Food Extrusion

In the food extrusion process, it typically means dried products, such as pet food kibbles and smaller breakfast cereals.

From: Extrusion Cooking, 2020

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Instrumentation for extrusion processing

Bradley Strahm, in Extrusion Cooking, 2020

2.1 Temperature

In food extrusion cooking processes, elevating the raw material temperature to accomplish cooking is probably the major objective of the process. Elevating temperature is required for cooking of starches, denaturing, and texturizing proteins and for increasing the temperature to above 100°C in order to induce water flash to steam and cause expansion of the product as it exits the extruder die. In addition, in today's food-safety conscious environment, measuring a lethal temperature of microbe reduction requirements sometimes becomes a critical control point (CCP) which requires accurate and reliable temperature measurement.

The extrudate temperature in the extrusion processes is not directly controlled. In other words, the extrudate temperature is a result of other inputs including mechanical energy, steam injection, and heating or cooling of the extruder barrel. Extrudate temperature is not directly a function of the temperature of the extruder barrel sections. However, during process startup, preheating of the extruder using external heating is needed and requires accurate temperature measurement of the barrel sections. In addition, during processing, cooling of the inlet section of the extruder is required, especially when it is receiving hot material from a preconditioner, to prevent premature steam formation and blowback which interferes with feeding materials into the inlet. This cooling requires temperature measurement.

To shorten the startup process, it is desirable to preheat the extruder barrel and screws from room temperature to the process temperature set points before starting the process of passing materials through the system and generating heat via mechanical energy input. To control this pre-startup heating process, it is necessary to measure the temperature of the metal barrel sections to control the preheating to a predetermined temperature.

Temperature is measured using either a thermocouple (TC) or a resistance thermometer detector (RTD). TCs work by coupling wires of two different metals together which creates a millivolt range voltage related to the temperature of the junction where the two dissimilar metals are joined together called the Seebeck effect. The millivolt signal is then translated into temperature using appropriate instrumentation. RTDs are constructed of materials whose electrical resistance varies according to the temperature of those materials. The resistance is then measured and translated into temperature using appropriate instrumentation. TCs have a larger temperature range and are less expensive, even though they are less accurate. RTDs are more expensive but are more accurate. Either technology is appropriate for use in food extruders.

Another option for temperature measurement in extruders is infra-red (IR) sensors which work by measuring the IR radiation that is related to temperature. IR temperature sensors have been used in experimental situations in polymer extrusion such as that described by Vera-Sorroche et al. (2015). However, the usefulness of IR sensors is dependent on the melt emissivity and the depth of penetration depends on the clarity of the melt. In most food extrusion, melt clarity is low and IR sensors do not provide a significant advantage over melt-bolt temperature probes for production extrusion and are not widely used.

Temperature sensors for extruders, no matter what technology they are based on, are generally housed within a standardized melt-bolt probe threaded with ½ in X 20TPI threads as shown in Fig. 1. In sections of the extruder where there is potential interference with the rotating screw(s), a flush tip probe is used where the face of the probe is positioned approximately flush with the inner wall of the extruder barrel. In other sections of the extruder, located between the end of the screw and the final die, a probe with an extended tip can be used to insert the probe into the extrudate flow and more directly measure the extrudate temperature.

Fig. 1. Melt-bolt temperature probes.

Whatever temperature measurement technology is used, it is important to understand what temperature is being measured. The relevant question includes is the product temperature being accurately represented, or is it some other temperature, for example, the temperature of the metal barrel sections that the probe is mounted in, or is it some combination of these temperatures? Research by Mulvaney and Tsai (1996) shows that when measuring temperature with a flush-mounted temperature probe in a jacketed extruder barrel, the difference between the temperature probe measurement and the extrudate temperature will be highly influenced by the temperature difference between the extrudate and the fluid being circulated through the extruder jacket. For example, when the temperature difference is 90°C, the temperature reading from the flush-mounted probe will be 28°C different from the extrudate temperature. If the temperature probe could somehow be immersed into the extrudate flow about 18 mm, then the temperature error by the probe under these same circumstances could be reduced to about 3°C. So the lesson is that when the extrusion process is being operated in a state where there is a large difference between the temperature of the thermal fluid and the extrudate, the temperature is measured by a flush-mounted temperature probe is neither the temperature of the metal that the extruder barrel is constructed of, nor is it the extrudate temperature, but rather it is somewhere between those two temperatures.

