Single Screw Extruders

Extruders are further classified into single and twin screw extruders depending upon the number of screws in the barrel. Single screw extruders are the most popular because of their low processing costs (Dziezak, 1989). Rossen and Miller (1973) reported that there are essentially five types of single-screw extruders used in the food industry. They are the pasta extruder, high pressure forming extruder, low shear cooking extruder, collet extruder, and the high shear cooking extruder (Harper 1986; Dziezak, 1989). The pasta extruder is one example of a single screw extruder and is used for forming various types of pasta products from a dough. The pasta extruder utilizes low temperatures and creates relatively low shear. It somewhat resembles a meat grinder in configuration in that the product is conveyed forward with a single screw to a flat, smooth die with several openings. The shape of the die determines the final shape of the product. The high pressure forming extruder is used for compressing and shaping pre-gelatinized dough into "half products" or products that will require another step in processing. An example of this is a snack product that after extrusion, would need to be fried, microwaved or processed by some other method before consumption. Also ready-to-eat (RTE) breakfast cereal pellets that need to go to a flaking roll, for example, can be formed using this type of extruder. The low shear cooking extruder is a continuous cooker for high moisture dough. The cooked product can then be processed further by forming or drying. Soft moisture pet foods and gelatinized starches are examples of the products made with this type of extruder. The collet extruder, as mentioned earlier, is a high shear, short barreled extruder used to make highly expanded snack food products. The collet extruder has no external heating source. Its energy comes from the viscous dissipation of mechanical energy. The low moisture input creates a highly expanded product with little or no additional processing necessary. The high shear cooking extruder is similar to the collet extruder, however a longer barrel is used to increase residence time and excess heat is removed by cooling. The final product is usually less expanded and requires a final drying process. A wide variety of dry ingredients and moisture contents leads to a variety of uses. Products such as texturized vegetable protein, dry pet foods and modified starches can be processed using this type of extruder. Table I compares the contrasts the different types of single screw extruders. The first step necessary in using any extrusion process is the mixing of the dry ingredients. After the dry ingredients have been mixed thoroughly, they are conveyed to the preconditioner. It is at this point that the ingredients are mixed with water or steam to the appropriate moisture content. The preconditioner can be either pressurized or a atmospheric chamber depending upon the specific needs of the process. The preconditioner utilizes steam when another heating source is necessary for the process. A preconditioner used with a extruder will increase residence time, reduce mechanical power consumption, increase capacity, optimize product quality and potability, reduce barrel wear and increase extruder efficiency (Harper, 1989; Wenger, 1992).

The preconditioner then deposits the ingredients into the screw barrel. The single screw extruder is composed of three distinct sections in the barrel; the feed section, the transition section and the metering section as shown in Figure 2 (Rossen and Miller, 1973; Harper and Harmann, 1974; Harper, 1989). The feed section receives the product from the preconditioner, compresses the feed material, works the material into a dough and convey it to the transition section (Harmann and Harper, 1974). The feed section has deep flighted screws and this aids in conveying the material forward. The transition zone further works the material into a dough that is partially cooked (121-177°C) and subjected to high pressure (Harmann and Harper, 1974). The metering zone function is to receive the compressed feed material, homogenize it and force it through the die at a constant pressure. The pressure is increased in this zone due to the die orifice and the small depth of the screw channel (Rossen

Table 1.

The typical operating data for the various types of single screw extruders (Harper, 1989).

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