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Perhaps the most interesting of these processes is the manufacture of pipe where not only is the outside diameter controlled but also either a fixed or floating die is also used to set the internal diameter and hence the wall thickness.

Commonly extruded materials are copper (pipe for plumbing), aluminium (various extrusion profiles for tracks, frames, rails), steel (rod, track) and a multitude of plastics (pipes, rods, rails, seals).

It is common in the plastic extrusion process to use plastic chip, which is then melted and rather than drawing the material through the die to squeeze the plastic out of the die in a similar fashion to the extrusion of toothpaste from a tube.

Extrusion has found a great application in Food Processing. Various products like pastas, breakfast cereals, ready to eat snacks, fry-ums etc. are now manufactured by extrusion. Softer foods such as meringue have long been piped using pastry bags.

Food Extrusion was used as a shaping tool since time immemorial. In India, it has been used to shape products like chaklis and sev. In Italy, it was used for the manufacture of pastas. The first industrial extruders came into existence around 75 years ago (Mercier, Linko & Harper 1989). Initially used only for mixing and forming pasta and for the mincing of meat, they have morphed into high temperature short time bioreactors that transform raw ingredients into intermediate or final products.

The first industrial food extrusions involved the use of piston or ram type extruders to stuff casings in the manufacture of sausages and processed meats (Harper 1981). These were followed by meat choppers and mincers, which consisted of a screw forcing the meat out of a small die plate. These were the first twin screw extruders used in the food industry. The pasta industry became the second food industry to use extrusion with the development of hydraulically operated batch cylindrical ram macaroni presses around 1900. However, the application of the single screw extruder which revolutionized the industry was its use as a continuous pasta machine in the 1930s. The pasta press mixes semolina flour, water and other ingredients to form a uniform dough. The screw of the extruder works the dough and forces the mixture through specially designed dies to create the variety of shapes that pastas are available in now.

In the late 1930s General Mills used the extruder in the manufacture of ready to eat cereals. Extruded corn collets were developed around the same time. However, the concept was not commercially developed till 1946. The desire to precook animal feeds to improve digestibility and palatability led to the development of the cooking extruder late in the 1940s, which has greatly expanded the application of extruders in the food industry.

Cooking extruders come in a variety of sizes and shapes and provide the capability to vary the screw, barrel, and die configurations as required by the product. Temperature is controlled by direct steam injection or heating through external barrels. Preconditioning of the feed in an atmospheric or pressurized chamber allows ingredients to be partially cooked and uniformly moistened before extrusion.

Modern food extruders can be designed to combine a range of unit operations into one process which does not require much pre or post processing. They can carry out one or more of the following in one step: transport, grinding, hydration, shearing, homogenization, mixing, compression, degassing, cooking with partial melting and plasticization of the mix, starch gelatinization, protein denaturation, destruction of microorganisms and anti-nutritional factors, pumping, shaping, expansion, formation of porous and fibrous texture and partial dehydration. Depending on their design, they can be used to make a variety of products including pastas, breakfast cereals, puffed snacks (corn puffs/collets, kurkure, cheese balls etc.), meat substitutes like soya nuggets, fry ums, breading substitutes, modified starches, soft-moist and dry pet foods and confections.

The second revolution in food extrusion came with the use of variable pitch single screw extruders. These extruders further improved the mixing versatility of the extruder. The most recent advance for the food extrusion industry has been the use of twin screw extruders. The screws either rotate in the same direction (co-current) or in opposite direction (counter-current) to each other. These extruders, while more complex than single screw extrudes, offer better control over residence time distribution and internal control of shear for thermolabile materials. They are also more versatile in that they accept lower moisture feeds and are self cleaning due to the wiping effect of the screws.

Food extruders today are all screw extruders and the early ram and piston type extruders have disappeared from the industry. The various components of an extruder are a drive, feed assembly, extrusion screw, extruder barrel and an extruder discharge. The drive consists of a support / stand, a drive motor, a set of gears for variation of speed, a gear transmission (to reduce speed and increase torque) and a thrust bearing (to support and centre the screw and absorb its thrust).

The type of feeder section depends on the material to be fed. Different feeders are available for dry, wet and slurry like materials. For solids and dry materials hoppers / bins, vibratory feeders, variable speed screw conveyers and weigh belts are used. Water wheels, positive displacement pumps, variable orifices and variable head feeding devices are available for liquid or slurry like feeds. These feeders can be batch or continuous feeders as per requirements. Often the raw materials are fed with such feeders into a preconditioner from where they are fed into the screw section.

