The physical quality of dry pet food

Raw materials and formulation

The source of raw materials and their quality and processing all affect physical pellet quality. For example, ‘feed quality’ wheat shows low Hagberg / falling number (lower viscosity) values from season to season and may contain varying levels of fibre and cereal grits. When used in economy dry pet food, these factors have a major influence on the binding capacity of wheat and can result in high levels of dust and breakage in the final product. Falling number and viscosity monitoring in raw materials can easily be performed with a rapid visco analyser (RVA). The RVA is a cooking, stirring viscometer with ramped temperature and variable shear capability optimised for testing the viscous properties of starch, grain, flour and foodstuffs. With the trend towards high levels of fresh meat included in the dry product, raw material quality is even more important than ever. To maintain the desired product quality, dry raw materials must effectively bind high levels of moisture and contribute to kibble texture.

Particle size reduction and mixing

Particle size distribution is another key quality parameter, as finely ground materials positively influence final kibble appearance and texture. Mixing efficiency and homogeneity is also important, with wide particle size variability causing segregation and separation during pre-extrusion mixing. This also has a negative impact on the nutritional value of the product, as each kibble should have the same proximate analyses. Mixing time should be optimised and monitored at least yearly, as short and long mixing times can cause segregation and affect production costs. Mixing times can often be reduced, thereby lowering costs without affecting homogeneity. Premix particle size can be measured with standard sieving equipment or by more laborious manual sieving. Laser particle-size analysers allow greater data collection on particle shape and size distribution. Mixing homogeneity can be assessed in different ways. Typical feed industry methods, based on the addition of marker substances into the premix, include mineral detection analyses (zinc or yttrium) using quantitative chemical or spectroscopic analysis and Microtracer™ detection methods. The Microtracer™ method can be carried out in-house or at certified laboratories, using iron particles of the same size as the premix (to minimise segregation), coated with food colour. The tracers are collected from the test samples with a magnet and sprayed with ethanol, causing the colour to leach onto filter paper. The number of coloured spots on the filter paper is then quantified.

Processing – conditioning and extrusion

Preconditioning and extrusion affect key parameters including strength, density and kibble cooking. Density control, kibble texture and kibble cook are important as these can affect digestibility and cause diarrhoea in pets.

Kibble texture is also important for palatability, with cats and dogs showing different preferences. Cats are especially sensitive to kibble shape and texture and texture analysis is essential for cat product development. Commercially available texture analysers can measure a wide range of parameters such as kibble hardness, elasticity/chewability, crunchiness, stickiness, etc. 

Post-processing operations

After extrusion, kibbles are dried, coated and cooled before being packed. The conveying process can be long and the kibbles are subjected to constant stress that weakens them, resulting in dust and breakage. Dust is an important quality parameter that affects product performance (pets don’t eat dust!) and nutritional value. It also causes plant hygiene problems. Dust is a total loss for the producer and pet owner. Levels in factories range from 0.2 – 1% or even 2%. Dust and broken kibbles delivered to the pet owner can generate an additional 1-5% waste (internal data). It is therefore important to minimise dust, both in the factory and with the owner. Some producers use in-line dust collection systems to minimise loss through rework. However, high rework levels can change product analysis and nutritional performance and must be monitored.Durability can also be measured with a range of equipment, including the Holmen tester (most common in Europe) and the DORIS durability tester. These should be selected for their ability to measure different shaped kibbles and fat contents. For example, the DORIS tester cannot analyse small, flat kibbles. Dust can be tested using sieving (for gentle removal of surface dust) and the more aggressive DORIS test method, which simulates the effect of longer handling / conveying times. Already widely used in fish and animal feed manufacture, physical measurements can bring many advantages to pet food manufacture. Extensive research is enabling the fish and animal feed industries to stay focused on best practices for maintaining product quality and satisfied customers. 

*source: (Hancock and Bennke,2001,