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Twin-screw extrusion of rice flour
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Factors Affecting Extrusion Cooking
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Manufacture of ready-to-eat breakfast cereals
Manufacture of ready-to-eat breakfast cereals Previous | Next
The above discussion describes two competing processes for the production of ready-to-eat breakfast cereals. It appears that at present the continuous extrusion process may offer some economic advantages, while the conventional batch process results in a superior product. You can probably test this for yourself by taste testing a generic store brand cornflake, which is probably extruded and a Kellogg’s cornflake, which is batch cooked. Notice the color, texture, and surface blistering of the products. You probably also notice that either one of these becomes fairly soggy in milk in a matter of seconds. However, consumers can apparently detect the difference.
As shown in the following Figure A6, either extrusion cooking or batch cooking can be used to obtain cooked cereal material for further processing. From this stage final cereals can be made by;
Direct expansion off the extruder - this process is a single step continuous process for cooking and shaping the product. Because of the lower moisture content used with the extruder combined with moisture flash-off during expansion minimal drying is needed to obtain the final product.
Instant porridges - If the above product is ground into a powder it will hydrate quickly in hot water and form a gruel, paste or porridge.
Gun puffed - Rather than direct expansion off the extruder, pellets (half-products) can be obtained via extrusion usually, tempered to a lower moisture content and gun puffed. This involves heating of the pellets in a pressurized steam environment and then "shooting" into the atmosphere where expansion takes place. This two-step process allows independent control of the cooking and puffing processes.
Oven puffed - Usually used for whole grain wheat or rice. Whole grains are preconditioned (softened) to an optimal moisture content and passed through a hot air conveyor oven for toasting and puffing. The advantage is that the whole grain appearance is preserved in the final product.
Flakes - This is the one area where the batch and extrusion process are currently in direct competition. The extrusion process can be a single-stage integral cooking/forming process or two-stage process. This is also a process where the importance of the cooking process and composition (biopolymer and moisture content) on the material properties of the viscoelastic melt becomes critical. This can be inferred from the description of the extrusion flaking process found in reference 2.
Cooking extrusion Cornell
Cooking extrusion Previous | Next
The above description of starch cooking (or conversion from partially crystalline to amorphous state) was developed for high moisture cooking systems to illustrate the basic phenomena. However, from a process point of view these are generally long time processes and also require tempering steps and ultimately final drying to obtain a shelf-stable cereal product. Alternatively, cooked starch slurries at lower solids content (10 - 20 % e.g.) can be spray dried to obtain pregelatinized starch powders for use as ingredients.
Cooking extrusion is a more versatile process that combines cooking, or conversion of starch, and forming into one continuous process. Co-rotating twin-screw extruders are increasingly being used in the food industry because of their modular screw design, high shear cooking capability and improved process control over single screw extruders. Cooking extrusion is a particularly interesting example of a thermomechanical process, where a viscoelastic biopolymer melt is actually formed within the extruder under high shear and temperature conditions.
A typical process used for puffed corn snacks is shown below (Figure A5). This example of a twin-screw extrusion screw profile was taken from Hauck and Huber (1989). "Dry" free flowing cornmeal (10 % moisture content) is fed into the feed end of the twin-screw extruder and mixed with added water inside the barrel. A typical in-barrel moisture content is about 15 - 17 % moisture content. This low-bulk density mixture is subjected to increasing pressure and temperature as the screw volume is reduced in the extruder. At some particular point in the extruder, the dough-like mass is "converted" into a "shiny, yellow’" viscoelastic melt. This melt has the appropriate rheological properties to expand and hold gas as it exits the extruder. The exit temperature of the melt is about 170 C, providing the thermodynamic driving force for steam expansion. In this process the density of the melt is reduced from about 1.5 g/cm3 to 0.1 g/cm3 during the puffing process. The residence time for this process is on the order of seconds due to the efficiency of viscous dissipation as a means of providing thermal energy for starch conversion. However, the high shear stresses and temperatures in this type of process also causes fragmentation of the starch polymer into smaller pieces and results in products with a characteristic "extruded flavor". The puffed cereal product could also be ground to produce an instant pregelatinized starch as described above.
