equipped with a 50.8-mm thick die having a 4.8-mm bore diameter was used for pelleting. Pellets were cooled in a California Pellet Mill3 Vertical Cooler. All experimental runs were performed using a warm die. Maximum production rates were obtained for all trials by holding the motor load of the pellet mill at 90%. The mash was conditioned to 77 C by adjusting the steam flow rate. Two conditioner blade pitches were used. In the standard setting, all mixing paddles were set at about a 45 degree forward angle. This procedure mixes and tosses the ration forward in the conditioner. For the second pitch, all mixing paddles, with the exception of the first and last set, were parallel with the conditioner shaft (parallel pitch). The goal of the second setting was to increase the residence time in the conditioner by increasing the fill volume and decreasing the forward tossing momentum. For the standard setting, the mean residence time inside the steam-conditioning chamber was estimated at 5 s, and the residence time for the parallel pitch was estimated at 15 s. The experimental design followed an unbalanced block design because of constraints (Table 2). The replicated runs performed in triplicate of Rations 3 and 4 were of primary interest in this study. However, the nonreplicated runs established the pelletability of a relatively new ingredient, mechanically expelled soybean meal, and general trends of the effect of oil and protein content on pellet quality. The effects of conditioner blade pitch configuration (standard and parallel) and steam pressures, 138 and 552 kPa (20 and 80 psig), were investigated. Replicated runs (Rations 3 and 4) and nonreplicated runs (Rations 1, 2, 5, and 6) were treated as separate experiments for statistical analysis. Separate analysis of variance (AN-
2
Ration
Oil (%) 6.2 3.1 2.9 4.9 5.6 7.5
OVA) was used to evaluate the results (pellet durability, temperature rise across the die, moisture gain of the ration caused by conditioning, and mill efficiency) of replicated and nonreplicated experiments. Because there was only one observation for each treatment for the nonreplicated experiment, no variability estimate within treatment was available. Therefore, the three-way interaction mean square term was assumed to be not significant based on statistical analysis of the replicated experiments and was used as the error estimator.
Data Collection Pellet durability index was evaluated at Kansas State University for each experimental run using procedures detailed in ASAE Standard S269.4 (ASAE, 1997a). Samples of the dry mash (preconditioned), conditioned mash, and pellets were collected for each experimental run for moisture analysis using the method described in ASAE Standard S269.4 (ASAE, 1997a). The mean diameters of ration particles were determined following methods established in ASAE Standard S319.2 (ASAE, 1997b). To obtain the temperature rise across the die, temperature measurements were taken prior to and immediately after the pellet die once the batch was operating at steady state. A stiff thermocouple was placed in the stream of the conditioned mash as it left the conditioner. To measure pellet temperature, pellets were collected in a foam-
FIGURE 1. Mean pellet durability index for rations (averaged over all treatments).
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PELLET QUALITY TABLE 4. Pellet durability indices for each treatment Standard pitch
Parallel pitch
Ration
552 kPa
138 kPa
Average
552 kPa
138 kPa
Average
1 2 3 4 5 6
28.6 72.0 85.8 79.2 88.1 35.3
55.8 63.3 86.8 84.9 88.4 77.1
42.2 67.6 86.3 82.0 88.2 56.2
75.8 83.9 91.1 85.1 87.0 64.3
68.5 84.1 91.4 86.8 87.6 61.5
72.2 84.0 91.2 86.0 87.3 62.9
insulated pail, and the temperature was taken using a stiff thermocouple after the temperature reading reached equilibrium. Production rate was calculated by dividing the weight of pellets (less fines) produced during the run time by the run time. Run times averaged 6 min. Fines were sifted, collected, and weighed separately. The production rate of fines was determined by dividing the weight of fines by the run time. Both pellets and fines were collected after the pellet cooler. Pelleting efficiency, expressed as kilowatthours per metric tonne (kWh/MT), was determined from voltage and amperage meter readings and production rate.
