By Dr. Warren G. Dominy
An extruder is a high-temperature short-time bio-reactor that transforms a variety of raw ingredients into modified, intermediate and finished products. The capability of an extruder to control a cooking process enhances the feed manufacturer’s flexibility to produce a wide range of products, but the economic considerations of extrusion cooking technology, compared with other mixed feed processing technologies, often preclude the use of an extruder for processing livestock ingredients and/or complete rations.
There are, however, numerous successful applications for extrusion processing in the feed industry, including dry pet foods, high-energy feeds, baby pig and calf feeds and floating aquatic feed. For example, pigs fed extruded cereal grains exhibit 5% to 15% improvements in growth performance and nutrient digestibility. A compilation of animal studies suggests that extruded soybeans may yield 2% to 5% greater feeding value than solvent-extracted soy plus feed grade fat. As lower-cost equipment and extrusion techniques are developed, opportunities for the continued evaluation and utilization of extrusion cooking in animal feeds and feed ingredients should increase.
The beneficial effects of extrusion include increased nutritive value by enhancing starch and fat digestibility and reduction of antinutritional factors (ANFs), including antigenic components and trypsin inhibitors. However, the extrusion process may also cause undesirable changes in the feed ingredients or finished feeds. These include changes in flavor, texture, functional properties or in nutrition, due to destruction of heat labile vitamins and amino acids through the Maillard reaction or oxidation. Cross-linking during extrusion may also reduce protein quality in some products.
SINGLE AND TWIN SCREW MACHINES
The single-screw cooking extruder has been the "heart beat" of the dry expanded pet and aquatic food industry for more than 45 years. Currently, single-screw extruders produce over 90% of expanded pet foods, which represent the greatest volume of extruded products on the market. Single-screw extruders are used to produce virtually all of the extruded soybeans in livestock feeding.
Twin-screw technology involves much higher capital costs — about 1.5 to 2.5 times higher than a state-of-the-art single-screw extruder of comparable hourly production capacity. This is due to the complexity of the screws, shearlocks, kneading elements, drive and heat transfer jackets. Maintenance wear on parts and energy requirements are 1.5 times higher than the cost for a single-screw extruder. Because of the increased investment and increased operating cost associated with twin-screw extruders, only special products with high margins can be processed profitably with them.
However, twin-screw technology has found application in certain areas of the animal feed industry with special needs. These areas are ultra-high-fat foods, high moisture (more than 17%) diets where high levels of fresh meat are added, ultra-small aquatic specialty foods and coextruded products.
DRY COOKING EXTRUDERS
Dry cooking extruders are designed to run without adding any moisture to the ingredients. These are used in processing low moisture, highly expanded starch products and in processing whole soybeans to full-fat soy flour for both the food and feed industries.
In dry extrusion there is no pre-conditioning of the ingredients with steam or moisture before entering the extruder barrel. All the heat produced by the dry extrusion process is generated by the mechanical energy applied to the ingredients via the screws, steamlocks and the barrel wall.
These simple dry extruders have since been transformed into more complex pieces of equipment with increasing product flexibility. Longer barrels can now provide increased capacity and cook. Increased selections of screws, steam locks and barrel liners allow high, medium or low shear configurations. Precision metering pumps for water and steam injection to the extruder barrel or to the conditioner provide additional capabilities, such as increased product cook and better shaped extruded products.
WET COOKING EXTRUDERS
Wet cooking extruders are more complex equipment with precision metering systems for steam or water injection into either the conditioner and/or the extruder barrel. Preconditioning is a very important part of the extrusion process. The single most important aspect of the preconditioning system is the potential for additional mixing and the retention time, which is necessary for chemical or physical reactions to take place.
Conditioner types are also varied with a single or double mixing shaft, and may include differential diameter or differential speed conditioning cylinder systems. Preconditioning of ingredients prior to entry into the extruder barrel increases the degree of cook and reduces the wear on internal extruder parts. To increase control of the cooking process, the extruder barrel of a wet extruder is also jacketed for steam or water injection for additional heating or cooling.
Like the dry extruder, the wet extruder has single and/or double flightscrews, shearlocks, and heads. However, the wet extrusion system has a greater variety of single and double flight screws. It also has a more complex array of single or double, cut-flight screws and cone head screws, shearlocks and heads for more intricate extruder configurations.
BASIC EXTRUDER COMPONENTS AND FUNCTIONS
Extruders are basically screw pumps through which ingredients are subjected to temperature, pressure, and shear as they pass through the extruder barrel and out through a restrictive opening called a die. The barrel of the extruder is comprised of barrel heads, screws, flow restrictors (called shearlocks) and a die.
The barrel heads can be jacketed for steam or cooling water to control the cooking process in the extruder barrel. The barrel wall in the heads is designed to provide resistance needed to keep the extrudate from sticking to the screw as it rotates. The inside wall of the barrel head can be smooth but is often grooved with either helical or longitudinal grooves. The helical grooves help propel the extrudate toward the die and act as an extension of the screw helix. The longitudinal, or ribbed, grooves are designed to increase the amount of slippage that the extrudate encounters as it passes through the extruder barrel to the die thereby decreasing the conveying efficiency and increasing the amount of shear.
The extruder barrel is divided into three sections: feed zone, transition zone, and the metering or final cooking zone. In the feed zone, deep flight screws are used to convey the material into the extruder barrel so the screw is completely filled. Feeding screws have a wide pitch and deep flights, and the screw flights are at nearly right angles to the screw centerline.
In the transition section, the screws and shearlocks compress and de-gas the feed material. The screws also work the ingredients from a mash to an amorphous continuous dough. In this section, compression increases rapidly, causing most of the mechanical energy used in turning the screw to be dissipated into the feed material as heat.
Compression of the ingredients in the extruder barrel can be achieved in several ways: by adding shearlocks; by decreasing screw pitch; or by decreasing the space volume between the barrel and the screws.
The reduction in screw displacement volume in the transition and final cooking zones increases the density and temperature of the extrudate and is necessary to obtain a continuous flow as the extrudate moves toward the die. There are other screw designs that use interrupted flighting, called cut flight screws. These screws are used to add mixing, increase backflow, or increase the amount of shear and extrudate residence time in the transition and the final cooking zone.
Pressure, temperature, high shear rates and degree of cook of the extrudate are highest in the final cooking zone prior to discharge from the extruder barrel.
The final cooking screw prior to the die may be conical in shape. This conical screw element is designed to cause a rapid pressure rise, uniform pressure distribution around the screw periphery, and dampen any pulsation that may be present in the final section of the extruder.
The die design and its effect on expansion, uniformity and appearance of the final product are often underestimated. The die design controls the expansion or shape of the product, but its open area is also a factor in controlling the retention time of the extrudate in the extruder barrel. Die shear rates can increase dramatically if the die open area is reduced from multiple openings to a single opening.