For most food extruders, it is not practical to extend a temperature probe into the extrudate flow in the extruder itself because the probe would interfere with the rotating screws and be damaged. However, this is possible in a space between the end of the extruder screw and before the final die. This space is often occupied by a die spacer containing open space in which channels flow from the extruder screw to the final die. In this area, it is possible to extend a temperature probe into the extrudate flow.

These same researchers set up an experiment where they mounted a temperature probe in a die spacer and when operating the extrusion process at a temperature less than 100°C to avoid steam expansion and evaporative cooling of the extrudate, compared temperature readings from the die spacer mounted probe and a handheld temperature probe applied to the extrudate after it exited the extruder die. Here they showed that with a flush-mounted probe in the die spacer, there was an 8°C difference in the measurements, but when the probe was extended into the extrudate flow by about 25 mm, the temperature difference disappeared. Here the lesson is that to obtain accurate product temperature readings, the temperature probe must significantly extend into the extrudate flow. A good rule of thumb is to extend the probe into the flow by at least 25 mm. In today's world where there is a need to document accurate extrudate temperatures to record a process preventive control temperature for a kill step in a Food Safety program, it follows that this is likely best done utilizing an extended tip temperature probe in a die spacer.

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Three-Dimensional Modeling of Food Extrusion Processes

Patrick Wittek, M. Azad Emin, in Reference Module in Food Science, 2017

Flow Domain Definition and Meshing

In a typical food extrusion setup, the flow domain is bordered by the twin-screw and the extruder barrel. Before the meshing step, the twin-screw and extruder barrel geometry is reproduced with CAD (computer aided design) software.

The complex geometrical shape of the flow domain in extrusion processing and the intermeshing characteristic of the twin-screws require advanced meshing techniques. Two main approaches exist: The first meshing approach is based on the use of a quasi steady-state approach. For each time step, a separate mesh of the flow domain is created and the flow characteristics are calculated on each of them. A time-dependent field is then obtained by interpolation between the meshes. This “remeshing” technique has been used in several studies (Bravo et al., 2000; Cheng and Manas-Zloczower, 1997; Ishikawa et al., 2000; Strutt et al., 2000), but exhibits some drawbacks: Many meshes and complex meshing tools are required because of the time-dependent geometry of twin-screw extruder. Additionally, the interrelation of flow characteristics between two timesteps is ignored. Nevertheless, this quasi-steady state method shows good agreement between numerical and experimental results in terms of velocity and pressure profiles at low screw speeds (Ishikawa et al., 2001).

A second approach is working with the generation of two separate meshes, one for the flow domain (extruder barrel without screw) and one for the moving element (extruder screw). These two meshes are then superimposed as they would be positioned at any given time. Based on this approach, several techniques have been developed.

The mesh superposition technique introduced by Avalosse (1996) has already been successfully applied to a twin-screw extrusion setup by Emin and Schuchmann (2013a). A static mesh is created for the extruder barrel (representing the flow domain) and a dynamic mesh to describe the moving screws (Fig. 1). In each time step, a procedure identifies the elements of the static mesh, which contain elements of the dynamic mesh. The velocity of the moving part is then imposed on the nodes of these static elements, using a penalty technique that modifies the equation of motion. For this purpose a penalty force term, H(ν−νp), has been introduced where H is either zero outside the moving part or 1 within the moving part and νp is the velocity of the moving part. The term is used by modifying the equation of motion as follows:

Figure 1. Computational domain: (A) barrel and (B) screw configuration meshed separately for mesh superposition technique.