It is also important to realize that the free expansion process occurs outside of the extruder. The extruder offers a convenient and versatile means of generating a viscoelastic melt at a specific temperature and moisture content. However, the result of this foaming process is determined by the thermodynamics of nucleation and growth of water vapor bubbles and the material properties of the melt. For example, if the moisture content is too high, then the melt will expand well, but will also collapse leading to a high-density product with unacceptable texture.
Low-shear batch cooking-Cornell
Low-shear batch cooking
The cooking of starches in excess water (> 60%), such as might be used to produce pregelatinized starches and flaked or shredded cereals is a two-stage process involving GELATINIZATION and PASTING. The starting material is whole grain cereals (e.g. corn, wheat, rice) at room temperature and about 10-14% moisture content. This type of cooking process has been described by Slade and Levine (1991). Paraphrasing their words; "The first stage is gelatinization, which involves a minimum temperature of about 60 C and a uniform moisture content of at least about 27 % on the molecular level. The second stage , usually termed pasting, involves major water uptake, swelling, softening, and loss of starch solids. The minimum processing requirements for pasting are typically a temperature of at least 85 C and a moisture content of at least about 45 % on a molecular level." Thus, cooking is a combined heat and mass transfer process.
It is also important to note that the longer cooking times of these type processes allows for flavor development, which is difficult to dupilcate in the shorter extrusion cooking processes.
Cereals subject to gelatinization and pasting as described above are suitable for either shredding (shredded wheat) or flaking (corn flakes). However, various tempering processes are usually needed to fine-tune the moisture content and temperature of the grains to achieve the material properties needed. There is a small window of moisture content-temperature where the cooked cereal grains are neither too hard nor too sticky to process.
The gelatinization and pasting behavior of starches can be conveniently characterized using specialized rheometers. The traditional rheometer being the Brabender viscoamylograph, while a newer rheometer is termed the Rapid ViscoAnalyser (RVA) (Newport Scientific). Figure 3 below shows schematically how the swelling, disruption of the starch granules and the colloidal dispersion that result with increasing temperature is reflected in the viscosity of the system. In the figure below the terminology is a little at odds with that presented before. This is due to the fact that the two step cooking process, gelatinization and pasting, is usually masked by the nature of cooking in excess water at temperatures above 100 C. It may be more appropriate to term the pasting temperature in this figure as the onset of gelatinization. Below this temperature starch is essentially insoluble in water. The peak viscosity corresponds to the maximum hydrated swollen volume fraction of the starch granules. Further heating and stirring beyond this point results in the rupture of the granules (and decrease in viscosity) and the release of macromolecules (primarily amylose) into solution.
It is of great technical significance that upon cooling again to room temperature (not shown in this figure) that this starch system would gel. This is the basis for starch-based puddings. This process termed RETROGRADATION involves the aggregation and eventual recrystallization of the starch macromolecules (primarily amylose). This process is also thought to be related to the staling phenomena in gelatinized starch systems such as bread, cookies, and tortillas.
Extrusion Processing Improves Waxy Barley Flour for Bread Making
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Design & performance of Pasta Dies
Design and Performance of Pasta Dies
Copyright©1993 American Association of Cereal Chemists, Inc. November-December 1993, Vol. 38, No. 11
Extrusion dies are used to extrude products in many different shapes and sizes. More than 250 configurations are possible, including dies for snacks, cereal, and animal feed. In a broader sense, extrusion dies can be used for metal and plastic products. This article traces the development of present-day dies and discusses considerations in designing dies for pasta production and some factors affecting die performance.
