The simple answer to the query posed in the title of this month’s column is, a lot. But first, what is an extruder? The word describes two rather different types of equipment: screw extruders and roll extruders. Roll extruders are commonly used in making confections and snacks by forming strips or ribbons of plastic material such as sugar-based taffy or cereal-based dough. The taffy or dough is placed in the nip between two counter-rotating drums, which may be smooth or grooved and are typically fairly far apart (as compared with crushing rolls, for instance). The wide ribbon is usually slit and then cut to the desired length and cooled in the case of confections or baked in the case of granola bars. The rectangular pieces may be coated in subsequent steps with chocolate or compound coating. That operation is relatively simple and is primarily used for forming although some compression may occur, changing the density and texture. Issues include the appropriate clearance or rotation speed and whether the rolls need to be heated or cooled. Empirical testing is usually required and scale-up needs to consider potential deflection of the rolls if they become too long and thin.
Screw extrusion involves one or two machined augers within a cylindrical barrel, which may have heating or cooling zones. A screw extruder combines several unit operations in one machine: transport, mixing, heat transfer, forming, and possibly, expansion. Broadly, extruders are further distinguished by the pressure levels they achieve and by whether energy imparted to the treated material is transported through the barrel or generated mechanically.
The modern screw extruder was developed initially to melt and form polymer resins into fibers, sheets, films, and other shapes. Polymer extruders are used in injection and blow molding of objects such as food containers. Screw extruders may be thought of as pumps. A single screw extruder behaves somewhat like a centrifugal pump in the sense that the output pressure it generates decreases as its flow rate increases just like the head curve of a centrifugal pump. Similarly, the pressure required for transport through a given die at the end of an extruder barrel increases as the flow rate increases. This results in two curves for a given combination of extruder, die, and material, which intersect at a single operating point.
Single screw extruders are typically flood fed, which means that their inlet hopper remains full of feed material and the open spaces between the flights of the screw and the barrel are full. To increase throughput, the screw speed is increased, requiring more power from the drive system. The pressure generated decreases, which creates an imbalance at the die. To compensate, the die must be adjusted by adding more holes or by reducing the thickness of the die plate; otherwise, the added energy is converted to heat and increased shearing, which normally is undesirable. Modifying the die is not easy to do routinely, so single screw extruders typically have a single operating point for a given material with a given die and screw configuration. Some extruders have a back pressure regulating valve just before the die to address this issue.
Many factors can affect the behavior of the material, of which moisture content is probably the most sensitive. The operating curves of both the screw and the die are affected by the apparent viscosity of the material, which is affected by temperature and moisture content. Temperature changes along the length of the barrel as mechanical and thermal energy is supplied or removed by cooling.
To make matters even more confusing, the apparent viscosities of common food materials change in different ways with temperature changes. Starches typically melt as temperature increases, with viscosity normally decreasing, while proteins often denature and cross-link as temperature increases with apparent viscosity increasing. Some raw materials have both starch and protein components.
Twin Screw Extruders
Twin screw extruders behave somewhat like positive displacement pumps in that their output pressure is independent of the flow rate. In operation, the barrel of a twin screw extruder is partially full, and adjusting flow rate involves providing an increased or decreased supply of raw material. The fraction of the barrel that is full adjusts accordingly.
Twin screws can be intermeshing or separate (in which case they behave like single screw extruders). The screws may counter rotate or co-rotate. Counter-rotating, inter-meshing twin screw extruders were originally developed to process polyvinyl chloride (PVC) polymers, which have resin beads that are slippery and hard for a single screw extruder to grab. They are more flexible in operation than single screw extruders but are more expensive. When single screw extruders were first applied to foods to make expanded ready-to-eat breakfast cereals, the only machines available were those developed for polymers. Over time, extruders that have provisions for cleaning and other features useful in food processing have been designed.
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Low-pressure extrusion is used to form but not cook or expand food materials—primarily cereal dough for pasta and some snacks. Durum wheat flour and water are mixed and then fed to a relatively short single screw extruder with an appropriate die and cut-off knife to make long and short pasta shapes. The pasta is then carefully dried at relatively low temperatures for long times to produce the typical dense and hard texture. Too rapid drying can cause checking or cracking of the pieces as moisture tries to escape faster than the solid, dry outside layer permits.
Die design for forming or cooking extruders is largely an empirical art because food pastes or dough are non-Newtonian fluids, which means their apparent viscosity is a function of shear rate as well as temperature. Further, many foods have elastic properties; after exiting a die, foods may swell so their cross-section area is not only larger than that of the die but also might even be differently shaped. As an example, when molten polymers are spun or extruded into fibers, the die holes are often star shaped, so the resulting fiber will be round. In low-pressure forming extrusion, non-ideal fluid behavior is less pronounced, but die design to get precise and consistent shapes is still tricky.
Defining cooking in extrusion is challenging. It is some combination of starch gelatinization, protein denaturation, polymer degradation, cross-linking, Maillard reactions, and caramelization—depending on the exact composition, moisture content, temperature, pressure, and shear. In an effort to separate somewhat a few of the complex functions of extruders, some manufacturers provide a preconditioning barrel before the feed to the extruder. This barrel is basically a continuous mixer/cooker in which a dry powder mixes with steam and water to hydrate and begin the cooking process under slower shear conditions than what occurs in the extruder proper. This operation frees the extruder to focus on the final heating and forming.
Some Products of Extrusion
Compared to traditional breakfast cereal processes, extrusion is simpler and less expensive. However, as in so many processes, there are tradeoffs. Some of the desirable flavors of prepared cereals come from long, slow cooking. Extrusion is a high-temperature, short-time process. Many extruded breakfast cereals are sugar coated, which helps mask their relatively bland taste.
Dry pet foods and treats for cats and dogs are some of the largest applications of both single and twin screw extrusion. A wide range of raw materials are used in pet foods as sources of protein, calories, and other nutrients. Pet foods are the sole source of nutrition for pets, so they must be complete and balanced. High-fiber raw materials for both pets and humans can be challenging to extrude because they do not contribute to a smoothly flowing fluid and can cause plugging in dies. The degree of expansion in pet foods is adjusted to meet local expectations. In some markets, pet foods are expected to be dense and crunchy while in others, they are more highly expanded, giving a lower bulk density and contributing to larger-volume packages for the same weight. Aquatic feeds for fish farms and home aquaria are smaller in size than the particles for dogs and cats. Also, they must be formulated to either sink or float, depending on the species being fed.
Most pet foods are coated with palatants, materials intended to have an early taste impact. Examples include fats, yeast, and proprietary mixes. The analogy is to salt and flavors coated on savory snacks for humans. Whether to coat fats on hot or cooled extruded pieces is debated. Hot pieces are thought to absorb fat while cooled pieces keep more of the fat on the surface. Hot pieces need to be cooled, which means that the cooler, if it follows the coating operation, will become especially dirty as compared with the cooler that precedes a coating operation.
Co-extrusion involves injecting a second material in the center of an extruded tube and is used to make snacks and biscuits. Dyes can be injected near the end of the barrel to provide streaks or differently colored pieces.
by J. Peter Clark,
Consultant to the Process Industries, Oak Park, Ill.