Extrusion Processing

6.1. Starch Modification

Transformation of starch in an extruder to a molten plastic-like product (destructurized starch) occurs in three steps; plasticizing the solid starch into a molten liquid through melting, shaping of the molten state with the use of a die, and cooking and texturization (Colonna et al, 1987). Starch undergoes strong chemical reactions during extrusion processing that result in depolymerization. The viscoelasticity of the starch is increased due to the sudden pressure drop, the molten mass expands. The is referred to as the "Weissenberg effect" and is independent of the "flash-off of the super-heated steam which occurs during the sudden pressure drop as starch leaves the die (Clark, 1978; Colonna et al, 1987).

The fluid flow of starch inside the extruder can be described in terms of food dough rheology. Under isothermal conditions, molten starch in the extruder is said to be pseudoplastic (shear thinning) in nature and is described by the power law:

where t = shear stress (N/m2), K = consistency index (n*s/m2), n = flow behavior index (dimensionless), and y = shear rate (s1).

Clark (1978) found a good fit between the experimental data and the above model, but did not account for a large range of shear stresses. Other models have been developed, but are too complex to be discussed in this chapter. Other methods take into consideration temperature and moisture content since they are both controllable parameters of extrusion operations (Colonna et al, 1987).

The granular and crystalline structure of starch disappears during extrusion cooking. The starch phase is homogenized by the shearing of the molten granules. Extrusion destroys the organized crystalline structure either partly or completely, depending upon the amylose-amylopectin ratio and on extrusion variables such as moisture content and shear (Mercier et al, 1979; Colonna et al, 1987; Chinnaswamy and Hanna, 1990). In a waxy maize starch, a reduction of crystallinity is observed at extrusion temperatures of 70°C. At a higher temperature the structure is destroyed and gave an X-ray diffraction pattern typical of an amorphous state (Colonna et al, 1987).

Starches with a typical amylose content of about 27% behave in a much different fashion. Two different structures are formed according to X-ray diffraction patterns. Below 170°C, the characteristic "V" starch structure appears, whereas when the temperature rises above 185°C, a new structure called "extruded" or "E" type structure appears (see Figure 5). The "E" type is characterized by three diffraction peaks slightly displaced from those of the "V" type. Both structures occur at 170°C, but reconditioning the starch to 30% moisture always transforms the "E" type to a more stable "V" type (Mercier et al, 1979; Mercier et al, 1980; Colonna et al, 1987; Watson and Ramsted, 1987). Because of the similarity between the "V" and "E" patterns, the assumption is made that the extruded starch structure is helical with six glucose residues per cycle in a hexagonal symmetry. The differences between the two are said to be due to the different interaxial distances in the two helices. The interaxial distance for the "V" structure in 1.38 nm while the "E" starch is 1.50 nm (Mercier et al, 1979). The results of X-ray diffraction suggest that during extrusion, the structure of the cereal starches is reorganized into the "V" structure and that the "E" structure is caused by the variation in the inter-helix distance rather than by the presence of another helix (Mercier et al, 1979; Colonna et al, 1987).

The amylose-amylopectin ratio determines the properties of the final product. Amylose provides lightness, elasticity, surface and texture regularities, but a sticky surface (Chinnaswamy and Hanna, 1988a). Amylopectin leads to a harder and less expanded extruded product (Mercier and Feillet, 1975; Colonna et al, 1987). Matz (1976) recommended an amylose content of 5% to 20% in starch to give an adequate crispness and acceptable texture in snack type products.

The gelatinization of starch is affected by several conditions set in extrusion processing. Chaing and Johnson (1977) reported that moisture content does not significantly affect starch gelatinization at low temperatures (65 and 80°C), but gelatinization increased at higher temperatures (95 and 110°C) when the moisture content is between 18% and 27%. Gelatinization of starch occurs at a higher rate when the moisture is between 24% and 27% than at 18% to 21% as the temperature was increased. Increasing the shear rate (screw speed) and /or the die size decreased starch gelatinization. Chaing and Johnson (1977) also reported that the (2,1) glycosidic bonds of sucrose and raffinose and the (1,4) glycosidic bonds of malto-oligosaccharides and starch are broken when cereal products are extruded.