History of Pasta Dies in the United States
In the early days of the pasta industry in the United States, there was a conspicuous absence of mechanized equipment. Dies were manufactured entirely by primitive hand methods. The first pasta dies in the United States were made of copper because of its physical properties bowed to human strength. Holes were hand-punched through a maximum thickness of 1 inch, and the outside diameter of the die was obtained by chiseling the excess metal and filing. These manual methods required a malleable material soft enough to yield to hand-punching. Hand-punching methods were subsequently replaced by hand-driven drill presses, and two years later power-driven drill presses and lathes were drafted into service.
With mechanization came the need for increased production by past manufacturers. Higher production was achieved by increasing the pressures in extruders. Copper, because of its malleability, was unable to withstand the greater pressures developed by the improved extruders. The problem, then, was to find a material that was not too difficult to machine yet strong enough to withstand the factors associated with the increased production. This problem was solved by the selection of a bronze alloy.
Profit-minded manufacturers, however, demanded a still better material with a higher yield point to prevent bowing under the higher pressures and number 303 stainless was selected. Stainless steel is more wear resistant than is bronze but has the distinct disadvantage of a low coefficient of thermal conductivity. Thus, stainless steel dies with bronze alloy or
Teflon inserts were developed. These materials were satisfactory until the production output of extruders was steadily increased to 4,000-8,000 lb per hour. The problem of die bowing was magnified by the fact that the European extruder manufacturers had no support under the die during operation. This problem was solved by the discovery of number 450 stainless steel, with an approximate yield strength of 177,000 psi. The maximum yield strength of aluminum bronze is approximately 40,000-45,000 psi, which is roughly comparable to that of number 303 stainless steel. Initially, all dies were manufactured from solid pieces of material, i.e., the outlets were machined in the solid piece itself. As dies became larger in response to greater production needs, weight and bulk became a decided factor in the development of inserts. Inserts can be described as miniature dies that are fitted into a support (or holder) that can accommodate a number of inserts. Inserts with the required outlet configuration can be manufactured and installed into a drilled support or holder. One advantage of the inserts is that they can be changed at the plant location, therefore eliminating the need for returning the weighty support back to the manufacturer. However, problems may be encountered should the chambers or holes in the support be distorted through die warping. Forcing the insert in the die might close the extruding outlets or distort the insert. If the insert does not fit properly, dough leakage can invariably change the outlet specifications. If the insert protrudes slightly, knife breakage occurs. If the insert is recessed, poor product cut results. Should the die be bowed or bent, a number of the above difficulties might be encountered. The extruding surface of the die might be damaged by knife action or during handling, and this condition must be corrected.
Depending on the extrudate and the process, cereal and snack dies are generally manufactured using stainless steel and bronze alloy inserts to reduce the effect of wear; Teflon inserts are extensively used for pasta dies.
Matching Dies to Production Requirements
In determining the specifications for a new die and/or product, calculations must take into consideration all pertinent factors that affect the final product. The first decision is the selection of the material to be used. This selection is dependent on the product itself, method of packaging, product appearance desired, and rate of production. The selection of the most suitable material for a given application is highly important and often difficult-many factors must be considered and balanced. In keeping with today's technological improvements and high-volume production, the basic material must be strong enough to stand up under a design that will provide maximum output. This usually means more outlets per die. The overall physical properties of the metal must be considered, with machine ability as a primary factor. General properties will include resistance to the flow of dough, and wear ability.
The following example illustrates the series of steps required for drawing up specifications of a spaghetti die. One common practice today is to submit samples with the order. The samples are carefully measured. Several measurements are taken over the entire length of the strand because the measurements at different points on a single strand will vary. Such variations may be caused by moisture content of the mixture, stretching during extrusion, drying, and condition of the die outlet. From these measurements on the dry product, an average figure is computed.
The next step-- and a truly important one--is to determine the dehydration or shrinkage factor for the product. This factor must be based on past experience and performance, for it varies with each manufacturer. Thus, it becomes a variable factor dependent on the method of production used and outlet material. For example, the use of Teflon will often necessitate a higher shrinkage factor over that for metal. This factor is added to the basic sample size to arrive at the final outlet size.
The next calculation, which often presents a problem, is the determination of the number of outlets per die.