H(v−vp)+(1−H)(−∇p+∇·τ¯¯+ρg−ρDvDt)=0

For H = 0, above equation is reduced to the normal Navier-Stokes equations, but for H = 1 the equation degenerates into v = vp. The value of H is determined by the generation of an “inside” field that depends on the position of the moving part. When an element or node is greater than 60% within the moving part, H is given the value of 1. To ensure the mass conservation and physically meaningful pressure in the zones where geometrical penetration occurs, the mass conservation equation is modified to become

∇·v+βη∇p=0

where β is a relative compression factor, and η is the shear viscosity and p the pressure.

The fictitious domain method introduced by Bertrand et al. (1997) is a finite element method for the analysis of incompressible flow problems in enclosures containing internal moving parts. The main advantage is that only one mesh (representing the enclosure, in this case the extruder barrel) needs to be generated. The internal moving part is accounted for by a set of kinematic constraints. These constraints are formulated on the classical Navier-Stokes equations using an augmented Lagrangian method.

The mesh immersion technique proposed by Valette et al. (2008, 2009) is a simplified application of the fictitious domain method. The rigid body motion of the extruder screw is imposed by using a multiphase/penalty method.

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Modeling extrusion processes

M.A. Emin, in Modeling Food Processing Operations, 2015

9.4 Concluding remarks

Although the history of food extrusion processing goes back to the late 1800s, the control of this process and the design of new extruded products are still mostly based on limited empirical knowledge. Considering conventional products, an empirical approach can provide a solution, but it probably will not be a cost-effective one. On the other hand, with increasing complexity of food products (e.g., health-promoting cereals, meat analogs), the success of the empirical approach is decreasing. While modeling food extrusion is still in its infancy, it provides tremendous advantages, making it well suited to dealing with the characterization and control of this process and its products. There is no currently available package or code that can supply a solution to the modeling of the whole extrusion process in detail. Nevertheless, available models (e.g., 0, 1, 2, or 3-dimensional) supply solutions to specific extrusion problems at different levels. Among others, the 3D modeling approach is eligible for analyzing the complex realistic conditions in the fully filled section of the extruder. Details of such an approach have been discussed in this chapter, and the importance of reliable experimental measurements, particle tracking analysis, and experimental validation is emphasized. 3D simulation of flow for materials of relatively simple rheological behavior (e.g., shear thinning) through a twin-screw extruder is currently possible. Thermomechanical stresses experienced by the materials and mixing characteristics in such extrusion conditions can be calculated. There is, however, a need for further work in modeling the flow of complex viscoelastic behavior and the melting of biopolymers and performing such modeling for flow in partially filled extruder sections, as well. To support and accelerate the advances in this field, developing strong experimental methods that can supply the foundational data for modeling complex flow conditions in extruders is also essential.

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Key technological advances of extrusion processing

M. Azad Emin, in Food Engineering Innovations Across the Food Supply Chain, 2022

7.5 Concluding remarks

The understanding of structural changes in food extrusion processes is still a major challenge because of the interactions between transient processing conditions, extensive structural changes, and material properties. To advance the state-of-the-art knowledge in this field, we presented an approach based on the analysis of the process at the mechanistic level. This approach involves the splitting of the process into coherent sections and the in-depth analysis of these sections using various numerical, rheological, and optical methods that we developed.

While the proposed methods still have several limitations, they offer enormous technological advancements and are therefore well suited for the characterization and control of the extrusion process. To support and facilitate the progress in this field, it is important to further improve the reliability of the methods available by critically applying them to a broad range of well-designed extrusion applications. Both standardization of current methods and the development of robust and sensitive new methods are also highly desirable. Such methods can provide essential information necessary to model complex flow conditions and structural changes of biopolymers in extruders. The main benefit of the mechanistic approach is that the focus remains on the critical processing mechanisms and not on the independent extrusion variables. As a result, the true interrelation between the independent variables and the final product characteristics becomes visible. Such information can then be efficiently used to adapt the process to perform a targeted product design or to transfer the process to different scales or materials more successfully. We therefore strongly encourage that future research efforts be directed toward the analysis of local processing conditions and material design properties in relation to these conditions by applying an interdisciplinary scientific approach at the interfaces between numerical modeling, rheology, sensor technology, and biopolymer science.

