The Extrusion Process

During extrusion, raw materials are forced to flow under controlled conditions along the length of the extruder barrel and through a shaped opening (called die assembly) at a defined throughput. First, raw materials are commonly ground to the preferred particle size. Frequently they are passed through a preconditioner in which other ingredients are added and steam may be injected. During extrusion the product is cooked and mixed by three separate energy sources: mechanical energy (shear caused by the screw elements), thermal energy that comes from the heating system, and self-heating due to the melt viscosity in the barrel. As the rheological behavior of the dough in the barrel greatly affects finished product quality, it is very important to control temperatures and process times to optimize food quality and heat transfer.

Variables that influence the extrusion process can be separated into three main categories: 1) Raw material composition and formulation (viscosity, moisture content, chemical composition); 2) thermomechanical cooking factors, including technology design (screw profile, length/diameter of the machine) and operating conditions (screw speed, temperature profile, water content, dry mix rate, residence time); and 3) die texturization factors (die design, insert shape, opening section).

There are four main categories of extrusion processes: cold, hot, steam-induced and co-extrusion. Cold extrusion is used to gently mix and shape dough without direct heating or cooking within the extruder. It is used mainly for producing pasta and dough. Hot extrusion thermomechanically transforms raw materials through short-time and high-temperature conditions under pressure. This type of extrusion is used mainly to cook raw materials to produce textured food and feed products. Steam-induced expansion defines the melt expansion at the die exit due to water flashing off, leading to highly expanded products. Subsequent processing then determines the textural attributes of extruded products. Examples of products produced using this type of extrusion are expanded snacks and breakfast cereals. Expanded co-extrusion combines steam-induced expansion and filling injection for expanded products with dual textures (usually crispy shell and soft filling).

Extruder manufacturers must offer highly flexible equipment to meet the broad scope of applications. For example, the material that the extrusion die is made from can have a significant effect on final product properties. Bronze dies are known to produce higher quality, rougher-surfaced pasta products than stainless steel dies. Flexible vertical cutting systems are popular with snack manufacturers to realize new product geometries, including cup-shaped snacks and a wide range of 3D shapes. As product development is usually performed on small-sized machines, it is critical to be able to scale up easily from an R&D machine to a production-sized extruder.