On one hand the extruder manufacturer has already set the rate of production for equipment and wants the die manufactured with as many holes as possible. The pasta manufacturer, on the other hand, expects a perfect product from the die. What are the problems? If the die is designed with too many outlets, the following scenarios are possible: 1) the die may be too weak, reach its yield point, and bend under pressure; 2) the dough may not have the opportunity to properly amalgamate before extrusion, which may result in a weak, low-density product; 3) the operator may think that the extrusion rate is too great and make the mixture a little harder (greater viscosity) with subsequent damage to the die; and 4) the strands may overlap too much and consequently be difficult to dry properly.
If the die is designed with too few outlets, too much back pressure can develop with possible damage to either the die, the extruder, or both. In this respect, collaboration with the pasta manufacturer is essential so that the requirements may be satisfied.
The die with a pin presents additional problems, for wall thickness must be considered. Shrinkage occurs on both the outside and the inside diameters. The shrinkage factor is greater for the outside diameter than it is for the inside diameter, and extreme care must be exercises in drawing up the specifications. This particular characteristic must be given close attention or wall thickness will be either too thick or too thin, which will present subsequent difficulties in drying , packaging, and cooking.
The trend today is toward more exotic shapes, including those that appeal to children. Difficult configurations are now designed using computer technology. Complicated configurations must be given special attention--the extruding pressure on each outlet must be adjusted to extrude a product that will have a uniform thickness after cutting at the die.
On the subject of cereals and snacks, shrinkage or expansion of the finished product cannot be determined empirically because of the many different processes and raw materials used.
Factors Affecting Die Performance
One of our more serious concerns today is die wear. Under normal circumstances, die wear becomes apparent through the warning signals associated with packaging. Too heavy a product yields less volume per unit weight, which results in too much slack in packages. This applies predominantly to solid and tubular products, in which gradual war can seldom be detected by visual inspection of the product but instead must be determined by actual measurement. The more elaborately shaped products generally give some indication of wear by a change in physical appearance. For example, sea shells tend to show greater curvature, mafalda shows a more pronounced wave, and rotini and twists show a tighter curl.
In a sea shell production, the flow of dough is at its maximum at the center of the shell, making this point more susceptible to wear than are the ends. As wear increases, the dough flows faster at the center, thereby increasing curvature. Today, the most common warning of wear in shell dies comes in the form of checking either during or after drying.
Checking can be described as minute cracks in the finished product as a result of improper drying or dehydration of the product. This may be a consequence of a heavy wall or improper diameter product (possible result of die wear) with no correction in the drying process. This checking can be attributed directly to die wear and can be eliminated by reducing the thickness of the die outlet.
In the wavy products, such as mafalda and rippled lasagna, die wear becomes evident visually by a more pronounced or closer-curled wave. A cross section of these products should present a flat, noodle-type appearance. The wave is the result of a greater flow of dough on the ends of the slots in the die, making the ends the points of greatest wear. An increase in wear is accompanied by an increased flow of dough, resulting in a more pronounced wave. A cross section of the product after wear made with a die exhibiting signs of wear appears as a flat noodle in the center with a spaghetti-like effect at the ends. This condition presents both drying and packaging problems and can be eliminated by proper die maintenance.
The rotini and twist products present cross sections analogous to those of lasagna--a cross section of the product before wear is a noodle type product whereas after wear, the ends (at the circumference) develop a heavier, spaghetti-like appearance. The increase in the flow of dough at these points results in a tighter curl or a greater degree of twisting.
Elbow macaroni is tricky because wear occurs at several points, and certain dimensional proportions must be maintained to obtain a standard product curvature. Wear takes place at the outlet, at the pin tip, at the base of the notch (where applicable), and, in the case of brass pins, at the pin stem between the notch and the tip of the pin. Many have been plagued with product splits on short-cut products and splits or weird distortions on long products. The cause, though not immediately detectable by visual inspection, can generally be traced to grit in the raw material. In the case of splits, the grit lodges between the pin and the outlet (the grit is too large to be pushed out) and results in a definite split in the extruded product. (A split is a term used in the pasta industry to describe a break in a tubular product in which the cross section looks like a split ring.) In the case of the weird distortions of long tubular products, the grit is forced through the die but in the process forces the pin to one side. Thus, off-center pins--directly attributable to grit--are the base cause.
