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The traditional definition of extrusion describes it as a process where a plastic material (meaning a material with plastic-like flow characteristics) is shaped or formed by forcing it through a hole or die under elevated pressure conditions.
However, in modern-day Feed and Food Processing the required function of extruders may be described as much more than just pushing a “plastic-like material” through a die or hole.
The reason for that is that we are utilising a multitude of different raw ingredients in Food and Feed Manufacturing, making a very large range of Food and Feed products, such as :
- Enhanced Animal Feed Ingredients (Gelatinised starch, Full Fat Oilseed Meals, Partially Defatted Oilseed Meals, etc).
- Shaped Animal Feeds (Horse Feeds, Pig Starter Feeds, etc)
- Aquatic Feeds
- Animal By-Product Meals
- Human Snack Foods
- RTE (Ready-to-Eat Breakfast Cereals)
- TVP’s (Textured Vegetable Protein) including TSP (Textured Soy Protein); and the list goes on……
This diversity in raw ingredients, along with their specific characteristics and processing requirements, leads to different designs in extruders. In general, extruders are designed to achieve at least the following Operating Conditions and work on the basis of
- Temperature generation by means of friction between feedstock (the ingredients or formulations we are extruding) and / or external heat input by means of steam and / or electrical heater bands;
- specific Pressure conditions, and
- specific Shear Rate conditions
The Diagram above gives a visual description of the Extrusion Process in general:
- Rotating screws convey Feed or Food Material in the direction as shown by arrows.
- Friction between Food or Feed Material and the internal components of the extruder barrel, generates temperauture and pressure.
- The geometry of the screw, barrel housing and choke plate design also delivers very specific shear forces and shear rates.
- The Choke Plates (or steamlocks) act as flow control valves regulating the time material spends in a specific barrel section.
- The barrel housing may have several internal profiles available depending on processing requirements.
- Temperature and pressure is controlled by utilising different options for choke plates; screws and internal barrel housing profile.
- The total retention time in a typical extruder barrel ranges from as short as 5 seconds to around 15 to 20 seconds, depending on set-up and barrel length.
- Food or Feed Extrusion is also referrred to as an HTST (High Temperature Short Time) Thermodynamic Process
- During this very short time temperature and pressure are elevated to what is required for the specific application, while very specific shear rates are generated.
- These operating conditions deals with Anti-Nutritional Factors some feed ingredients may contain while maximising the digestibility of Protein, Fat & Fibre