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Extruder screw, barrel, and die assembly: General design principles and operation

Waleed Yacu, in Extrusion Cooking, 2020

4 Extruder design for specific food applications

Extruders, both single- and twin-screw, are used in vast food processing applications. The above discussion has described in simple terms how to select the appropriate extruder, specify, and design its configuration.

Water is perhaps the most important ingredient in food extrusion applications, as it can:

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decrease the formed melt/dough viscosity and resulting mechanical energy input through viscous heat dissipation;

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decrease the melting/gelatinization temperature of starch;

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evaporate at the die exit, thus producing low density expanded products;

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be added in a liquid or vapor (steam) form, thus impacting the system's energy input;

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be removed from an open venting port within the extruder and at the die exit, thus inducing accelerated extrudate cooling; and

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be used as an important control tool in the ingredients formulation to achieve the desired product functional properties. It is typically removed afterward and does not impact the finished product ingredients listing.

The following product examples are categorized on the basis of feed material moisture concentration. They are intended to provide a starting point on how to specify and develop the extruder design.

4.1 High viscosity cooking at very low formula moisture concentrations (< 17%)

In this case, the energy input to the product is likely to come entirely from the extruder motor. The added water to the formula is very limited, which makes it hard to substitute some of it in the form of steam. At such low moisture content, the starch melting/gelatinization temperature is also high (> 160°C). This would require high mechanical energy input to the product (~ 0.160 kWh/kg), necessitating large extruder motor per unit output. The mechanical energy input rate (through viscous heat dissipation) is very fast. Therefore, the required ART for melting and forming functions will berelatively short (< 30 s). An extruder screw with no flow restricting elements before the die would be recommended. As such a short cooking extruder (< 9D long) is adequate for such an application.

Corn puff snacks are extruded at moisture levels in the range 13%–17% and produce a highly expanded product. It utilizes very short single-screw extruders (L/D ≤ 6). The very short extruders (L/D = 2.5–4) limit their feed ingredient intake rate by using short pitch (~ 0.33D), two starts, and a relatively shallow screw (H < 0.13D) with a compression ratio of 1. That is to say, it has no deep or tapered screw sections. The die assembly is the only flow resistance device. In some cases, these extruders are choke fed to maximize their output. They utilize good flowing, coarse particle sized raw ingredients, typically termed “meal.” The meal is prehydrated and allowed to rest for > 30 min for adequate moisture equilibration. These extruders do not operate well with fine particle size in feed material such as flour. Wetted flour has a greater tendency for bridging in the feed port, causing operation interruption and instability.

Short twin-screw co-rotating extruders (L/D ~ 6) are also used for making corn puffs. They provide improved conveying and better mixing than single-screw extruders, thus allowing the use of unhydrated flour ingredients and direct water addition to the extruder.

Short single- and twin-screw co-rotating extruders are used to make other highly expanded snacks, breakfast cereals, and pet foods. The optimum extruder length depends on the formula complexity, the extent of necessary mixing, product temperature and resulting expansion, output, and extruder use flexibility for multiple products. An extruder length in the range of 9–15L/D is common for many of these applications. Longer extruders in this category would likely include additional non-restrictive mixing elements such as forwarding kneading paddles as well as restrictive elements such as a disc or a neutral kneading paddles section. These elements will provide added retention volume that may be necessary for providing adequate retention time in extruders operating at high screw speed and/or high production output.

4.2 Medium viscosity cooking at medium formula moisture (18%–24%)

The moisture concentration in such applications should allow utilizing steam for the part of the total water. This can be added at 2%–5% of the dry mix feed rate so long as there is still > 5% of liquid water to be added. The option of adding steam can:

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shorten the needed extruder length, especially if both the steam and water are added in a preconditioning cylinder;

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increase production output in relationship to the substituted energy by steam (this assumes that the output is limited by the extruder's energy input from either or both motor power and barrel heat transfer);

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reduce the needed motor size and perhaps the gearbox duty;

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improve product expansion and other product attributes, particularly when the feed formula contains appreciable oil/fat concentrations; and

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soften the dry mix and reduce the extent of abrasive wear.