In an extruder, liquefaction of starch occurs without the use of enzymes due to the development of high pressure and high shear. Starch is broken down into small molecular weight sugars at high temperatures and pressures (Chinnaswamy and Hanna, 1988a,b,c). However, the formation of monosaccharides has not been observed (Lorenz and Johnson, 1972; Chaing and Johnson, 1977; Colonna et al, 1987). During extrusion cooking, amylose and amylopectin are degraded into lower molecular weight materials. However, the percentage of a (1,6) linkages as compared to a (1,4) does not change during the extrusion process. Therefore, only the a (1,4) links are not affected. This could be explained through the theory that there are fewer a (1,6) linkages or that they are not as accessible as the a^M^Mercier and Feillet, 1975). It is known that the macromolecular degradation occurring in the extruder is a function of extrusion parameters such as temperature, moisture and screw speed (Colonna et al, 1987).

Figure 5. The X-ray diffraction patterns of corn starches extruded at the temperatures indicated. Initial moisture content before extrusion was 22% (db). Final moisture contents are indicated on the figure (Colonna et al, 1987).

The overall functional properties of starch after extrusion vary with the amylose-amylopectin content (Chinnaswamy, 1993). The water absorption index decreases with increasing extruder temperatures for waxy maize starch, while little change is observed for amylose until the temperature increases above 200°C. At this point the water absorption index increases sharply with increasing temperatures (Mercier and Feillet, 1975). The water solubility index also decreases with increasing amylose content. Native starch is, of course, insoluble in cold water. During extrusion, the molecules are broken down into smaller segments which make the overall solubility greater, and the starch becomes soluble in cold water (Mercier and Feillet, 1975). Chinnaswamy and Hanna (1988a) reported that the optimum expansion ratio for starch was at 160°C for 70% amylose starch. The expansion ratio decreased at 140°C when amylose content of starch was greater than 50%. Chinnaswamy and Hanna (1988b) also found the optimum moisture content for expansion of starch to be 14%.

Chinnaswamy (1993) studied the effect of length to diameter ratio (L/D) of the die nozzle and how that affected the final expansion of extruded starch. Starch consisting of 25% amylose was used for this study. The expansion ratio increased sharply from 4.5 to 13 as the nozzle L/D ratio increased from 2.5 to 3.4 and then declined gradually to 8.5. The extruder pressure (measured in the extruder compression section), however, increased with increasing L/D ratios from 4.6 to 14.6 MPa. The increase in expansion ratio with L/D ratio and extrusion pressure may be due in part to a increase in the degree of starch gelatinization. It has been published that starch expansion is dependant upon the degree of gelatinization. The operating pressure increased with increasing length of the die and decreasing diameter of the die orifice. The results indicated that the operating pressure for maximum expansion of extruded starch was 7 MPa. However, it should be noted that L/D ratios for the die nozzles are not the best method for judging maximum expansion and pressure for the extrusion of starches because different combinations of length and diameter can give the same expansion and pressure results.

Chinnaswamy (1993) also reported the effect of chemical additives to the extruded starch at various amylose concentrations. It was found that the addition of NaCl increased the expansion ratio of starch as compared to the native starch at both 25 and 50% amylose contents with the expansion ratio of 50% amylose being slightly higher than the 25% amylose starch. The addition of sodium bicarbonate slightly lowered the expansion ratios for 25 and 50% amylose starches, but was higher than the native starch at 0% amylose content. Urea was also studied for its effect on expansion and was found to have the lowest expansion ratios for all four (0, 25, 50, 65%) amylose contents studied.

Chinnaswamy (1993) and Sokhey and Chinnaswamy (1992) further studied the effect of radiation treatment (10, 20 and 30 kGy) on expansion ratios of extruded starched with various amylose contents. At amylose contents of 25 and 50%, the expansion ratio increased with increasing radiation treatment. Chemical additives such as eerie ammonium nitrate, potassium persulfate and hydrogen peroxide which are known to induce or enhance free-radical formation were mixed with the starches treated with 20 kGy and all of the chemically treated irradiated samples showed significantly less expansion than the irradiated starches with no chemical treatment. All these chemical modification treatments have altered the starch molecular profiles which had irreversible effects on starch functional properties.

The partial depolymerization of starch in extrusion cooking leads to a low viscosity at 95°C compared to native starch, and subsequently signifies an absence of gelling ability. The paste stability at 95°C, however, is improved and starch rétrogradation is reduced (Colonna et al, 1987). A loss of paste viscosities in extruded starches presents a major disadvantage in some product formulation. However, their slow dispersion in cold water and rapid dispersion in hot water to form products of various consistencies permit the commercial use of this type of modified starch for instant foods (Colonna et al, 1987). The overall digestibility of the starch product is also greatly increased as the temperature of extrusion is increased. However, the potential substitution of pregelatinized starches by extruded starches is based only on the estimation of their functional properties, rather than their nutritional value. This is due to the lack of information available on the nutritional quality of extruded starches (Colonna et al, 1987).