RESULTS AND DISCUSSION Pellet Quality The ANOVA indicated that main effects of ration and pitch and the two-way effect of pressure × ration interaction significantly affected PDI. However, when data from nonreplicated runs were analyzed, the results indicated that ration was the only factor significant at P < 0.05. The mean squared error term, which is based on variability within treatments, was calculated to test significant effects from replicated observations of Rations 3 and 4. If Rations 1, 2, 5, and 6 had been replicated and a mean squared error term had been determined based on this data also, statistical results similar to those found in the analysis of replicated observations would have been likely. Clearly, trends in the data suggest this. Increasing the protein content increased the pellet durability. From proximate analysis (Table 3), the protein contents of Rations 2 and 3 were 16.3 and 21%, corresponding to an average pellet durability of 75.8 and 88.8, respec-
tively (Figure 1). Similar results were observed by Winowiski (1988) and Stevens (1987). In contrast to the effect of increasing protein, increasing the oil content had a negative effect on pellet quality. From Table 3, the oil content of Rations 3, 4, 5, and 6 varied from 2.9 to 7.5%, and the protein content ranged from 20.3 to 21.0%. Among Rations 3, 4, and 5, the average PDI for all treatments showed little change, ranging from 84.0 to 88.8 (Table 4 and Figure 1). However, the pellet durability decreased greatly as the oil content reached 7.5%, as was found for Ration 6. Ration 1 had an oil content of 6.2% because of soybean oil added to the ration, which resulted in an overall average PDI of 57.2. The low PDI for Ration 1 was due to a combination of low protein and high oil content. Based on the findings of this work, pellet quality is not compromised when the oil content is below 5.6% and the protein content is about 20%. Richardson and Day (1976) evaluated the effects of adding animal fat to a corn-soy broiler finisher diet that originally contained 2.9% fat. After the addition of fat to the rations prior to conditioning, the fat contents ranged from 3.9 to 7.9%, with corresponding PDI ranging from 82.0 to 49.2, respectively. Fat content totaling above 4.9% yielded poor quality pellets. Our research suggests that an oil content limit is slightly higher than previously reported. Unfortunately, the ratio of corn:soybean meal was not stated in the work by Richardson and Day (1976); therefore, it is not possible to make a direct comparison between their data and ours. The degree of pitch on the mixing paddles (residence time) also affected pellet quality (Table 4). The PDI produced with the parallel pitch paddles averaged five points higher than durabilities of pellets conditioned using the standard pitch. The longer residence time obtained using the parallel pitch allowed the mash to be more adequately
TABLE 5. Rate of fines produced Standard pitch
Parallel pitch
Ration
552 kPa
138 kPa
Average
1 2 3 4 5 6
311 107 122 107 80 178
120 125 71 93 125 130
216 116 96 100 102 154
552 kPa
138 kPa
Average
116 112 86 88 82 141
100 116 72 109 69 136
108 114 79 98 76 138
(kg/h)
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BRIGGS ET AL.
FIGURE 2. The negative relationship of the rate of fines produced to the pellet durability index (R2 = 0.58).
conditioned prior to pelleting. Based on these findings, operators should adjust the configuration of the mixing paddles to obtain an optimum residence time to produce quality pellets. Alternative methods for increasing the residence time are to decrease the rotational speed of the mixing paddle axle or to invest in longer length conditioners. The amount of fines correlated negatively with the PDI (Table 5 and Figure 2). Rations resulting in relatively poor quality pellets (Rations 1 and 6) produced, on average, 57% more fines. Ration 3 containing high-oil corn and solvent-extracted soybean meal produced similar amounts of fines as Ration 2, which contained regular dent corn and solvent-extracted soybean meal in the same proportions. The rate of fines produced was also correlated with PDI. Moisture addition, temperature rise across the pellet die, and mill efficiency did not correlate with PDI.
