by Diego Clivio, Clivio Solutions, Argentina
The extrusion process is a thermo-mechanical process that consists of forcing a product to pass through one or more holes of a specific size under pressure and temperature thanks to the Archimedes screw concept (also known as "endless screw").
The extrusion processes in the processing of extruded pet food and aquaculture aim to cook a mixture of raw materials and ingredients, giving it the form of croquette (pellet/kibble) that facilitates its subsequent handling.
The most important processes that take place inside an extruder is the
gelatinisation of starches and texturisation of proteins. Both processes help
aid in greater digestibility, as well as to form a structure and texture that
hosts other ingredients, minerals and fats in a stable and uniform way.
For a good gelatinisation of starches and texturisation of proteins, moisture is needed (which is added in the form of liquid water and steam in pre-conditioning and/or extrusion). Suitable temperatures are also needed, which can be provided through the addition of steam, as well as being produced by the mechanical work and friction produced by the screws. Mechanical energy is also needed, granted by the shearing action of the screws.
Today, there are two technological forms of extrusion, single-screw (SSE) and twin-screw (TSE). In this article we will talk about the features, advantages and disadvantages of each of these technologies.
Single-screw extruders
In single-screw extruders we observe as a characteristic that the screw moves the materials through the channel by developing a “drag flow”, the speed of which is directly proportional to the speed of the screw. Due to the constraint of the die in the discharge of the extruder, there is a "backpressure flow”, which goes in the opposite direction to the drag flow. Therefore, the flowrate in single-screw extruders is equivalent to the drag flow rate minus the backpressure flow rate.
The flow dynamics in the screw channel shows us that the fluid particles travel at different speeds and that they do not interact in their entirety, resulting in a dispersion of dwell times and an uneven mixture.
Read more HERE.
The extrusion process is a thermo-mechanical process that consists of forcing a product to pass through one or more holes of a specific size under pressure and temperature thanks to the Archimedes screw concept (also known as "endless screw").
The extrusion processes in the processing of extruded pet food and aquaculture aim to cook a mixture of raw materials and ingredients, giving it the form of croquette (pellet/kibble) that facilitates its subsequent handling.
For a good gelatinisation of starches and texturisation of proteins, moisture is needed (which is added in the form of liquid water and steam in pre-conditioning and/or extrusion). Suitable temperatures are also needed, which can be provided through the addition of steam, as well as being produced by the mechanical work and friction produced by the screws. Mechanical energy is also needed, granted by the shearing action of the screws.
Today, there are two technological forms of extrusion, single-screw (SSE) and twin-screw (TSE). In this article we will talk about the features, advantages and disadvantages of each of these technologies.
Single-screw extruders
In single-screw extruders we observe as a characteristic that the screw moves the materials through the channel by developing a “drag flow”, the speed of which is directly proportional to the speed of the screw. Due to the constraint of the die in the discharge of the extruder, there is a "backpressure flow”, which goes in the opposite direction to the drag flow. Therefore, the flowrate in single-screw extruders is equivalent to the drag flow rate minus the backpressure flow rate.
The flow dynamics in the screw channel shows us that the fluid particles travel at different speeds and that they do not interact in their entirety, resulting in a dispersion of dwell times and an uneven mixture.
Read more HERE.
The Global Miller
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which is published by Perendale Publishers Limited.
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