These types of starches, referred to as pregelatinized starches, quickly rehydrate in water and can be incorporated into cold food products to increase viscosity and binding. They are useful in food products that do not require cooking (Whistler and Daniel, 1984). Other possibilities for modifying starches using extrusion cooking is the direct injection of chemicals such as acids, cross-linking agents, and phosphate groups into the extruder barrel. This decreases the processing time for the modification of such starches.

6.2. Ready to Eat Cereals

Extrusion cooking of ready-to-eat cereals (RTE) provides several advantages over conventional processing methods. Extrusion cooking allows for faster processing times, lower processing costs, less square footage of the plant required for processing equipment, and greater flexibility leading to more types of end-products (Bailey et al, 1991). One needs to go no further than the local grocery store to see the wide array of cereals currently available in the market place. The creation of a wide range of cereal shapes and sizes is possible by changing the processing conditions such as moisture, temperature, ingredients, die orifice and screw configuration. The ability to coat the final product with various colors and flavors increases the variety of extruded RTE cereals even more. Bailey et al (1991) summarizes the steps required to make either expanded RTE cereal or a flaked product using a forming extruder.

The cereal ingredients are first screened to remove any large fragments that may interfere with flow in the barrel. Next, the product is combined in a mixer to ensure uniform dispersion of all dry ingredients. From the mixer, the dry mix is moved to the bin discharger which is mounted to the top of the extrusion cooker. The discharger holds and maintains a predetermined amount of dry mix so the mix can then be moved to the preconditioner by the screw feeder. The screw feeder assures that the flow of product is uniform to the preconditioner. The preconditioner allows increased residence time and mixing in the extrusion process. Heat can be added to the system in the form of steam which also increases the moisture of the dry mix and allows for even distribution of that moisture and thermal energy through the extra mixing that takes place. The moisture of the product assists in overall flavor development and affects the final texture of the product. It also improves extruder efficiency.

Bailey et al (1991) recommends the fully intermeshing co-rotating twin screw extruder for RTE cereals because of its versatility. The twin screw extruder can be adapted to perform several different tasks including heating, cooling, conveying the product, feeding, compressing, reacting, homogenizing, melting or rendering amorphous, cooking, texturizing and shaping of the product (Bailey et al, 1991). However, considering the most RTE cereals are of low moisture and high expansion formed under high shear and pressure, single screw extruders can also be utilized for this type of processing. The initial moisture content would be higher which in turn requires more energy for drying, but the same end product can be produced (Harper, 1986). The screw configuration is designed so that the feed zone conveys the product forward to a processing zone. In this zone, the ingredients are compressed using shear locks and a decreased shear pitch to make a homogeneous dough. The temperature of the dough increases dramatically in the last 2 to 5 sec of the total residence time of 35 to 40 sec. The barrel has a length to diameter ratio of 16.5 to 1 and is typically the same for direct expanded cereals as well as flaked products. The moisture content of the expanded product is around 10 to 15% at the die whereas the flaked product moisture contents are from 20 to

30%. The direct expanded cereal is ready to be dried further or toasted and coated if necessary before it is ready to package. The flaked product goes through several other processing steps.

The processing of corn flakes without the use of extrusion cooking is a long process that is manufactured in batches, and not a continuous process as described by Fast (1990). First, the germ must be removed from the corn kernel and the endosperm broken into two or three pieces depending upon the size of the kernel (the process varies slightly for wheat flakes and rice flakes). The other processing ingredients are added next. The product is then moved to batch cookers which are filled to 50-67% capacity to allow for expansion of the cooking product. The cookers are generally 4 ft in diameter and 8 ft long and built to withstand direct steam injection under pressure. The product is normally cooked at 15 to 18 psi for 2 hours. The products is considered finished cooking when the grits have transformed from a "hard, chalky white to a soft golden brown product when is translucent". The product is then dumped and moved onto further processing (Fast, 1990).