With steam injected in a preconditioner, the extruder length can be shortened to 9–15D. The screw design may include nonrestrictive forward mixing as well as one or two neutral restrictive sections. Without a preconditioner, the extruder length may need to be increased by ~ 4–6D to allow for direct steam injection into the extruder barrel. To prevent the steam from escaping through the feed port, the injection port should be placed in a partially filled section (screw pitch = or > 1D) following a low or non-conveying screw element. This can be a short pitch screw, a disc, or a kneading block. Food extrusion cooking applications such as breakfast cereals, pet foods, textured plant proteins, and others have benefited from steam addition.

In the absence of steam energy, the extruder is likely to be longer (15–25L/D). The optimum extruder length increases with increasing product temperature and expansion, higher oil concentration, and production output. Increasing oil/fat concentrations in the feed material (> 4%) decreases the melt/dough viscosity and makes it harder for the extruder to extract energy from the motor. The screw design would likely include forwarding, neutral, and reverse screw elements to provide the needed retention time and desired energy input.

4.3 Low viscosity cooking at high formula moisture (25%–38%)

Applications in this category include high moisture breakfast cereals, pet foods, snacks, and others for the purpose of making unexpanded products (pellets).

Extrusion-formed pellets for snacks and breakfast cereals are in most cases regarded as intermediate products. Without significant processing, they may be flaked, shredded, or sheeted. Alternatively, the formed pellets can be dried to a shelf-stable moisture in the range of 10%–12% before the final processing step. For snack products, such dried pellets can be expanded by frying, hot air, microwaving, or hot mold processing. For breakfast cereals, they can be expanded by hot air or steam gun puffing.

The extruder design is dependent on the formula ingredients, cooking and forming temperature, cooling requirement and difficulty, production output, and flexibility. Starch type, concentration, form (native or pregelatinized) and other formula factors are important deciding factors in the extruder design process. In the presence of an adequate portion of precooked starch, snack pellets, particularly those including heat-sensitive ingredients, may be formed without further cooking in the extruder. Thus, a cold-forming process may be adequate.

Potato and tapioca starch-based snack pellets can generally be cooked and formed at temperatures < 90°C. Thus, an extruder (single or twin) with 15–20D length that includes forwarding and neutral screw elements would be adequate. The discharge end screws elements of the extruder (~ 3–5L/D) are likely to be mostly conveying screws.

In the above-suggested moisture range, cereal grains (corn, rice, wheat, and oats) starch gelatinization require relatively high cooking temperatures (110–130°C). To avoid product expansion at the die exit, considerable cooling is required after the cooking section. Small-scale (pilot plant scale) extruders, especially those operating at relatively low screw speeds (~<200 RPM), may be capable of providing adequate barrel cooling and form unexpanded pellets. Large extruders (production scale) are typically unable to cool the dough sufficiently before exiting the die with barrel cooling alone. The cooling process can be accelerated by the venting part of the water before forming the unexpanded pellet product. There are two ways of utilizing such a venting process:

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Venting within a single cooker extruder (single- or twin-screw). A specifically designed screw is needed to allow for a venting port in a partially filled section. In this case, the extruder (~ 20–25D long) may have: (i) a cooking section (~ 12–17D long) that will include forwarding, neutral, and perhaps reverse screw elements; (ii) a high volumetric flow capacity screw section (~ 2D long) with an open venting port; and (iii) a conveying (metering) screw section (~ 4–6D long) at the extruder discharge (Fig. 19B). Steam energy may be successfully used to shorten the extruder's cooking section (with a preconditioner) and/or to supplement the extruder's mechanical energy input. The venting port can be open to the atmosphere or subjected to vacuum to increase the evaporative cooling rate.

The melt rheology and temperature affect the behavior of the melt and extent of volatiles removal from a venting section. With reduced initial moisture/fat content and/or excessive vapor release, the vented melt may produce a highly expanded mass that is hard to convey forward. The use of a stuffer should help in such circumstances. Fig. 20 shows different styles of vent ports used for co-rotating extruders.

Fig. 20. Different styles of vent port used in twin-screw co-rotating extruders.

