In the poultry industry, for example, pelleting has become the most extensively used thermal processing method in feed production, based on the use of pelleting to agglomerate ingredient particles by mechanical action, in combination with moisture, pressure and temperature. Pelleted diets, produced in this way, have been shown to increase feed intake, feed conversion ratio and weight gain, when fed to broilers. Further analysis indicates that the positive effect of pelleting on broiler performance is due, at least in part, to improved nutrient digestibility, increased feed consumption and increased resting time in the broilers being fed, resulting in lower energy expenditure by birds during their feeding activities(Kiarie and Mills, 2019).

However, this better performance can only be achieved if the pellets being fed to birds maintain their integrity until they’re eaten. In this context, pellet quality is defined according to the ability of the pellet to resist fragmentation and abrasion during handling without breaking up, enabling the pellet to reach feeders without generating a high proportion of fines or fragments (Keysuke Muramatsu et al., 2015).

Pellet durability index (PDI) is one of the key parameters used to determine pellet quality, indicating the percentage of pellets that remain intact after being exposed to mechanical forces. Pellets are subject to friction, impact and pressure during storage, transport and dispatch from the feed mill to farms. Poor-quality pellets disintegrate under such pressures, resulting in feed consisting of fewer complete pellets and an increased percentage of fines. If, in such circumstances, the geometric mean diameter (GMD) of fine particles is equal or lower than that of mash diets, then these particles may cause a nutritional imbalance in feed chemical composition, potentially having a negative impact on animal performance. The evaluation of diets with different combinations of pellets and fines, showing that birds fed a 100% pellet diet achieved the highest weight gain and feed conversion figures while the worst results were seen in birds which were fed 100% fines (Keysuke Muramatsu et al., 2015).

Analysis of the range of factors influencing pelleting quality, indicate that heat processing and feed formulation, especially fat inclusion levels, are the biggest end product influencers. That was the conclusion reached by researchers at Brazil’s Universidade Federal do Paraná, Curitiba, and Universidade Federal de Santa Catarina. They also suggested that alternative procedures, such as the expansion process, increasing moisture level addition, fat inclusion restriction or post pellet liquid fat application, could be incorporated into feed milling to produce high physical quality pellets.

Meanwhile, analysis by R. Yasothai of India’s Tamilnadu Veterinary and Animal Sciences University, focusing on fat free and low-fat diets, found that using the highest possible conditioning temperature improved pellet durability and reduced pellet mill energy consumption. It also improved pellet durability in high fat diets (Yasothai, 2018).  In addition, increased steam conditioning was found to have led to a decrease in mechanical friction during pelleting, decreased electrical energy consumption and improved pellet durability.

The primary objective of pelleting, of course, is to produce a palatable, high quality product with minimum production expense. Factors influencing pellet quality, such as grinding, mixing, conditioning, pelleting techniques and cooling are therefore important and potentially fertile areas for securing further pellet quality improvements (Figure 1). This includes pellet press settings, which have been shown to impact on the technical quality of pellets and therefore the profitability of the pelleting process.