The screw is the central portion of a food extruder. It accepts ingredients at the feed port, conveys and works on them and forces them through the die. It is further divided into various sections. The first section is the feed section. The flights in this section are deep so that the product can easily fall in these and be pushed forward.. The compression section which follows is characterized by decrease in the flight depth or a decrease in the pitch. This leads to a compression of the material and its working into continuous dough. Heat is applied to this section and cooking of the material begins. The metering section of the screw is the section where the maximum heat is applied and where the material faces the maximum shear and pressure. This section is characterized by various types of screw conformations to get the optimum product quality. Some screw configurations used in the industry are given in Fig. xx ( Zuilichem, Kuiper, Stolp & Jager 1999; Chuang & Yeh 2004)

The extruder barrel is the cylindrical member which fits tightly around the rotating screw. Although it seems to be a simple piece, there are various designs available for barrels. They may be in one piece or in segments which can be detached and attached as per requirements. The inner walls of barrels often have grooves or splines to prevent slippage and improve the ability of the extruder to pump food material against high back pressures. These grooves may be straight (running axially down the barrel) or spiral (helical grooves in a direction opposite to that in which the screw rotates). The barrel also has a jacket for the heat transfer medium to flow in. A vent placed at an intermediate point on the barrel is used to allow the escape of steam, air or other volatiles from the extruder. Vented extruders tend to act as two extruders in series. A recent addition to the barrel has been an inlet valve towards the end of the metering section of the screw. This is used to add thermosensitive materials to the screw and is often used when extrusion is used for microencapsulation.

The extruder discharge consists of a die head assembly (holder for the die and support for the cutter), a breaker plate (a perforated plate which serves as a seal between the barrel lining and the die and provides an even pressure distribution to the die and the die (a narrow orifice of variable shape and size) or the die plate (a heavy plate which can receive individual die inserts containing the actual die opening) and die inserts.

Expanded RTE cereals are manufactured from mixtures of cereal flour and starch combined with small amounts of malt, fat, sugar, emulsifiers and salt. Extruders are used in these products to either get the puffed and cooked product which is face cut to give the cereal after dehydration or to get pellets which are sheeted and cut to give the desired cereal. The uniform high moisture cooking reduces starch damage and gives a full bodied cooked grain flavour. The equipment used for the pellet formation consists either of two single screw extruders (one for cooking and the other for pelleting) or a twin screw extruder with a vent in between to let out hot air and steam and cool the product.

For the manufacture of snacks by extrusion cooking, lower moisture feeds are used. The temperatures used are much higher with greater screw speeds to reduce the residence time. Due to the higher pressures, the moisture in the feed does not get converted to steam at the prevalent high temperatures. However, once the material comes out of the die, it suddenly comes to atmospheric pressure and therefore a lot of steam is let off. This expands the product. The latent heat of evaporation for the steam is taken from the product as a result of which it cools and the starch sets giving a puffed product. Due to the low moisture content and the puffed structure, the product becomes crisp. Ready to fry snacks are also produced by extrusion processing. In these cases higher moisture contents and relatively lower temperatures are used to get cooked, shaped snacks which are dried and sold to be fried at home. Twin screw extruders find great use in the manufacture of such products especially the former because of their greater versatility, better mixing & heat transfer and self cleaning ability.

Majority of the research carried out on food extrusion focuses on this area since it deals with high food margin products with high demands. The added advantage of lower fat contents in the finished goods with the same product quality has just increased interest in this area. Various authors have worked on the development of new products or on the improvement of product quality in case of these products (Ding, Ainsworth, Tucker & Marson 2003; Santiago & Areas 2000; Ohtsumo, Suzuki, Yasui & Kasumi 2005; Guha, Ali & Bhattacharya 1997; Thakur & Saxena 2000; Ding, Ainsworth, Plunkett, Tucker & Marson 2005; Thymi, Krokida, Pappa, Maroulis 2005; Guha, Ali Bhattacharya 2003).

Pet foods (semi moist and dry) can also be manufactured by extrusion cooking. The dry products are manufactured using cooking extruders and moist feeds and then dried. Due to the lower profit margins and greater toughness demanded by such products, cheaper and rugged equipment is often used and twin screw extruders are rarely used. For the high value end of the pet food business (semi moist foods), twin screw extruders with lower temperature and pressure requirements are used. The feed water activity is carefully adjusted and processing conditions are controlled to ensure the microbial stability of the product.

The confectionary industry has found a few applications for twin screw extruders in the manufacture of toffee, caramel, peanut brittle, wine gums and licorice. Some of the twin or more layered confections also use extruders. Extrusion is also being looked up as a method to reduce time in the production of chocolate by removing the step of conching which takes hours to complete and replacing it with a single extrusion step requiring a few minutes. Single screw extruders have not yet found any use in the confectionary industry.

Extruders are also used for the manufacture of texturised vegetable proteins where protein isolates are given the texture and mouthfeel of meat using the process of extrusion. Pasta products, which were one of the first food products to be made by extrusion are manufactured by feeding high moisture semolina feeds to a single screw extruder at low temperatures. These are used mainly as mixing and shaping equipment. Of late, extruders have found use in the manufacture of modified starches and proteins and other food reactions which require accurately controlled high temperatures and pressures over relatively short time with proper homogenisation.