The product moves along to a delumping stage to break the grits into single pieces. Delumping is important in that it insures uniform drying of the product. The grits next move to the drying stage. The temperature used is generally 121°C (250°F) under a controlled humidity. The humidity must be controlled to "impede the removal of moisture from the center of the grit" (Fast, 1990). Controlled humidity prevents this phenomena known as case hardening. It also speeds up the drying process to the desired end moisture of 10 to 14%. The product is then conveyed to a cooling and tempering step when the moisture of the grit is allowed to migrate from the center of the grit to the outside making for a more uniform moisture distribution. Even with controlled humidity drying, the tempering step takes a few hours to complete. The grits are now ready to be rolled out in the rollers which are hollow to allow for chilled water to pass through removing any excess heat. From here the final step is to toast the product at 274-329°C (525-625°F) for 90 seconds to drive the final moisture down to 1.5 to 3%. This step creates the blistering effect that is desired in the final flaked product.

Making corn flakes with extrusion processing eliminates the long cooking processes required with the grits, the several hours of tempering to equilibrate the moisture and the bottleneck created in production between the cooker and the controlled humidity drying. These steps are eliminated because extrusion processing begins with flour instead of a whole corn grit and follow the same processing procedures used for extrusion processing of RTE. The flaked product, after leaving the cooking extruder, goes to a forming extruder. This extruder has low shear and a deep flighted screw to allow the dough to be extruded through a die that makes little beads. The barrel temperature is controlled using a jacket to circulate water around the system to remove excess heat. This process has a long residence time and since the extrudate in hot and moist, it adds to the flavor development (Bailey et al, 1991). The beads are formed by cutting the product with a rotating blade at the end of the die. The same type of blade is also used to cut the direct expanded RTE cereal at the end of the cooking extruder. The beads are conveyed with a negative air conveying system to insure the outside of the bead dries significantly enough to prevent the product from sticking together (Bailey et al, 1991). The beads are then transferred to a long cylinder called the bead conditioning reel which allows air to cool the temperature of the product to 38 to 63°C. The residence time can be changed by altering the angle at which the reel sits. The steeper the angle, the shorter the residence time (Bailey et al, 1991). From the conditioning reel the product goes to the tempering screw where the objective is to maintain proper moisture and temperature of the bead. If the beads are too cool, heat is applied to the wall of the tempering screw (Bailey et al, 1991). Corn and rice beads tend to flake best between 43 to 54°C while bran and multi-grain beads flake best at 46 to 63°C (Bailey et al, 1991). From here the product is passed through a metal detector to ensure product safety and eliminate sparks that may form on the flaking roll if metal fragments are present. This will also increase the wear life of the flaking roller. The beads go to flaking roller where the final shape is reached. From here, as in the direct expanded cereal products, the flakes go to ovens to be toasted for additional flavor and to coaters to add flavor and sugar to the outside of the product. The product is then dried and cooled and is ready for packaging (Bailey et al, 1991). This process cuts the final floor time needed to produce a flaked cereal from several hours to 30 to 60 minutes.

Another area where extrusion cooking can cut the time necessary to process a RTE cereal is in the production of gun puffed whole grain cereals (Fast, 1990). The traditional method calls for rice or wheat grain. Wheat requires a pretreatment step to avoid loosing the bran in a ragged manner. This can be accomplished by treating the product in a 4% brine solution (26% salt). The salt toughens the bran during preheating and make it adhere to itself better and makes it stronger.

In the traditional process, the product is then loaded into a steel pressurized barrel built to withstand 200 psi that has a internal volume of 0.4 to 0.5 ft3. The puffing gun has one opening into which the product is loaded into and fired for puffing. The gun is closed and sealed after loading. Heat is applied to the walls of the barrel through the use of gas burners and the barrel is rotated to insure uniform heating. The water inside the grain is converted to steam. When the pressure is released, the internal steam is released from inside the grain causing puffing to occur. The final product is about 5 to 7% moisture and must be dried to 1 to 3% moisture in the final product.

Extrusion processing of puffed cereal products also begins with flour instead of a whole grain which eliminates the preconditioning step. The extruder cooks the product and forms it into the desired product with the use of a die, with cooking and extruding a one step process instead of a two step process. An example of a product made in this fashion is General Mill's Kix (Fast, 1990). The dough goes from a cooking extruder to a forming extruder to make the general shape for puffing in the same manner as before. This completely eliminates any pretreatment and also eliminates the worry of broken kernels not adequate for puffing using the whole grain (Fast, 1990).