Moisture Addition Analysis of variance results for replicated runs (Rations 3 and 4) indicated that ration significantly affected the amount of moisture gained during conditioning (P < 0.05). Pressure, pitch, and higher order interaction terms were not significant. Analysis of the nonrepeated runs showed that the effect of pressure was statistically significant. The average moisture gains of rations processed using 138
and 552 kPa were 3.35 and 3.71%, respectively. However, it is believed that the small difference of 0.36 points would not impact the process or the quality of the product significantly. Moisture is added to the mash in the conditioning chamber by direct steam injection. The average moisture gain for all rations ranged between 3.3 and 4.3 points, a relatively small differential (Table 6). For high starch feeds containing 50 to 80% starch, the moisture gain may be increased by up to six points when the mash is conditioned to 82.2 C (Maier and Gardecki, 1992). Numerous factors affect the amount of moisture added, including the absorption diffusivity of the ration ingredients, steam quality, steam quantity, degree of mixing during conditioning, and the conditioning chamber dimensions. Steam is used not only to increase the mash moisture but also to increase mash temperature. In this study, all rations were conditioned to 77 C by adjusting the steam flow rate. We hypothesize that the increase in mash temperature caused by conditioning softens the protein polymers and may cause some starch gelatinization. Unfortunately, the pilot plant feed mill is not equipped with a steam flow meter; therefore, the exact steam flow rate is unknown. We recommend that mills be equipped with steam flow rate meters to permit closer monitoring of the conditioning process.
Temperature Rise Analysis of data from repeated experimental runs (Rations 3 and 4) indicated that the main effects of ration and pitch and the two-way interaction of ration × pitch were statistically significant (P < 0.05). As found by analyzing the effects of moisture addition, results from the nonreplicated runs revealed that temperature rise across the die was affected by steam pressure. However, without replications of Rations 1, 2, 5, and 6, statistical inferences implying that the main effect of pressure is significant may not be valid. In previous research by Stevens (1987), the temperature rise across the die was correlated with degree of gelatinization. Furthermore, condensed steam acts as a lubricant to decrease frictional heating in the die, causing the reduced temperature differential between conditioned mash and pelleted feeds. In our research, the temperature rise across the die ranged from 0.1 to 7.9 C (Table 7). On average, the temperature rise that resulted from mash conditioned using the parallel pitch was 2.8 C higher than
TABLE 6. Percent moisture gain caused by conditioning Standard pitch
Parallel pitch
Ration
552 kPa
138 kPa
Average
552 kPa
138 kPa
Average
1 2 3 4 5 6
4.1 3.2 3.6 3.7 3.1 4.2
3.0 4.0 3.8 4.6 3.1 3.2
3.6 3.6 3.7 4.1 3.1 3.7
3.8 3.8 3.9 3.6 3.4 3.7
2.9 3.4 3.7 5.2 3.7 3.5
3.4 3.6 3.8 4.4 3.6 3.6
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PELLET QUALITY TABLE 7. Temperature rise (C) across the pellet diet Standard pitch
Parallel pitch
Ration
552 kPa
138 kPa
Average
552 kPa
138 kPa
Average
1 2 3 4 5 6
2.2 2.4 2.5 2.6 4.3 0.1
2.6 5.4 2.5 2.7 7.6 4.4
2.4 3.9 2.5 2.6 6.0 2.3
4.4 4.6 7.2 4.2 5.1 3.2
7.5 5.6 7.2 5.1 4.4 4.9
5.9 5.1 7.2 4.7 4.8 4.0
the temperature rise from mash conditioned using the standard pitch. The additional residence time in the conditioner for the parallel pitch might have allowed the moisture from condensing steam to leave the particle surface and penetrate the particle. Reducing the surface moisture resulted in less lubrication effect across the die. Hence, the increase in temperature was observed. It was expected that increasing oil content in the ration, which would also act as a lubricant, would decrease the temperature rise across the die. However, this trend was not consistent (Table 7). Among treatments, Ration 5 had an average temperature rise of 2.9 C, whereas Rations 4 and 6 had average rises of 3.6 and 3.2 C, respectively. This result showed that oil and moisture gain are not solely responsible for limiting the temperature rise, because Rations 4 and 5 contained similar quantities of oil. Therefore, it is proposed that particle size may affect the observed temperature rise. The mean diameters of Rations 5 and 4 were 1,030 vs 890 µ, respectively, which is a 14% difference (Table 8). The increase in particle size would explain the higher temperature rise caused by greater frictional heating in the die.
