The Technology of Extrusion Cooking

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A screw configuration consisting of a variable pitch, constant depth, increasing root diameter, increasing number of flights, shearlocks, and decreasing end diameter is most frequently used in the food industry. A single-screw barrel can be divided into three processing zones: feeding zone, kneading zone and the final cooking zone Mercier et al. The feeding zone generally has deep channels which receive the feed.

The preconditioned or dry material entering this zone is conveyed to the kneading zone. Water may be injected at this point to help develop a dough and improve heat transfer in the extruder barrel. As the material is conveyed into the kneading zone, its density increases because of water and steam addition.


Screw pitch in this zone decreases and the flight angle also decreases to facilitate mixing and a higher degree of barrel fill. This zone applies compression, mild shear and thermal energy to the feedstock, and the extrudate begins to lose some of its granular definition. The flight of the screw may be interrupted in this area to further increase mixing via leakage flow Rokey, The mechanism of shear begins to play a Selecting the right extruder 35 dominant role because of the barrel fill in this zone.

Steam and water can be injected in the early part of this zone. Steam injection increases thermal energy and the moisture content of the extrudate. As the extrudate moves through the kneading zone, it begins to form an increasingly cohesive flowing dough mass, which typically reaches its maximum compaction.

The material exhibits a rubbery texture similar to a very warm dough. At this stage, the material enters the extruder final cooking zone. Screw flights in this zone are typically shallow, and have a short pitch. The function of this zone is to compress and pump the material in the form of a plasticized mass to the die. Temperature and pressure typically increase very rapidly in this region because of the extruder screw configuration.

Shear is highest in this zone, and product temperature reaches its maximum and is held for less than five seconds before the product is forced through the die Harper, The product expands as a result of moisture vaporization as it exits through the die into a region of lower pressure. The extruded material can then be cut into desired lengths by the knife attachment. This low-shear, low-temperature-forming process first found commercial production in the s and s, and remains a standard process although equipment has improved Huber, Several new developments in the single-screw extruder have further increased its efficiency and versatility.

Single-screw machines cost about half the price of twin-screw extruders and maintenance costs are lower. Fewer complications exist in assembling screw configurations as compared to twin-screw extruders, because intermeshing between two screws is not required. Wet extruders have higher capacities than dry extruders due to large drive 36 Extrusion cooking motor requirements per unit throughput on dry extruders.

Since the single-screw extruder has only one shaft, it will not self-clean as completely at the end of the operation. However, if formulas of pet food or feed products are nearly similar, a prepared second swing-away die plate assembly can be mounted at the discharge end. Usually, the extruder can be stopped, the die plate loosened, swung out, and replaced by another with a very short downtime. If not allowed to cool, the extruder can be started up. Also, increased water or oil and ground corn or cracked soybeans may be added to help clear the cooling extruder at shutdown.

Typically, production extruders operate 24 hours a day, and shutdowns for cleanup do not occur except for required sanitation when meat ingredients are used in food products. Very fine powder will not feed in this type of extruder as well unless preconditioned, and very coarse material will not cook properly.

Final particle diameter sizes can also be a factor, fish feeds smaller than 1. This type of extruder was developed initially for processing whole soybeans on the farm. A typical dry extruder is shown in Fig. Dry extruders can process ingredients which have a wide range of moisture contents, i. If the ingredients have sufficiently low initial moisture content, drying of the product after extrusion cooking may not be necessary. Moisture loss in dry extrusion is in the form of steam flash-off at the die, and the extent depends on initial moisture in the ingredients and product exit temperature.

Dry extruders have the option of water injection during extrusion.

Extrusion Cooking: Technology and Applications

Usually starchy material requires some moisture in order to gelatinize. Considerable advances have been made in the design of dry extruders and their components. Longer barrels will work much better in some applications than the shorter barrel formerly used for soybeans. Dry extruders are single-screw extruders with screw segments and steamlocks choke plates on the shaft for increasing shear and creating heat. When material moves through the barrel, and comes up against these restrictions, it is unable to pass through, pressure increases, and a back flow is created.

Usually Selecting the right extruder Fig. This build-up of pressure and temperature, together with the shear stresses developed, plasticizes the raw materials into viscous paste or puffed shapes, depending on the raw material. In dry extrusion, pressure and temperature are at their maximum just before leaving the die.

The die design and opening also play very important roles in pressure build-up. As soon as the material exits the extruder dies, pressure is instantaneously released from the products, causing the internal moisture to vaporize into steam and making the product expand and results in sterilization of the product. A major use of the dry extruder is in preparing oilseeds for screw pressing of oil — primarily soybeans and cottonseed, although they have been applied to sunflower, peanut and canola seed processing.

In the process, soybeans and cottonseed are extruded using a dry extruder, followed by pressing 38 Extrusion cooking in a parallel bar screw press to remove the oil. Extrusion prior to screw pressing greatly increases throughput of the expeller over the rated capacity. Oil and meal produced by this method are remarkably stable because extrusion also releases natural antioxidants in oilseeds. This process is used around the world for processing raw soybeans into full fat soybeans and partially defatted soybean meal.

Cereal grain fractions and other starchy raw materials can be preceded by dry extrusion. Less training is required as compared to twin-screw extrusion systems. Dry extrusion is a good choice where steam is not available, although it does have the flexibility of adopting a steam preconditioner for applications such as complete feed or shaped products.

The Technology of Extrusion Cooking

A typical dry extruder with preconditioner is shown in Fig. Dry extruders are able to grind whole soybeans during extrusion, and therefore grinding steps can be eliminated in the case of soybeans.

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Dry extruders require relatively high horse power to operate in some applications as compared to other extrusion systems. Higher wear occurs on the screw because of lack of lubrication from steam injection. The final size of the product may be limited to certain sizes, as the high pressure involved with this type of extruder makes it extremely difficult to shape product which is less than 2 mm.

They are not as flexible as wet segmented single and twin-screw extruders. Maintenance costs are higher when whole soybeans are ground.

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Grinding soybeans by hammer mills before dry extrusion may be a more costefficient operation. Initial moisture content of the recipe is very critical. Formulas with high fat contents may not cook properly because of product slippage inside the barrel. Highly viscous material is hard to process through this type of extruder compared to twin-screw extruders. Expanders were exported to Brazil for stabilizing rice bran in , Ecuador in , and Mexico in This design was applied to preparing soybeans and cottonseed for solvent extraction in Brazil in the early s.

Brazilian-made expanders were brought back to the United States for processing cottonseed in the late s. A typical interrupted flight expander extruder is shown in Fig.

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An interrupted flight extruder is mechanically different from other extruders, because it was developed from a screw press. However, a screw press is a more massive and costly machine; it generates more pressure, and it is equipped with a barrel section that allows oil to flow away from the solids Williams, The purpose of the intermingling of the rotating worms with stationary pins is to provide a high shear, turbulent mixing action, which kneads the solid formulation with the injected water and steam to provide rapid and uniform absorption of the injected moisture into all of the solid matter.

As the steam is absorbed, it releases its heat of vaporization which elevates the temperature of the ingredients.

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