The die used can also have an affect on the expansion of RTE cereals. Chinnaswamy (1993) experimented with 25% amylose starch to determine the effect of the die shape on expansion. As the length to diameter ratio (L/D) of the die increased, expansion increased until the L/D reached 3.4 mm. As the L/D increased further, expansion slowly decreased. Chinnaswamy (1993) also found that an increase in extrusion pressure increased the expansion of 25% amylose starch. Optimum expansion was achieved at starch blends of 50% amylose. This is very important in choosing optimum conditions for the desired expansion of an RTE cereal product.

6.3. Snack Products

Snack product are very similar to RTE cereals in that they are cereal starch based products. However, snack products are generally extruded at a lower moisture content, higher shear and higher temperature to cause significant starch modification as opposed to the RTE cereals. This leads to a more expanded product that melts in the mouth (Harper, 1989). As mentioned previously, the collet extruder is primarily used for the forming a snack products with high expansion. The internal pressure, as well as the size and shape of the die orifice, leads to the final shape of the product. Extruded snack products are produced in a variety of shapes and sizes which include rings, stars, curls, balls, lattices and squiggles (Tettweiler, 1991). There has been little interest in producing corn-curl type products with a twin -screw extruder because of the high capital costs of these machines and the wear that occurs to the twin screws due to the low moisture content used during processing. Briefly, the single screw extruder performs adequately in making this type of product at a lower cost. Also, the control of temperature and shear are not necessarily desired in this case, so the use of a twin screw extruder is not cost effective (Harper, 1989).

Twin screw extruders play a role in second and third generation snack products that need to be fried, baked, microwaved, reformed, etc. by the consumer or processor before consumption. The twin screw extruder in these cases is used more for forming a product. The single screw extruder in this case eliminates processor control necessary to properly maintain a consistent product. Twin screw extruders are also responsible for creating a whole new line of low-calorie crispbread snack foods (Linko et al, 1983; Harper, 1986). Extrusion replaces the old baking lines used to make this type of product which were more expensive and required more drying and cooling than the extruded product.

Extrusion technology also plays a role in the processing of tortillas. After the dough has been mixed, extrusion can be used to form the tortilla. The extruder is designed for minimum kneading of the dough and works a sheeting mechanism. The extruder sheets the dough into a layer that is dusted with flour, rolled, and further thinned by a crossroller. From this point, the dough is cut to the desired size with a rotary cutting unit with the scrap returning to the extruder to be reprocessed (Serna-Saldivar, 1988).

Piston and roller extruders play a dominant role in the baking of cookies and other baked snack goods. Many bakeries use piston type dough depositors that either directly deposit dough into a pan or use a wire to cut a layer of dough to be baked on a cookie sheet. Generally, short dough is used for these types of products to ensure a clean cut. Roller extruders can be smooth and used to make thin sheets that are required to make cracker type products. Here>the rollers move back and forth perpendicular to the cookie sheet to layer the cracker dough. The layers are held together with docking pins during baking and are responsible for the holes found in saltines. The thin sheets of dough can also be used to make figure cookies, such as animal crackers, by cutting the dough layers into various shapes and sizes. They can also be perforated to various shapes and sizes to make formed products such as Oreo type cookies. Sugar and shortening are used at lower levels than those for conventional cookies to ensure the dough does not spread while baking and yet the design of the perforated roller is not lost. This leads to a crumbly dough that is just wet enough to work with.

6.4. Texturized Vegetable Protein

Texturization of vegetable protein is the restructuring of protein molecules (usually soy protein) into a layered, crosslinked mass which is resistant to disruption upon further heating and/or processing (Harper, 1986). Texturized vegetable protein (TVP) is divided into two classification; extrusion cooked meat extenders and extrusion cooked meat analogs. Meat extenders are produced from defatted soy flour (52% protein) and soy concentrate (70% protein). These products are rehydrated to 60 to 65% moisture and are blended with meats or meat emulsions at levels of 20 to 35 % or higher (Rokey and Huber, 1987). Meat analog processing involves using one or two extrusion cookers in series to convert vegetable protein into varieties of meat analogs which can have both the texture and appearance of meat (Rokey and Huber, 1987).