Mill Efficiency Statistical analysis (P < 0.05) indicated that ration and pitch were the only significant factors affecting mill efficiency of replicated runs (Rations 3 and 4). For the nonreplicated runs (Rations 1, 2, 5, and 6), pitch was the only significant main effect. It was expected that increasing the oil in the rations would improve milling efficiency; however, a consistent trend was not observed (Table 9). The average milling efficiencies for Rations 4 and 6 (all treatments) were 10.5 and 10.2 kWh/MT, wheras the average milling efficiency for Ration 5 was 13.7 kWh/MT. It is not known why
TABLE 8. Mean diameter of particles Ration
Mean diameter
SD
1 2 3 4 5 6
(µ) 1,023 1,020 946 881 1,029 1,052
50 77 95 52 52 44
Ration 5 required more energy to pellet. Comparison of production rate and particle size revealed no consistent cause. The mill demanded less energy, by up to 17% on average, to pellet rations when the standard pitch was used during conditioning as opposed to the parallel pitch. Again, the increased residence time (parallel pitch) may allow moisture to penetrate the particle, resulting in less surface moisture. As a result, more energy was used in pelleting because of the decreased lubrication effect caused by surface moisture.
Production Rate Statistical analysis showed that pressure, pitch, ration, and higher order interactions did not significantly affect the rate of production for replicated or nonreplicated runs. The production rate averaged 1,180 kg/h with a standard deviation of 190 kg/h for all experimental runs (Table 10). Generally, these production rates had no effect on the PDI, with the exception of Ration 1. A production rate of 1,460 kg/h was calculated for Ration 1, standard pitch, and high pressure. The unusually high production rate might have prevented the mash from becoming adequately conditioned. Even though the pellet mill was able to handle this large production rate, the pellets were lower in quality.
Steam Pressure The effect of steam pressure was not statistically significant when evaluating pellet durability, moisture gain, temperature rise across the die, mill efficiency, or production rate for repeated runs (Rations 3 and 4). These results agree with the findings of Stevens (1987). The reason for lack of effect may be found by examining the thermodynamic properties of steam. Assuming that the steam enters the conditioner as saturated vapor (100% quality), the enthalpies of steam at 138 and 552 kPa are 6.45E5 and 6.58E5 cal/kg, respectively. Enthalpy represents steam energy that can be transferred to the mash to raise the mash temperature. There is only a 2.2% enthalpy difference between the two pressures investigated. Therefore, we recommend that operators run steam pressure between 138 and 552 kPa, such as 241 to 276 kPa. Often, steam systems cannot adequately separate condensate collecting in the pipes at low pressures (138 kPa), causing excessive water in the mash, which may lead to plugging
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BRIGGS ET AL. TABLE 9. Mill efficiencies for pellet mill operating at 90% of motor load Standard pitch
Parallel pitch
Ration
552 kPa
138 kPa
Average
1 2 3 4 5 6
14.9 11.7 12.0 9.9 11.3 7.5
12.6 10.2 11.5 8.1 12.6 10.6
13.8 11.0 11.8 9.0 12.0 9.0
of the pellet mill. Running pressures at the other extreme, 552 kPa, is needlessly wasting energy required by the boiler. The two-way interaction of pressure × ration affected (P < 0.05) PDI. However, the importance of a two-way interaction must be evaluated on a case-by-case basis (Neter et al., 1990). Plotting the PDI vs pressure for each ration revealed that the lines are nearly parallel to each other, indicating an unimportant interaction. Moisture addition caused by conditioning and temperature rise across the die both were affected by pressure for nonreplicated runs. Because of the statistical design of these experiments (one observation per treatment), little emphasis should be put on the significance of pressure based on the lack of effect of pressure found in the replicated experiments. However, running the nonreplicated rations did reveal information regarding the pelletability of high-oil corn and mechanically expelled soybean meal.