A rather mystifying condition is presented by uneven wall thickness in short-cut products in which grit definitely does note enter picture.
When proper and standard operational procedures are not carefully adhered to, the die yields during production. This bending follows an elliptical pattern tending to distort the outlets and disturb the concentricity of pin and outlet. The result is uneven wall extrusions.
In these days of high-volume extrusion operations, a major complaint is wear vs. number of hours production. Number of production hours can no longer be used as a yardstick for wear. Statistics must be based on tonnage pushed through the die, which will give a more realistic basis for rate of wear.
How often should dies be returned for repairs and reconditioning? Every three months? Every six months? The answer depends on a number of production and handling factors. A die in a continuous production system must be repaired more often than must a die in limited production. The responsibility of setting tolerances for the product sizes rests with the manufacturer. Once these tolerances have been set for the various products, it is easy to determine wear on the die. This task may be facilitated by the use of gauges within the specified tolerances on the die or by enlisting the aid of quality control on the product. In view of the many variables that influence final product size, it may be beneficial to work out some program involving die wear, although it most certainly will be more practical to work on product tolerances, which may result in a colossal headache.
All difficulties are not the result of die wear. Improper maintenance will result in serious problems. For example, a die not properly cleaned will have a thin crust of dough left on the outlet. This will affect both product size and appearance. Pressures may have a decided effect on the extruded product. One problem that has gained prominence is the noodle with a slight twist. A die extruding 1,000 lb per hour manufactured to identical specifications as a die extruding 3,000 lb per hour will not give the same twist as the latter die as a result of different extruding pressures. The twist will be in direct proportion to pressure. Similarly, pressure may affect the wall thickness of the product. The same things apply to all products that curve or twist, such as elbows, rotini, and sea shells.
Among the factors affecting the quality and appearance of the extruded product are raw material, moisture content, pressure, die outlet finish, number of outlets in die, material of die, and drying procedures.
Copyright © 2001 D.Maldari & Sons, Inc.
Extruders not just for snack food.
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Feed extrusion program Texas A&M
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Oilseed extrusion-Texas A&M
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Food extrusion Program-Texas A&M
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Process Instability in Food Extrusion
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Developing Product Through Extrusion
Developing Product Through
July 1992 -- Process Engineering
By: Massoud Kazemzadeh, Ph.D.
In the highly competitive market of variety foods, the greatest challenge for any researcher in the product and/or process development area is to produce unique, innovative products based on current market demands. This continual effort has lead to an unprecedented number of new product introductions in recent years. This particularly has been the case with products that are low-calorie, low-fat, low-cholesterol, low-sodium and high in fiber. Not only does this constitute the largest new category filling the supermarket shelves, but indications are that it will continue to do so.
In fact, according to Shopping for Health, a 1992 study conducted by the Food Marketing Institute in cooperation with Prevention magazine, 58% of shoppers say they've made major changes in their diets during the last three years for health reasons.
It is estimated that among all foods, snacks and breakfast cereals have the shortest product life in the market. Therefore, there is a greater challenge for researchers in these areas to keep up with increasing market demands. Because many snacks and breakfast cereals are made via twin screw extrusion (T.S.E.), it is critical for product designers to fully understand how to apply this technology.
Where to learn
Among its many varied purposes, a research technical center with twin screw extrusion capabilities is usually designed and staffed so that it can assist researchers and customers in meeting growing demands in the development of new concepts and ideas in the area of new food products and process development.
In recent years, the use of T.S.E. in new product/process development has proven essential, and its full range of application still has not been totally investigated. Future-oriented extruder manufacturers are aware of the tremendous potential in these areas for T.S.E. and are offering services and facilities to meet the demand.