When the mechanical and thermal energy of extrusion cooking is applied to proteins during extrusion processing, they tend to lose their structure, unfold and become denatured, forming a continuous visco-elastic mass (Harper, 1986; Rokey and Huber, 1987). The design of the extruder barrel, as well as the screw configuration, align the protein molecules in the direction of the flow. This realignment exposes bonding sites which lead to cross-linking and a reformed expandable structure (Harper, 1986; Rokey and Huber, 1987). The increase in shear, temperature and retention time will cause cross-linking to occur between the protein molecules. Texturization is resistant to disintegration upon rehydration and leads to a chewy end product (Rokey and Huber, 1987). The temperature in the barrel is usually between 140 and 160°C as that range leads to the best chemical cross-linking between vegetable proteins and gives the final fibrous meat-like structure. The final product can be rehydrated to approximately three times its weight (Harper, 1986).

In addition to retexturizing vegetable protein, Rokey and Huber (1987) discuss several other functions extrusion cooking serves. (1) Extrusion cooking denatures proteins which lowers solubility and makes the product more digestible. It also inactivates enzymes and destroys the activity of any toxic proteins. (2) Texturization with extrusion cooking deactivates heat labile growth inhibitors native to raw vegetable protein. (3) Extrusion cooking controls bitter flavors associated with soy proteins. Most of these flavors are volatile and are vented off during extrusion processing. Others are lost due to the compression of the protein at the die orifice. (4) Extrusion cooking also provides a homogeneous, irreversible bonded dispersion throughout a protein matrix. This ensures the final product is uniform and all ingredients have been incorporated thoroughly. (5) The shape and size of the extruded vegetable protein is convenient for packaging and shipping.

A typical setup for the processing of TVP includes a bin or feeder, preconditioner, and extrusion cooker (Rokey and Huber, 1987). The raw ingredients are added to the bin and are metered to the preconditioner after being thoroughly mixed. It is important that the flow is constant and the rate of flow controllable. The preconditioner controls the rate of flow and moisture through the injection of water or steam (Rokey and Huber, 1987). The addition of steam controls the raw material moisture content and temperature. The raw material requires a moisture content ranging between 10-25%. The preconditioner can either be pressurized or atmospheric. A pressurized preconditioner can create a higher outlet temperature but can destroy nutrients (Rokey and Huber, 1987).

Either single or twin screw extruders can be used for texturizing vegetable protein. In both cases, the texture is affected by screw speed, barrel, temperature, moisture, raw material quality, and die orifice (Rokey and Huber, 1987). The advantages and disadvantages of the twin screw and single screw extruders have already been mentioned. The protein is conveyed down the channel of the barrel and converted into a dough. The compression ratio, which refers to the volume in the flights of the screw, decreases as the product moves closer to the die which increases pressure. The product at this point has been plasticized. As the product passes through the die, the product suddenly reaches room temperature and expands (Rokey and Huber, 1987). The die for processing TVP emits a smooth streamlined flow that eliminates shearing so to not disturb the newly cross-linked protein (Rokey and Huber, 1987).

6.5. Pasta Products

Pastas are generally wheat-based products that are formed from a dough, where no leavening is necessary. They are generally made of flour and water, although eggs are sometimes added (Hoseney, 1986). Durum semolina is the best material for making flour for pasta. Durum wheat is a hard wheat and gives pasta its yellow color and is generally different from common wheat. Most durum wheats are white spring wheats that are translucent and high in carotenoids which gives it a slight amber color (Hoseney, 1986). Generally, durums have poor wheat gluten and do not make good bread products.

Figure 6 demonstrates the extrusion equipment used for processing pasta products. The semolina is mixed with water until 31% moisture is reached. The product will form into small dough balls. This mixing occurs in a air tight mixer in order to control the incorporation of air (Hoseney, 1986). A degasser can also be used to aid in keeping air out of the dough (Cantarelli, 1985). Small air bubbles in the product will weaken the final dried pasta product. The incorporation of air also leads to the activation of lipoxygenase enzymes that bleach the dough white. Hence, elimination of air in the system will limit the activation of these enzymes (Hoseney, 1986).

A single screw extruder with deep flight channels to eliminate shear is used for the extrusion of pasta products. It acts as a mixer, kneads the dough and exerts pressure. The dough moves down the barrel to the extruder die. This process produces a lot of heat which needs to be eliminated. Generally, the pasta extruder is equipped with a jacket that is usually filled with cold water, keeping the temperature below 45°C. The low temperature combined with the low moisture content leads to little or no expansion which is desired in pasta production (Hoseney, 1986). A high level of extrusion velocity leads to less damage to the gluten network and better starch retention upon cooking while lower extrusion temperatures produce less damage to the protein fraction thus yielding better hydration upon cooking (Cantarelli, 1985). The smoother the die plate, the better the production. Therefore, stainless steel and teflon dies are preferred over bronze dies which are more expensive and tend to wear out faster. The die must be thoroughly cleaned and kept frozen after use to prevent the growth of bacteria which could greatly effect the final product (Hoseney, 1986). Several different die shapes can be used to make the various types of pastas available in the market.

