In the production of petfood, extrusion is fundamental, both as a potential bottleneck to factories and as a physiochemical process.
Generally, extrusion is a clean process, one that uses very little water but is energy efficient and productive. Additionally, the machinery is robust and durable. It must comply with the following processes, which will be explored in detail in this article:
- Work flow
- Product safety
- Product digestibility
- Appearance, shape and size
- Hardness and palatability
Work flow
An extruder’s work flow is a primary concern for plant and production managers – the maximum flow rate of an extruder is dependent on its design, but in order to achieve this, certain parameters have to be kept in mind:
- Die exit area: the number of holes in the die determine the exit area. If there are few holes, this can lead to a rise in internal pressure and potentially destabilise extrusion. In worse cases, reverse flow occurs and the product exits through the extruder inlet, rather than through the die. For petfood, the ideal range begins at 200mm per ton, to 400mm per ton
- Extruder threads and sleeves’ wear level: during the extrusion process, it is important to ensure a necessary level of friction and shear, but in doing so, risks wear. It reduces energy transfer, the carrying capacity of the thread, which can then generate backflow and instability. To prevent this, the recommendation is that then space between the thread and sleeve doesn’t exceed the range of 3mm to 6mm, extruder depending. Additionally, the extruder tip should be replaced three to four months in the case where the machine is used at maximum flow for 20 hours a day
Product safety
Owing to extreme pressure and temperature conditions during extrusion, the product that exits the extruder is generally sterile. To monitor these conditions within the barrel you can utilise expensive probes, but the general recommendation is to monitor the temperature of the conditioner. At above 71 degrees celsius, with 18 percent humidity, bacteria are eliminated.
It’s equally important to prevent cross contamination, both with dry flour that has yet to go through the conditioner and by flour residue projected from the conditioner. The recommendation is to work at a steam pressure not exceeding 1.5 BAR. Any potential powder leaks that are sucked up by the pneumatic transport system by the exit of the extruder must be sealed.
Project digestibility
The product digestibility is influenced by protein sources used in the formulation of the food, but the carbohydrates involved are regulated during the extrusion process. Raw starch, for instance, cannot be digested by dogs and cats but gelatinised starch can, which is a process involving the extruder. The gelatinisation of the starch by the end of the process should not be less than 85 percent.
The expansion of the product also influences its digestibility. If the product is compact, for example, it becomes difficult to chew and digest. To ensure adequate expansion, the advice is to work in the exit area, examining the temperature and humidity levels in the extruder and conditioner, and product density:
- The die exit area should not exceed 400mm per ton, so that protein and fat content can increase, thereby increasing the expansion of the product
- Conditioning temperature is advisably a minimum of 93 degrees celsius to ensure rapid starch gelatinisation
- The total humidity level in the conditioner needs to be at a minimum of 24 percent, to allow for adequate expansion
- By measuring the density of the product by the end of the process, you are ensuring it does not exceed 400g/l
Appearance, shape and size
The shape of the product is dependent on the wear of the extrusion dies. If the dies are made of a particularly hard material, they have the ability to last through 5000 tonnes of production before they wear.
The size can also be influenced by the die’s wear but is primarily influenced by the extrusion system’s cutting speed. The faster the cutting speed, the smaller the size of the product. The speed of the rotation of the thread is another factor to consider, in extruders with a variable frequency drive that allows for the handling of different speeds. Increasing the speed of the thread increases expansion, and therefore the size.
Hardness and palatability
The product’s hardness is closely linked to its texture, which directly influences palatability. The texture reflects the internal structure of the food. The size and arrangement of expansion bubbles determine the texture, which can be measured via a texture analyser.
Hard products are less palatable, particularly in the case of dogs. Cats prefer crunchier products, but this does not mean hard. It is recommended to also analyse the interior of kibble through a microscopic, to verify the size and shape of the expansion cells, or ‘bubbles’.
Particularly dense pellets with compact cell structure have difficulties absorbing surface liquid, which proves a problem when it comes to applying fats and flavourings, reducing the impact of the palatants.
What has the greatest impact of the product’s texture is the configuration of the extrusion thread, as this dictates the amount of mechanical energy transferred to the product during the cooking process. High mechanical energy settings are best for low starch and high fat products. Just from this overview of the processes involved in extrusion, it has a huge impact on the quality and performance of the product.
By properly calibrating the mechanical and operational parts of the extruder, we have more control over modifying the final product. Maintenance is highly important, in keeping the parts free of wear, calibrating temperature sensors and flow metres, regulating humidity and controlling the thread configuration properly. These all have the greatest impact on achieving adequate quality.