Physical pellet quality

Durability and hardness

Physical quality is often described as an important factor influencing the overall quality of pet food. The amount of fines and crumbled particles in pet food determines its physical quality. The durability and hardness of pellets are considered the most important factors that are taken into account in quality control prior to further logistics or distribution. Pellet durability is defined as the ability to handle the final product without unacceptable breakage, thus producing dust particles that most likely will not be eaten by the animal. 

Physical quality of pet food

The significant effect of raw material composition and manufacturing technologies on pellet quality is well-documented. It is important to understand all of the factors that influence the physical quality of pet food in order to raise awareness of its final quality. There must be increased control of factors influencing the final physical and nutritional quality. Combining various analytical results rather than using a single analytical approach might help to determine physical and nutritional pellet quality. Such an approach could include water activity (aw) measurements along with durability and hardness analyses.

Non-destructive method

Water activity is an analytical tool for assessing safer, stable pet food products. Water activity measurement defines the equilibrium of relative humidity in a sample at a given temperature. Such analyses are vital for evaluating the microbial growth, texture, flavour stability and chemical reactivity or enzyme activity in a solidified pet food matrix. Stabilized pet food products should have aw values of 0.45. However, scientific evidence correlating aw values and physical characteristics of pet food is scarce. Pet food quality characteristics should be correlated by measuring moisture, aw, hardness and durability. This is a non-destructive method, so the same sample can also be used for measuring the physical characteristics of pellets.  

Durability analyses

Factors influencing the durability of pellets relate to the percentage of different nutrients in the materials and their interaction within the morphological structure of the pellet. Factors influencing optimum pellet durability are the manufacturing technology, production parameters and capacities, and the compaction forces that create the network between nutrients. The mechanical stability of newly extruded pet food pellets has been shown to be lower than that of pellets stored for three weeks. This could be explained by an increased moisture level during storage. In order to improve the physical quality of pet food pellets, the use of the protein cross-linking enzyme transglutaminase or protein-based binders should be considered. The optimum tool for measuring durability should be selected according to the amount of fat used for coating the pellets. Durability analyses free of human error can be successfully carried out on dry pellets using the New Holmen (NH) durability tester. 

However, the NH is difficult to clean and static electricity rapidly builds up due to pellet friction, which increases the likelihood of contaminating the next sample. What’s more, non-coated pellets are not likely to be offered to the pet food market, and thus alternative techniques are needed. Promising techniques that are well known in fish-feed manufacturing are the DORIS tester and the SeedBuro PDI tester. However, running the SeedBuro PDI tester is time-consuming (10 minutes per sample) and it does not represent the real-life dynamics involved in handling pet food, even when hex-nuts or ball bearings are added during analyses. Durability analyses with the DORIS tester are quicker and represent more realistic pellet fracture and dust formation. 

Assessing pellet hardness

The hardness of the pellets affects their appearance and storage properties. Analysing pellet hardness allows us to understand which forces are necessary to break the pellet after manufacturing and therefore helps us to define which ingredients and processes are best to use during manufacturing. The hardness analysis is performed on 30 individual pellets by inserting them between a pressure piston and a static plate. Increasing the pressure of the piston shows us the force needed to break the pellets. All of the techniques mentioned are effective tools for assessing and subsequently maintaining the stability, quality and safety of pet food products before and after manufacturing.