The pasta then goes through a long drying step lasting 10 to 16 hours. Drying is the most important process in pasta production as cracking or checking can occur if the product is not dried properly (Hoseney, 1986; Cantarelli, 1985).

Due to the high cost of durum wheat flour and the availability of other types of starches in developing countries, there is a trend to develop pasta products based on cereal products other than wheat (i.e., rice, starch , potato flour, maize and legumes) (Giese, 1992). These technologies require a high temperature pretreatment of a fraction of the dough starches or flour, which is then mixed with the remainder of the ingredients. This could be done with a preconditioner and a cooking extruder and allows for proteins to form a coagulated network. The operation works well to improve pasta quality with raw materials with inferior protein quality (Giese, 1992). Extrusion cooking could also play a role in pregelatinizing starch in pasta products leading to gelatinized levels over 95%, a low microbial count, and the ability to rehydrate in cold water (Giese, 1992).

Raw material infeed

Mixing trough, shafts and paddles all made of stainless steel.

Stainless steel feed mounted at right angle to mixing trough, allows a large mixer discharge openmg and conveys loose product, with* out compacting, to the main extrusion screw.

Screw feeder for accurate metering of dry floury or granular material plus water.

Raw material infeed

Mixing trough, shafts and paddles all made of stainless steel.

Screw feeder for accurate metering of dry floury or granular material plus water.

Air pumped from vacuum chamber here for de-aeration of product as it transfers from the feed screw to the main ex* trusion screw.

Stainless steel mam extrusion screw builds up pressure, necessary for forcing product through die.

- Main screw equipped with cooling (or heating) chamber around sleeve m this portion.

Extrusion die for forming product shape. Rotary cutter -on bottom die surface cuts product to required length.

Figure 6. An example of a single screw macaroni extruder (Harper, 1981).

Air pumped from vacuum chamber here for de-aeration of product as it transfers from the feed screw to the main ex* trusion screw.

Stainless steel mam extrusion screw builds up pressure, necessary for forcing product through die.

- Main screw equipped with cooling (or heating) chamber around sleeve m this portion.

Extrusion die for forming product shape. Rotary cutter -on bottom die surface cuts product to required length.

Figure 6. An example of a single screw macaroni extruder (Harper, 1981).

6.6 Meat Products

The best example of the meat product extrusion is the standard meat grinder. The meat grinder is similar to the pasta extruder in that there is one solid screw with deep channels used for conveying the meat forward, and a smooth die is used to save on wear and forces the product through the desired die size. Simply put, it is a forming extruder. One of the drawbacks to using a meat grinder is that more surface of the meat is exposed to the metal of the grinder. This leads to microorganism contamination of the product and incorporates oxygen thus decreasing the shelf life of the product (Urbain and Campbell, 1987). Besides the obvious production of hamburgers, the use of a meat grinder is the first step to the production of fresh sausage products. Spices and flavorings are added after grinding in a mixer.

Fresh ground sausage is packaged in either non-edible or edible casings. For both types of packaging, a piston extruder, screw extruder or what some call a ram type extruder are used. The piston and ram types are very simple in design. A certain amount of product is forced through an die opening and into the casing. These machines are designed to twist the sausage casings to a desired length after a designated amount has been forced in, leading to the long sausage links available in meat markets. At this point the fresh sausage is ready for consumption.

Another method in which extrusion is used in meat processing is the production of meat emulsions such as frankfurters and bologna. Although the simple piston extruder will stuff the casings in a similar fashion as the fresh sausage, co-extrusion is also used. Figure 7 demostrates that co-extrusion is essentially the extruding of two products at the same time (Potter, 1986a). A premixed collagen dough casing is extruded around the outside of the emulsion and formed around the product. Then the emulsion is twisted to desired lengths the same way the fresh sausage is and the frankfurters go to the smoke house to be cooked (Potter, 1986a).

Direction of Flow

Figure 7. Co-extrusion processing set up for frankfurters (Potter, 1986).