Conclusions Producing quality pellets is largely thought of as an art rather than science by many feed mill operators. This study demonstrates that the process may be manipulated to produce quality pellets. Increasing the residence time of the mash inside the steam conditioner, which may be accomplished by adjusting the mixer paddle configuration, slowing the rotational speed of the mixing paddle shaft, or investing in longer conditioners increased PDI. From repeated observations (Rations 3 and 4), our research revealed that adjusting the mixing paddle pitch caused the PDI to increase by up to 4.5 points. Ration ingredients strongly affected pellet durability. Increasing protein content caused an increase in pellet quality, whereas increasing oil content resulted in a de-
552 kPa
(kWh/MT) 16.5 14.2 12.7 12.1 15.6 12.6
138 kPa
Average
11.4 15.9 13.1 12.1 15.4 10.3
14.0 15.0 12.9 12.1 15.5 11.4
crease in pellet quality. In this study, the upper oil limit to obtain acceptable pellets was about 5.6% when the protein content was about 20%, which was representative of Ration 5. Although Ration 5 was not repeated in the unbalanced design, the pellet durability was consistent across all treatments. Evaluating the combination of oil and protein in a ration may give feed mill operators an indication of the expected pellet quality prior to processing. The objective of the nonreplicated experiment (Rations 2, 5, and 6) containing high-oil corn and expelled soybean meal was to evaluate a general trend of the pelletability of these emerging high energy feed ingredients. The pelletability of both ingredients depended on the other ingredients in the ration. When high-oil corn was pelleted in about a two-thirds ratio with solvent-extracted soybean meal, the pellet durability decreased only slightly compared with a ration having the same ratio of regular corn to soybean meal. Similar results were found when expelled soybean meal was mixed at about a one-third ratio with regular corn. However, pelleting high-oil corn and mechanically extracted soybean meal together greatly reduced pellet quality. This work showed that high-oil corn may be successfully pelleted if, when combined with the other ration ingredients, an upper oil limit (5.6%) is not exceeded. However, this limit may depend on residence time in the conditioner and protein content.
ACKNOWLEDGMENTS We would like to thank the U.S. Poultry & Egg Association for their support. We also appreciate the assistance of Jared Froetschner. He provided valuable advice with regard to pelleting experiments conducted at the Grain
TABLE 10. Production rate determined after pellet cooling Standard pitch
Parallel pitch
Ration
552 kPa
138 kPa
Average
1 2 3 4 5 6
1,460 1,150 1,400 1,230 1,080 1,110
1,100 1,110 1,180 920 860 1,170
1,280 1,130 1,290 1,080 970 1,140
552 kPa
138 kPa
Average
1,160 2,000 1,250 1,290 1,190 1,260
940 1,260 1,070 1,230 1,180 1,160
1,050 1,630 1,160 1,260 1,180 1,210
(kg/h)
PELLET QUALITY
Science and Industry Department, Kansas State University.
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Maier, D. E., and J. Gardecki, 1992. Feed mash conditioning field case studies. Paper No. 92-1541. Am. Soc. Agric. Eng., St. Joseph, MI. Neter, J., W. Wasserman, and M. H. Kutner, 1990. Applied Linear Statistical Models, 3rd Ed. Irwin, Boston, MA. Proudfoot, F. G., and H. W. Hulan, 1982. Feed texture effects on the performance of turkey broilers. Poultry Sci. 61:327–330. Proudfoot, F. G., and A. E. Sefton, 1978. Feed texture and light treatment on the performance of chicken broilers. Poultry Sci. 57:408–416. Richardson, W., and E. J. Day, 1976. Effect of varying levels of added fat in broiler diets on pellet quality. Feedstuffs 48(20):24. Runnels, T. D., G. W. Malone, and S. Klopp, 1976. The influence of feed texture on broiler performance. Poultry Sci. 55:1958–1961. Salmon, R. E., 1985. Effects of pelleting, added sodium bentonite and fat in a wheat-based diet on performance and carcass characteristics of small white turkeys. Anim. Feed Sci. Technol. 12:223–232. Scheideler, S. E., 1991. Is pelleting cost effective? Feed Management 46(1):21. 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. 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, KS. Winowiski, 1988. Wheat and pellet quality. Feed Manage. 39 (9):58–64. Wood, J. F., 1987. The functional properties of feed raw materials and their effect on the production and quality of feed pellets. Anim. Feed Sci. Technol. 18:1.
