Extruders are used for making a wide range of food products. Their design and specifications should be tailored for the intended application, from simple forming applications such as pasta products, to very short cooking extruders for corn curls, to more complex and long extruders involving multiple operations and resulting in significant modifications of the extruded material. The extruders feed material may include a single solid or many solid and liquid ingredients. The solid ingredients are mostly grain, pulse, and root-based foods, sweeteners, minerals, and vitamins. They are utilized in a variety of particle size distributions. The liquids include water (almost always), oil, liquid sweeteners, and colors. The ingredients are typically mixed, transformed into a melt/dough, and formed into the desired product shape. There is a great deal of interaction between the feed ingredients, extruder design, and operating conditions. Understanding the extruder design principles in a simplified and practical approach should lead to faster product development, quality improvement and consistency, processing efficiency, and optimum operation.
There are two major types of extruders; single and twin screws (co-rotating and counter rotating). These come with a wide range of screw diameters (D), lengths (L), and designs. The single screw and co-rotating twin screw are inherently axially open-channel extruders. They can be regarded as drag flow pumps. Their output or degree of fill (if not running at maximum volumetric rate) can be impacted by the pressure flow within the extruder. Closely intermeshing counter-rotating twin screw extruders form closed channels in the intermeshing region. Their output is less vulnerable to the pressure flow within the extruder. As such, they can be considered as positive displacement pumps.
This chapter intends to build on a previous chapter also written by Yacu (2012). It aims to break down the extrusion process to discrete sections, describe their function and operating parameters, address the process needs, and provide a logical and simple approach to the selection and design process. Extruder design should improve with experience and better understanding of the material characteristics, rheology, transformation reactions, and interactions with the system design and operating parameters.
Food extruders utilize thermal and mechanical energy. Understanding energy consumption and input requirements is very important for improving performance and economical system design. Water is a common ingredient in almost all food extrusion formulations. It impacts the characteristics of the ingredients, melting behavior, and formed dough rheology. Part of this water can be sometimes applied in the form of steam, thus impacting the total extruder energy input, extruder output, system design, and operating conditions. Both forms of water can be directly metered into the extruder and/or into a preconditioning mixing device. The steam incorporation option can have a significant impact on the extruder selection, design, and performance, as well as on the product characteristics.
The extruder’s important design parameters include appropriate selection of the screw elements and barrel sections. The screws and barrel design impact their functional performance, namely conveying, mixing, melting, and metering, as well as the product characteristics. In practice, one may be able to suggest more than one system design and operation to make the same product. The optimum selection is likely to be governed by availability, flexibility, and economics.
Combined extrusion cooking and cold forming of unexpanded products is a common process used to make snacks, cereals, and other products. Accelerated cooling before the final forming step is critical in such operations. This is typically achieved by evaporating part of the liquid water in a venting stage either within a single extruder or in between a cooker and a forming extruder. The selection and design of such a step are briefly described in this chapter.
When the extruded product is developed on a small production basis, scale-up becomes part of the total extruder’s selection and design process. The impact on the scale-up process is briefly discussed, identifying important factors, limitations, and additional process options.
The die plate assembly is a part of the total extruder design. It is responsible for forming the desired product shape and acts as flow resistance, thus impacting the extruder output/degree of fill, energy input, and resulting material transformations. The die assembly design will be briefly described and its important components and its operating functions identified.
History of extrusion in brief
Extruders have been used for the manufacture of cereal grain products for many years. The origin of extruders dates back to Archimedes continuous screw, which was used to deliver the materials from one end of the screw to the other end, and then for the application of oil press (Harper and Clark, 1979; Oliver, 1906). Conveying is an important function of food extruders. Extruders have evolved into today’s systems in which the hardware has been modified so that the materials are cooked while they are being conveyed.
It is believed that General Mills Inc. (USA) was the first to introduce a single-screw extruder in the processing line of ready-to-eat cereals in the late 1930s (Riaz, 2000). This type of extruder was like a pasta press system which is being used in the food industry today. The purpose was to only precook the cereal dough for subsequent drying, flaking, or puffing.
Direct-expanded corn curl product was first marketed by Adam Corporation (USA) in 1946 (Bouvier and Campanella, 2014). In the 1950s, extrusion processing was developed for pet foods and from the 1960s, ready-to-eat cereals and texturized vegetable protein were continuously developed (Riaz, 2000). All these applications used single-screw extruders to continuously process cereal-based raw materials.
The food industry has explored the capacities of the single-screw extruder to a great extent. Some examples of single-screw extruder systems are depicted in Figs. 3 and 4. Fig. 3 shows a typical disc extruder, which is a single-screw system with a disc as the die. Since this extruder does not have a well-defined die, it produces extrudates that are not uniform in shape or size. Fig. 4 shows a typical single-screw extruder. The image on the right shows the cross-sectional view of the system, where the raw material is fed from the right end, and the die and the face cutter are at the left end of the screw.