To take full advantage of these services, product designers must follow a specific plan when working with the extruder manufacturer. First, the extruder manufacturer and the client company should work together on determining which market the product will serve. In conjunction, the designer will need to determine the desired texture, surface structure and internal structure of the end product.
Next, the two parties will need to prepare an ingredient list according to the customer's specifications. These specifications should reflect both the requirements of the finished product parameters and the labeling requirements.
The team will then propose the product shape and process design after isolating and defining all parameters for the product identity.
At this point, the extruder manufacturer's technical staff should prepare a research outline that will serve as a blueprint for setting up the processing line. Using such an outline lends objectivity to the research because it results in several samples that are evaluated only after final testing. Choosing the "best" product or process then is at the customer's discretion.
The process itself
The twin screw extrusion process normally can fulfill the following tasks in a continuous and efficient one-step process:
• Cooking. Most T.S.E.'s are capable of heating the dough material either by frictional or viscous heat dissipation, providing for a very energy efficient heating method; via transfer of heat energy through barrel walls depending on the convective heat transfer coefficient; or by direct steam injection into the product during extrusion while under mixing.
• Cooling. Two methods are used to cool a viscous product in the extruder. One method is through venting, which provides efficient cooling if moisture is present and the product is above water's boiling point. The second method is cooling through the barrel wall which, due to limited surface area, is minimal and can be improved by closer tolerance between screw and barrel for continuous refurbishing of the surface.
• Kneading. Various screw elements have been designed and tested for maximum efficiency of such a process. The most important aspect of such elements is the distance provided between the elements and the barrel. The tighter the distance, the more shear area at the tip of the paddles will be generated, thus decreasing kneading efficiency. In situations where extensive kneading is required to develop the protein dough -- such as wheat gluten -- long sections of the screw are provided with such paddles.
• Venting. Usually a portion of the barrel top is removed via a cap in order to expose the screws and doughy material to the atmosphere. Because of screw geometry the product remains in the screw channels and volatiles and vapors escape. A vacuum line can also be incorporated in order to enhance this process.
• Mixing. Special elements should be provided for the T.S.E. line which are designed to be very thin and highly efficient in mixing various dry materials or dry and liquid products forming a dough like consistency. A screw profile may contain more than one such zone and can also be incorporated with paddle elements to give improved results.
• Reaction Chamber. T.S.E. barrels and screw elements may be designed and manufactured to provice continuous mixing and reaction chambers for caustic reactions. By injecting and adding various raw materials throughout the barrel length, and by designing special screw configurations, the product can be mixed, reacted, pressurized and vacuumed, all within the same barrel length.
• Forming. The final stage of the T.S.E. is the die area and the cutter. This determines the shape and size of the final product. By various double or triple techniques, a semi three dimensional product can be achieved. This can be interchanged easily to provide various designs, shapes and lengths of product.
There are a few other process capabilities that are mainly derivative of the above steps, such as high shear versus low shear, steam injection into the extruder, as well as injection of liquid carbon dioxide or nitrogen into the product to provide more efficient cooling or foaming reactions.
With such a substantial list of capabilities, it is only reasonable to imagine that the various applications of such a machine can go beyond snacks and breakfast cereals to milk candies and refried beans. Non-food products from this process range from soaps and air fresheners to low explosive and solid rocket fuels. In most cases, operating such a machine requires neither extensive expertise nor an advanced degree because comprehensive training and hands-on experience should be provided to the operators by the extruder manufacturer. However, in order to achieve the extruder's full potential for a given product or process, the assistance of an extrusion process specialist is needed. The extruder manufacturer should offer both types of assistance.
Such a complicated process is not without shortcomings or problem areas. With twin screw extrusion systems, the greatest shortcoming occurs during process scale-up. This is because many of the twin screw extruder parameters are interdependent and inseparable, and varying just one parameter affects many other process parameters. An example is RPM, which affects residence time, degree of screw fill, extent of mixing and extent of shear rate, thus varying the temperature profile.