6.7. Confectionery Products

Twin screw extruders are more common in the processing of confectionery products. This is due to the twin screw's ability to convey material, renew material at the heating surfaces, control temperatures of heat-sensitive materials, and incorporate fat, milk solids, nuts, color, and flavor portends (Harper, 1989). Twin screw extruders used for confectionery products generally have a longer length to diameter ratio (25:1) to increase the heat transfer area that is required to produce products such as licorice, peanut brittle, caramel and toffee (Harper, 1989). For example, licorice is produced by adding flour, starch, corn syrup, sugar, and a gum such as carboxymethyl cellulose (Harper, 1989; Schuler, 1986). The ingredients are blended together to make a low viscosity slurry before being pumped to the extruder. In the extruder, high pressure and temperatures dissolve the sugar and gelatinize the starch. Flavors and colors can be directly injected into the barrel to improve product characteristics. The finished product comes through the die and is cut to the desired length (Schuler, 1986). By producing licorice continuously, labor and energy is reduced compared to conventional methods.

Extrusion cooking is also used in the production of milk chocolate. After production of chocolate liquor from processing the cocoa beans, the liquor goes through several steps before it becomes milk chocolate. The chocolate liquor is first mixed with sugar, milk, cocoa butter and other flavors. This mixture is then subjected to a fine grinding step called refining by passing through revolving rollers (Potter, 1986b). The mixture then goes to a special heated mixing tank to be "conched" (Harper, 1989; Potter, 1986b). These tanks have pressure rollers that grind and aerate the melted liquor to improve smoothness, viscosity and flavor. This is usually performed at 60°C for 96 to 120 hours (Potter, 1986b). Chaveron et al. (1984) reported that this entire process is being replaced using a twin screw extruder and the total processing time runs between 2 to 3 minutes. A preconditioner is used as a mixer in this process and incorporate air into the system. The mixed product is then fed to the barrel where heat would be applied and melt the mass to form milk chocolate.

Extrusion can also be an alternative for liquid depositing of confectionery jelly materials such as "jelly bean" and "gummy bear" type products (Moore, 1989). The product is first cooked and evaporated down to its final moisture content with no extra drying being necessary. The cooked material goes to a forming extruder which is used to convey a viscous material that is thick and plastic like. Cooking extruders can also be used in this field. It has been demonstrated that jelly formulations can be cooked, extruded, and die face cut with a high speed rotary knife (Moore, 1989). This is done with instantized gelling starches that can be processed at moderate temperatures without boiling or flash-off (Moore, 1989). One problem with this technique is the temporary hot resting stage of belt travel, where the gel can most effectively form, is lost in this process. One way to prevent this is to use a chilled water spray on the die face, the emerging hot candy and the knife cutting edge. This can serve as a temporary release agent, preventing stickiness and buildup on the work surfaces (Moore, 1989).

A forming extruder is also used in the processing of jelly confectionery product that would be a "continuous rope" to be cut into pieces (Moore, 1989). The pieces are then coated with sugar or a glaze and be ready for consumption. The flavors and colors are directly injected into the preconditioner of the extruder and mixed thoroughly before entering the extruder barrel. The belt travel after extruding is cooled to allow the product to be workable. Sticking and hang-up in production will occur in this process and thus the reason to cool the product immediately after coming through the die opening. As mentioned earlier, chilled water sprays in appropriate areas will aid in processing. The blades used are guillotine or ferris-wheel type cutters that are not at the end of the die orifice. They work after the product is cooled to a workable temperature (Moore, 1989).

6.8. Co-extrusion

The concept of co-extrusion was discussed briefly in the meat products section of the chapter. Since co-extrusion is used in several areas of food processing, we decided to separate it from the other sections and discuss it now. In co-extruded products, the outer layer is processed with a cooker extruder with a die that forms a hollow center. The center is then filled with a viscous filling which will not flow freely at ambient temperatures (Bass, 1985). The uniformity of cooking and operation of the twin screw extruder make co-extrusion more practical in the processing of snack type products as well as center filled confectionery goods (Harper, 1989). It is now possible to pump the filling directly into the center of the expanded outer product with the use of special dies developed for this process (Bass, 1985).

Several commercial products consisting of outer layers made by cooking extrusion and filled with pumpable centers have been introduced to the market. Examples of these types of products are Cadbury's "Criss Cross," Mars' "Cornquistos" and "Dooleys," and Frito-Lay's "Stuffers." No information was available on the success of these products (Abbott, 1987). It is assumed that these products are post filled or the filling is not introduced when the outer layer is expanded. The product handling and injection filling equipment necessary for these products is obviously complex and expensive.

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