(Courtesy of Coperion, Inc.)
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In applications where the material temperature is relatively high (> 120°C) in the cooking section and at high production capacities, the use of a “two extruders” system is typically preferred. The first extruder cooks the material at a high screw speed. Its length could be 15–25D long having forward, neutral, and reverse screw elements. It forms an expanded product at the die exit. Venting and cooling are achieved between the two extruders. The second, typically a low shear single-screw extruder, operates at a low screw speed (< 50 RPM) to cool and form the unexpanded desired product. The two extruders can thus be designed and operated independently in an optimum manner. In this way, the cooking and the forming extruders are utilized to their maximum throughput and efficiency without having to compromise, as may be the case in a single cooking/forming extrusion system.

Fig. 21 shows an example of the suggested screw configuration for twin-screw co-rotating extruders.

Fig. 21. Suggested examples of co-rotating twin-screw configurations used for different cooking applications depending on the feed formula moisture concentration. (A) For low formula moisture (MC &lt; 17%). (B) For medium formula moisture (MC 17%–24%). (C) For high formula moisture (MC &gt; 24%) with steam conditioning. (D) For high formula moisture (MC &gt; 24%) without steam conditioning.

4.4 Low viscosity cold forming at high formula moisture (25%–38%)

Cold-forming extruders are typically a low shear rate single-screw type used in pasta (Dalbon et al., 1998) and other product applications. They are operated at a low screw speed (< 50 RPM) to form the product with minimum mechanical energy input (< 0.03 kWh/kg). The most common cold-forming extruder screws have ~ 9D length, single flight start, a constant deep channel (~ 0.25D), and a channel pitch of ~ 0.67D (helix angle ~ 12 degrees). Note that the compression ratio is 1 as there is no tapered nor shallow metering section. The emphasis here is on maintaining a low shear rate throughout the extruder. A mixing head element is typically placed at the discharge end to minimize the material flow oscillation/pulsation to the die assembly. An example of a forming extruder with downward discharge assembly and such a forming screw is shown in Fig. 22. A multiple flight screw element with three starts or more flights is sometimes placed at the discharge end instead of a mixing element.

Fig. 22. A forming extruder with downward discharge die assembly. Note that the forming screw has a mixing head at its discharge end.

(Courtesy of FEN s. r. 1.)
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Food Extruders

Mian N. Riaz, in Handbook of Farm, Dairy and Food Machinery Engineering (Third Edition), 2019

The following terms and phrases are the ones most commonly used in food extrusion (Riaz, 2000; Harper, 1981):

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Barrel: This part is in between the preconditioner and knife assembly. This part contains shafts (single or twin) and screws with shear locks for cooking and processing the food.

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Breaker plate: This serves as a mechanical seal in between the die and the end of the extruder.

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Compression ratio: This is also called C/R ratio. It is the volume of the full flight of the screw at the feed opening, divided by volume of the last full flight before discharge.

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Compression section of the barrel: The main function of the compression section is to plasticize the raw material into a dough-like state. This section is in between the feeding section and metering section. This section should have a gradual decrease in the screw flight depth in the direction of die.

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Cut flight screw: This is also called interrupted flight screw. In this case, a section of the flight is missing. This is used to increase the shearing action and for more cooking of the raw ingredients.

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Die: These are small openings at the end of the extruder to shape the products.

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Die insert: Individual die openings exist as inserts, which slip into holes in a die plate.

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Pellet: This is a discrete particle that is shaped and cut by the extruder, regardless of shape, sometimes referred to as a “collet.”

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Die land length: This is a ratio between the length and diameter of the die through which the product passes. Longer die land length provides more backpressure, and the product will be denser.

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Die plate: This plate contains several holes that can receive individual die inserts containing the actual die opening.

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Feeder: This device provides a uniform delivery of the food ingredients to the preconditioner. These feeders are either volumetric or variable-speed augers.

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Feeding section of the barrel: This section usually contains deep flight screws to carry the raw material immediately to the next section. The main function of the feeding section is to ensure that raw material is moving quickly to the barrel. Otherwise, the barrel will plug up and we have to shut down the extruder.

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Flight: This is a helical conveying surface of the screw, which pushes the raw material forward in the extruder barrel.