A good overall view of variation and interdependency of parameters within the twin screw extruders was done by Chang and Halek, 1991. In this paper, a computation procedure was developed for calculating contributions of frictional and viscous heat generation during the extrusion process. Application of energy balance to experimental data, chemical energy input, feed rate, feed moisture, screw speed, die pressure, etc., was evaluated and recorded. Other parameters not totally studied were the effects of wear and tear and screw configurations, and most importantly, the final effects of the above parameters on the texture, viscosity, mouthfeel, flavor, etc., of the final products, which are subjective and cannot be easily quantified.
Existing product re-design
The development of a new process using twin screw extrusion for the production of an existing traditional product is far more difficult than development of a process/ product with new market identity. Therefore, when developing a new process for a well-established product, a complete study of the traditional process technique must be conducted. In order to be more explicit, the following is an outline of a specific example of twin screw extrusion treatment of an existing, well-established product -- refried beans.
In the case of refried beans, many different types and recipes exist and consumer product identity is relatively wide. An initial project examination reveals the largest users to be fast-food chains and ethnic restaurants. The examination further identifies that the main problem area with the old process is that it's an inefficient batch process that is labor-, space- and energy-intensive. Furthermore, consumers seem to care more about the final taste, texture, aroma and mouthfeel of the product than the procedure by which it is made. In particular, the mouthfeel and the taste stand out as the most important characteristics.
Along with organoleptic properties, other functional aspects prove to be important. For example, the product is to be sold in dry form from a central processing facility, received by the user, then hydrated and heated to produce the final marketed product. For this to be rapid and easy for the end user, the water absorbability and consistency after rehydration plays a big role.
Studying the old process demonstrates that hydration of the raw beans appeared to be important in the development of good aroma and taste in the finished product. For the purpose of this new study, a Buhler T.S.E. 62mm, 28/1 L/D/ ratio with a specially designed screw configuration is used.
In order to fingerprint the new process, the main characteristics of the product and processes are considered. These include the longtime hydration of the beans, slow cooking of the starch and protein, and the addition of certain ingredients to decrease shear and increase kneading action and residence time in the extruder. The major limiting factor is the slow cooking of the traditional method. This is overcome by reducing the size of the raw beans to a fine powder. This also serves as a benefit to the bean growers and may open a market for their broken beans -- usually sold at a loss. With the addition of various fats, oils and spices to generate low shear and high pressurized conditions to increase heat transfer efficiency at low moisture levels, a high-quality product with intact starch granules is achieved.
The screw itself is designed to provide four specific chambers, including the mixing chamber; the steam injection area; the high kneading, mixing and pressurizing section; and a highpressure zone.
The final product exits the die at a low moisture of 12 to 14% end can be cut easily into the required size and shape. These shapes are further dried and milled to 20 mesh consistency. This material is then packaged and sold to the customer at a lower cost, higher quality and at reduced shipping charges.
Adding previously run chunky soya and wheat protein in the form of texturized vegetable protein produces a chunky meat appearance and texture to the final product designed for vegetarian palettes. Extensive conditioning of the raw material before extrusion, coupled with high temperatures and low RPM, proves to be beneficial and flavor enhancing. High shear provides a beany, unpleasant taste and aroma.
A knowledgeable and qualified staff is needed to be able to translate process requirements based on the traditional method readily and easily into the parameters, screw design and barrel length required by the T.S.E., thus reducing speculative procedures which are time consuming and frustrating to the researcher.
The above procedures are merely a brief outline of the hours of expertise required under normal conditions to fully develop a new product or process. Usually, between the time the customer places an order for a given process system, and the time he begins production at his facility, fine-tuning of the product/ process can be conducted at the manufacturer's technical center.
Dr. Kazemzadeh is manager, technical center, for Buhler, Inc., Minneapolis.
Developing Product Through ExtrusionDeveloping Product Through Extrusion