Extrusion processing is a commonly used processing technology in the food industry with a wide number of applications. It is a processing system that utilizes a single screw or a set of screws to force food materials through a small opening. While food is being forced through the extruder, foods are cooked by the high pressure, high shear, and high temperature environment created by the screws, encased in the barrel. Upon exiting, materials often puff due to the release of pressure and conversion of water into steam. The entire process is continuous and capable of happening in less than a minute. The most commonly used extruders in the food industry include single-screw and twin-screw systems, with twin-screw systems more widely used because of their flexibility. A brief overview of extrusion processing systems is provided in this publication, including applications of extrusion in the food industry, different parts of the extruder, and the concept of extrusion as a multiple input and multiple output processing system. This publication serves as an introduction to the understanding of food extrusion processing.
Extrusion is defined as a system of pushing mixed ingredients out through a small opening, called a die, to form and to shape the materials (Launay and Lisch 1983). The formed products are then referred to as extrudates (Berk 2009). The first extruders for food processing were piston and ram-type extruders for processing meats and sausages (Harper 1981). While still functional today, piston extruders have seen little development, but are still useful in specific applications. In the 1930s, pistons were replaced with a single screw to create a continuous process as opposed to the previous batch process. The single screws were applied to pasta production and revolutionized the food industry with the speed and functionality a screw extruder offered (Ainsworth 2011). Later in the 1960s, twin-screw extruders were established. The increased potential of a twin-screw system led to a diversification of options in food extrusion that expanded into a large variety of new snack and cereal products in the 1980s (Mercier et al. 1989).
Modern extruders now have incredible amounts of variability and functionality. Figure 1 shows a typical food extrusion system. Extruders are popular due to the creation of a rapid, continuous process that can be used in the food industry to make numerous food products such as snacks, breakfast
Figure 2 is a broad chart of what a food extrusion processing production line encompasses. The process begins with characterizing and receiving the raw ingredients. The raw ingredients used are crucial to the product consistency at the end of the processing line. The raw ingredients then undergo mixing and/or preconditioning, which can be done with the equipment such as ribbon blenders and preconditioners to ensure uniformity as they enter the extruder. However, mixing and preconditioning is optional for certain products.