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Hopper or bin: This part holds the food ingredients above the feeder. They are usually made up of stainless steel with a glass window to see the level of the ingredients.

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Jacket: This is hollow cavities outside the barrel in which we can run cooling or heating media such as water, oil, or steam to cool or heat the barrel surface.

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Knife cutter: This is the assembly that cuts the food into desired sizes and lengths.

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Length to diameter ratio: This is also called L/D. It is the distance from the internal rear edge to the discharge end of the barrel, divided by the diameter of the bore.

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Metering section of the barrel: This section is nearest the discharge end of the extruder. The main function of the metering section is to increase the shear rate and cooking of the raw material. Therefore this section should have a very shallow flight screw.

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Pitch: This is the angle of the flight, relative to the axis of the root of the screw.

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Preconditioner: This part is in between the extruder barrel and the feeder. In the preconditioner we can add moisture and steam to partially cook and mix the raw material before it goes to the extruder barrel. Not all extruders have a preconditioner.

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Recipe: The ingredients or mixture (protein, carbohydrates, lipids, minor ingredients, etc.) to be processed in an extruder to make food.

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Root: This is a continuous central shaft of the screw, which is usually a cylindrical or conical shape screw, around which the flight is wound.

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Screw: This part accepts the food ingredients at the feed port, conveys, works, and then forces through the die. There are several different types of screws, i.e., single flight screw, double flight screw, cut flight screw, shallow flight screw, deep flight screws and feeding screw, etc.

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Segmented barrel: It refers to a barrel made of several segments.

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Shear: This is a working, mixing action that homogenizes and heats the raw ingredients.

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Shear ring: Also called shear-lock or steam-lock or ring dam. This is a round device that locks together individual screws in the barrel.

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Venting concept: This is used when we need to make dense food like pasta, third-generation snacks, or noodles. Toward the end, one of the barrel sections will have an opening to release the pressure and steam to densify the food products.

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Microstructure and its relationship with quality and storage stability of extruded products

Kasiviswanathan Muthukumarappan, Gabriela J. Swamy, in Food Microstructure and Its Relationship with Quality and Stability, 2018

Abstract

Industrial applications of extrusion have originally emerged from the need to process various polymeric materials. Food extrusion processing along with the reported product quality and microstructure, on the other hand, is fairly new. In addition, food extrusion inclines to be more segmented and application oriented. Therefore, a multidisciplinary approach is necessary, for which the comprehensive estimate of the process performances is not yet available. Due to a great interest in visualizing the various aspects of structure via such techniques as light microscopy, confocal light microscopy, transmission electron microscopy, scanning electron microscopy, atomic force microscopy, and computed tomography, it is now possible to probe into the process of food extrusion in more detail.

The purpose of this chapter is to provide extensive microstructural information postextrusion processing on starch- and protein-based feed. The chapter has been inspired both by existing applications of extrusion technology and by emerging visualization techniques. The objective of this effort is to bridge the space between long-term experience in extrusion and the science of extrusion, in order to create a link between the world of extrusion practitioners and that of imaging engineers in the field of food engineering. For this purpose, the generic extrusion process idea has been applied in order to represent and converse the extrusion processing culture and to shape the structure of the chapter content.

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Volume 3

Mohammed Shafiq Alam, Raouf Aslam, in Innovative Food Processing Technologies, 2021