Until the 1970s, food extrusion was more an art than a science and therefore trained and experienced personnel alone could handle such equipment. However, over the past 30 years, many models have been developed to predict the throughput, energy requirements, mixing patterns and residence time distribution of extruders. The volumetric throughput of a single screw extruder as given by Janssen in 1989 is: Qv = (W * Uz * H * Fd / 2) – (H3 * W * Fp * (dP/dz) / 12µ) where Qv is the volumetric flow rate, W and H are the width and height respectively of the channel formed between the barrel and the screw, Uz is the speed when the particle is touching the barrel, µ is the viscosity, P is the pressure and Fd & Fp are the correlation factors for the drag flow related to the rotational speed of the screw and the pressure force due to the die respectively. This equation is arrived at by assuming that the screw is stationary and the barrel is rotating. The channel is further unwound to make it similar to two parallel plates. Inertial and gravitational forces are assumed to be negligible as compared to viscous forces, the flow profile is assumed to be stationary with respect to time and the screw is considered to be of uniform cross-section.

The energy needed for pumping (Ep) is given by Ep = 3µ(?ND)2 * W * L * [a (1 – a) * cos2?] / H sin ? where, N is the screw RPM, D is the screw diameter, ? is the screw angle and a is a dimensionless form obtained by dividing the pressure flow component by the drag flow component. The pumping efficiency of the extruder defined as the fraction of the motor energy used to pump the material towards the die is given by ? = [3a (1-a)] / [1 + 3a + 4 tan2 ?] When d?/da is zero, the efficiency is maximum. In the case of most single screw extruders with pitch equal to the screw diameter (? = 17° 40?), the maximum efficiency is 27.9 at a throttle ratio of 0.36.Therefore, in a single screw extruder, more than 72% of the energy is wasted on the generation of heat and other energy losses.

A simpler model has been developed based on one dimensional modelling (Li 2001). This model can simulate and predict extruder behaviour (such as pressure, temperature, fill factor, residence time distribution, shaft power, degree of cook) under various operating conditions (such as feed rate, screw speed, feed temperature/moisture, barrel temperature). With a very feat and efficient solution algorithm, this mode1 runs fast on a PC.

The process time is very important for the reaction in an extruder and every element of feed material is supposed to be subjected to the similar residence time, but particles experience variations in residence time due to screw geometry and rheological effects. This results in the residence time distribution (RTD) that gives information about the degree of mixing, the residence time expectancy of fluid and the degree of uniformity of reaction the feed material undergo during the their passage through an extruder.

Seker (2003) studied the effect of moisture content on the residence time distribution in a single screw extruder. It was observed that the residence time distribution was hardly affected by the variation in moisture. This may be due to the fact the moisture content of feed affects the rheology of feed in two opposite ways at the extruder. In the first way increasing the moisture content of feed material results in the decrease of viscosity of feed material in the barrel of an extruder, and lower specific mechanical energy is required to pump the melt through the die. Consequently in the second way, temperature in the die due to viscous dissipation is lower, and the lower temperature of feed increases the viscosity at the die, which tends to increase the restriction of flow through the die with dependence on temperature control. The effect of moisture content on the mean residence time is expected to be the result of two opposite effect of moisture content on rheology of feed material in the barrel and die of the extruder.

However, with respect to the effect of screw speed on residence time distribution, there are certain contradicting reports. While Unlu & Faller (2002) state that screw speed has a relatively small effect on the decrease in RTD, Seker (2003) reports that this effect is more pronounced. The variation in these results may be due to the different screw designs, lengths (1.16m v/s ~20cm), moisture contents of the raw material (11.2% v/s 28-40%) or other feed characteristics and processing conditions. The reports also contradict when the residence time distributions are considered. Narrower residence time distribution (RTD) is preferred in extrusion processing since one of its main purposes is to homogenize the product. While Unlu & Faller (2002) have observed that an increase in the screw speed has a negligible effect with a small increase in the spread of the RTD curve, Seker (2003) reports that with an increase in the screw speed the curves narrow down rapidly. However, Unlu & Faller have quoted many other reports which match with their findings.

Extruders are usually run in a starved fed condition. Therefore, an increase in the feed rate leads to an increase in the degree of fill in the extruder and therefore a decrease in the residence time.

Gonzalez, Torres, Greef & Guadalupe (2005) studied the effect of feed moisture, screw speed, die length, hardness of raw material and composition of feed on the extrusion responses of die pressure, mass output, specific mechanical energy and melt viscosity. They concluded that there is a complex and combined relation between all the factors and each response and that Hence each material behaves in a different mode depending on the particular level of each factor. These interrelations between factors add more complexity to the analysis of starchy systems and particularly to the melt viscosity.

One of the reasons why extrusion is gaining importance is that it is a high temperature short time (HTST) process. HTST processes are known to cause lower degradation of nutrients. However, during extrusion, nutrients are subjected to high mechanical shear and pressure and this may cause greater degradation.

Even as rugged as extruders can be, they can still fail. This is why operations personnel should have a good mental grasp of basic scientific principles, such as mechanics, physics, and electrical priciples as well. Extrusion processes are increasingly technical and not designed for the marginal or employee of low-functioning intelligence.