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Factors influencing pellet quality

Factors influencing pellet quality
The ultimate indicator of pelleting system performance is the pellet quality analysis and measurements.

By Wilmer Pacheco, Adam Fahrenholz and Charles Stark

Sixty-five years ago, a broiler required roughly 17 weeks to reach 3 pounds (lbs.). Today with the improvements in genetic selection, nutrition, health, management of the growing conditions, and good feed manufacturing practices, a broiler can reach 4 lbs. in around 5 weeks with less feed needed per unit of gain. However, the animal industry continuously faces challenges such as higher ingredient prices or supply chain glitches caused by the global pandemic. Based on current ingredient prices, the value of FCR (unit of feed needed to produce a unit of gain) is becoming more important. One of the initial alternatives is to focus on improving pellet quality; therefore, the main goal of this short article is to discuss some factors that influence pellet quality.

Good quality pellets improve broiler performance and reduce feed wastage, dustiness, selective feeding and nutritional segregation, and bacterial load (contingent on conditioning temperature and retention time). In a nutshell, pellet quality is the ability of pellets to resist disintegration and abrasion during handling, storage, and transportation. At the farm, better pellet quality reduces bridging in silos, increases flowability in feeders, and reduces physical and nutrient segregation as feed moves through the feeding system. Pellet quality is influenced by several factors such as formulation, conditioning temperature and retention time, particle size, pellet die specifications (relationship between die thickness and holes’ diameter), mixer fat addition, and production rate. Fahrenholz (2012) reported that die thickness, conditioning temperature, and mixer fat addition had greater influence on pellet quality compared to production rate or retention time during conditioning. Many of these factors are beyond the control of feed mill managers and pellet mill operators; therefore, we will focus on factors that can be controlled and modified once the mash feed enters the conditioner.

Conditioning is the first step prior to pelleting; steam addition softens feed particles, extracts natural binders, and increases pellet die lubrication which reduces frictional heat (measured as the difference between conditioned mash and hot pellet temperature). Retention time in the conditioner is important for heat and moisture migration into the core of feed particles. However, most feed mills are already configured to have single, double, or triple conditioners and the only option to modify retention time is to adjust the angle of the paddles. Conditioning temperature is easier to modify by adjusting steam mass flow rate into the conditioner (here you need to pay attention to steam pressure and steam valve opening). Smith et al. (2021) reported that as conditioning temperature increased, conditioned mash moisture and pellet quality increased (Table 1). However, as conditioning temperature increases, there is a higher likelihood to damage heat labile nutrients (e.g., amino acids and vitamins) and deactivate exogenous enzymes. A good alternative to measure enzyme stability is to take samples after the mixer and pelleting and evaluate enzyme activity. For instance, if the level of phytase is 500 FTU/kg and 450 FTU/kg after the mixer and pelleting/cooling respectively, then the enzyme stability is 90%. Although exogenous enzymes are very stable during thermal processing, it is important to have a schedule of analysis within the quality control program. One common question is “What is the optimum conditioning temperature?” However, the optimum conditioning temperature is not a “fixed value” as it will be influenced by other factors such as retention time in the conditioner, initial mash moisture, and level of mixer added fat.

Immediately following the conditioning process, conditioned mash enters the pellet press where it is forced through a ring die consisting of multiple cylindrical holes and shaped into whole pellets. Since the conditioning process is not long enough to allow full hydration of feed particles, the moisture remaining on the surface contributes to the formation of wet bonds between feed particles and starch gelatinization which occurs on the surface of pellets. The formation of solid bonds along with starch gelatinization on the surface of pellets contributes to pellet hardness and durability. Pellet dies with a greater compression ratio have a positive impact on pellet quality, but can have a negative effect on production rate. However, if your feed mill only has a single conditioner and it does not have post pellet application systems for fat, one alternative to increase pellet quality is to increase conditioning temperature and compression ratio of the pellet die, either by using thicker dies or dies with small openings.

Pellets leaving the pelleting chamber need to be gradually cooled and dried for safe storage. During cooling, soluble components recrystallize and solid bonds are formed between feed particles. Air-flow and residence time (e.g., pellet bed depth) determines the degree of heat and moisture removal and the temperature and moisture in the finished product. Another factor usually overlooked is the temperature and relative humidity of the ambient air used to cool and dry pellets. High air-flow in combination with low residence time (shallow pellet bed depth) can cause cracks on the surface of pellets and increase their susceptibility to abrasion, negatively affecting pellet durability. Furthermore, if the surface of pellets is cooled quickly, it can prevent moisture migration from the center leading to moist pellets with lower nutrient density. As a rule of thumb, pellets leaving the cooler should not be more than 5°C above ambient temperature and contain ±0.5% of the original mash moisture. During cooling, feed mill managers must focus on the management of bed depth and airflow; for instance, pellet bed depth should be increased if the moisture content of the finished feed is 1% higher than mash moisture. Increasing pellet bed depth will increase contact time between air and pellets, which increases the temperature of the air and its water holding capacity leading to drier pellets. Additionally, you need to monitor temperature and moisture of the finished products if you increase the production rate drastically. Higher or lower production rates will decrease or increase the filling rate of cooler and hence its feed retention time.

The ultimate indicator of pelleting system performance is the pellet quality analysis and measurements which should be conducted on a regular basis, particularly once the system is running steadily, to make opportune adjustments if necessary. To maintain a good balance between pellet quality and poultry performance, it is essential to understand the role that each component of the pelleting and cooling system plays in the overall quality of finished products.

Table 1. Effect of conditioning temperature on moisture content of the conditioned mash and pellet quality

Conditioning temperature, °C

Moisture of conditioned mash, %

Pellet durability index, %

Tumbler

Holmen, 60 s

60

14.06

85

29

75

15.44

86

39

90

16.46

91

58

 

References

Fahrenholz, A. C. 2012. Evaluating factors affecting pellet durability and energy consumption in a pilot feed mill and comparing methods for evaluating pellet durability. Ph.D. Dissertation. Kansas State University, Manhattan, KS.

Smith, L. C., S. M. Bonilla, J. P. Gulizia, M. S. Rueda, F. K. Ovi, J. Escobar, J. Froetschner, and W. J. Pacheco. 2021. Effect of conditioning temperature and pellet die compression ratio on pellet durability index. M52. International Poultry Scientific Forum.

 

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