3.02.4.2.1 Snack Foods

The adoption of extruded foods by consumers is primarily due to the ease, taste, desirable appearance, and texture considered to be unique to these foods, especially when it comes to snack items (Harper, 1981). Snack food extrusion involves chosen grains being exposed to a variety of complex physical processes to create snacks of varying shapes and textures (White, 1994). In the extrusion of snack foods, grain and other materials are combined and cooked under pressure, with shearing in a barrel, which is also called a pipe, at high temperatures. The resultant mass is compressed through a die and sliced into individual pieces and takes different shapes that customers expect in the retail snack food aisles (Harper, 1981). Current materials, state-of-the-art extrusion technologies, and creative processing methods are merged to deliver new snack foods with ever-increasing appeal to health-conscious customers searching for different textures and mouth sensations in convenient foods (Pamies et al., 2000). The processing variables, such as feed rate, screw speed, barrel pressure, and temperature, determine the quality characteristics of extruded snacks that determine the acceptability of that product (Harper, 1981). A modern extruded snack food product's success or failure is directly related to sensory characteristics, where texture plays an important role. Texture is of great importance in such foods, in which expansion is required and crispness is one of the most important attributes (Pamies et al., 2000). Snacks were prepared from normal maize and high-quality protein maize in a single-screw extruder (Martinez et al., 1996), chickpea flour (Carrillo et al., 2000), and sweet whey solids in combination with cornmeal and potato flour (Onwulata et al., 2001). To improve expansion, sometimes high-temperature short-time air puffing is incorporated to give better extruded products. For example, in the production of potato–soy RTE snack foods, air puffing was used to produce lightly textured and highly porous snacks, ideally at puffing temperature in the range of 185°C–255°C and puffing time of 20–60 s (Nath and Chattopadhyay, 2008).

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Raw material behaviors in extrusion processing II (Proteins, lipids, and other minor ingredients)

Wesley Twombly, in Extrusion Cooking, 2020

1 Introduction

Extrusion of proteins has traditionally been seen as challenging. However, there are a variety of protein-based products commercially available, demonstrating that it is possible to achieve a desired product density, shape, and structure. Understanding the mechanisms that drive structure formation allows an extrusion process to be designed to deliver the desired characteristics. Looking outside of the common food extrusion literature has yielded concepts that can be leveraged to optimize high protein extrusion processes. For example, the concept of phase separation is useful in understanding the anisotropic (directional) structures in some texturized proteins. Foaming processes outside of food extrusion can give guidance for how to control cell structure in high protein products. Utilizing these and other resources will help make high protein extrusion more successful in future endeavors.

Extrusion of products with high levels of proteins or lipids is generally viewed as challenging. Part of this perception may be due to a restricted viewpoint that does not fully recognize the system's capabilities, instead limiting itself to systems configured for starchy ingredients. While the same model of extruder can be used for extrusion of starchy ingredients or for petrochemical polymers, the system configuration and operating conditions for the two products will be different. A change in system configuration and operating conditions may also be needed between starch and protein ingredients.

The extrusion systems and conditions have been optimized to work with starch as the primary component over the course of decades. Most cooking extrusion literature, and most of the experiences of people working in cooking extrusion, have focused on starchy products. This can lead to researchers assuming the extrusion system setup and conditions for the starch-based products are also the correct system setup and conditions for creating a protein-based product. It is commonly overlooked that the optimum setup and conditions for products with high protein content may be significantly different.

When a product fails to meet expectations, one useful approach is to identify the mechanisms driving the desired characteristics and then move the process in the direction needed. The basic mechanisms driving the desired characteristics are similar among starches, proteins, and petrochemical polymers.

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Volume 1

A.C. Smith, in Handbook of Waste Management and Co-Product Recovery in Food Processing, Volume 1, 2007

8.9 Sources of further information and advice

aguilera, j. m. and stanley, d. w. (1990), Microstructural Principles of Food Processing and Engineering, London, Elsevier.

frame, n. d. (1994), The Technology of Extrusion Cooking, London, Blackie.

kokini, j. l., ho, c. t. and karwe, m. v. (1992), Food Extrusion Science and Technology, New York, Marcel Dekker.

ledward, d. a., taylor, a. j. and lawrie, r. a. (1983), Upgrading Waste for Feeds and Food, Kent, Butterworth.

liese, a., seelbach, k. and wandrey, c. (2000), Industrial Biotransformations, Weinheim, Wiley-VCH.

loncin, m. and merson, r. l. (1979), Food Engineering: Principles and Selected Applications, London, Applied Science.

ohlsson, t. and bengtsson, n. (2002), Minimal Processing Technologies in the Food Industry, Cambridge, Uk, Woodhead.

richardson, p. (2001), Thermal Technologies in Food Processing, Cambridge, UK, Woodhead.

whitaker, j. r., voragen, a. g. j. and wong, d. w. s. (2003), Handbook of Food Enzymology, New York, Marcel Dekker.

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