Bibliography

1. E. W. Schüler, "Twin-Screw Extrusion Cooking Systems for Food Processing," Cereal Foods World 31, 413 (1986).

2. J. M. Harper, Extrusion of Foods, Vol. I and II, CRC Press, Boca Raton, Fla., 1981.

3. Q. Lu et al., "Model and Strategies for Computer Control of a Twin-Screw Extruder," Food Control 4, 25 (1993).

4. F. Hsieh, I. C. Peng, and H. E. Huff, "Effects of Salt, Sugar and Screw Speed on Processing and Product Variables of Corn Meal Extruded With a Twin-Screw Extruder," J. Food Sei. 55, 224 (1990).

5. F. Hsieh et al., 'Twin-Screw Extrusion of Rice Flour With Salt and Sugar," Cereal Chem. 70, 493 (1993).

6. B. W. Garber, F. Hsieh, and H. E. Huff, "Influence of Particle Size on the Twin-Screw Extrusion of Com Meal," Cereal Chem. 74, 656 (1997).

7. M. K. Kulshreshtha and C. A. Zaror, "An Unsteady State Model for Twin Screw Extruders," Trans. I. ChemE., Part C, Food and Bioproducts Proc. 69, 189 (1992).

8. G. Delia Valle, J. Tayeb, and J. P. Melcion, "Relationship of Extrusion Variables With Pressure and Temperature During Twin-Screw Extrusion Cooking of Starch," J. Food Eng. 6,423

9. F. Martelli, Twin-Screw Extruders, Van Nostrand Reinhold, New York, 1983.

10. M. Liang et al., "Barrel-Valve Assembly Affects Twin-Screw Extrusion Cooking of Corn Meal,'V. Food Sei. 59, 890 (1994).

11. D. W. Stanley, "Protein Reactions During Extrusion Processing," in C. Mercier, P. Linko, and J. M. Harper, eds., Extrusion Cooking, American Association of Cereal Chemists, St. Paul, Minn., 1989, pp. 321-341.

12. "Extruder Directory," Feed Tech 2, 32 (1998).

13. R. B. Fast, "Manufacturing Technology of Ready-to-Eat Cereals," in R. B. Fast and E. F. Caldwell, eds., Breakfast Cereals and How They Are Made, American Association of Cereal Chemists, St. Paul, Minn., 1990, pp. 15-42.

14. S. A. Matz, "Extruding Equipment," in S. A. Matz, ed., Snack Food Technology, AVI, Westport, Conn., 1984, pp. 203-230.

15. A. Noguchi, "Extrusion Cooking of High-Moisture Protein Foods," in C. Mercier, P. Linko, and J. M. Harper, eds., Extrusion Cooking, American Association of Cereal Chemists, St. Paul, Minn., 1989, pp. 343-370.

16. O. B. Smith, "Extrusion Cooking of Corn Flours and Starches as Snacks, Breadings, Croutons, Breakfast Cereals, Pastas, Food Thickeners, and Additives," in G. E. Inglett, ed., Maize: Recent Progress in Chemistry and Technology, Academic Press, New York, 1982, pp. 193-219.

17. P. L. Noakes and W. A. Yacu, "Extrusion Cooking of Wheat Flour to Process Breadings," Cereal Foods World 33, 687

18. 0. J. Banasik, "Pasta Processing," Cereal Foods World 26,166 (1981).

Fu-hung Hsieh University of Missouri Columbia, Missouri

See also Extrusion processing: texture and rheology.

EXTRUSION PROCESSING: TEXTURE AND RHEOLOGY

In recent years extruders have been widely used in the human food and animal feed industry to manufacture products such as textured proteins, snack foods breakfast cereals, pasta, confectioneries, and pet foods. A list of products presently produced using the extrusion process is given in Table 1. The history of food extrusion has been reviewed in Ref. 2.

The two major functions of a food extruder are cooking and forming. In the cooking process, the product is heated through the transfer of heat energy, which is applied by steam injection or heaters and/or by the dissipation of viscous energy through shearing action between the rotating screw and the material. As a result of the cooking process, starches are gelatinized, proteins are denatured, undesirable enzymes are inactivated, and antinutritional substances (eg, trypsin inhibitors in soybeans) are destroyed. Because the temperature reached during the process can be quite high (ca 200°C) and the material takes a relatively short time to travel through the extruder (5 s to a few min), the cooking process is often referred to as high-temperature, short-time (HTST) extrusion. The forming extruder is used to produce special shapes from precooked material. Typically, the operating temperature of forming extruders are lower than that of cooking extruders.

Two major types of extruders are used in the food industry: single-screw and twin-screw extruders. The twin-screw extruders can be further classified according to the type of screws, ie, nonintermeshing or intermeshing and the type of rotation, ie, corotating or counter-rotating. Use of single-screw extruders began in the early years of the industry and they are still used today, though the twin-screw machines are becoming increasingly popular. Twin-screw extruders are comparatively more expensive than single-screw machines of the same production capacity. The flow of material in single-screw extruders depends on the friction between the barrel wall and the material. Hence the type of material that can be used is limited by its properties; proper conditioning of the material is necessary to reduce or eliminate process instability. The corotating, intermeshing twin-screw extruder uses the positive pumping characteristics created by the two intermeshing screws to convey the material forward. This type of mechanism reduces the need to precondition material. Other potential advantages of the corotating twin-screw extruder over the single-screw extruder include (1) narrow residence time distribution, leading to uniform processing, (2) better mixing capabilities, and (3) wide formulations of product, particularly low bulk density and high fat material that cannot be processed using a single-screw machine. The twin-screw extruder provides better control of the process and more uniform product characteristics than does the single-screw extruder. These advantages of the twin-screw machine compensates to some extent for the

Table 1. Extrusion Application in the Food and Feed Industry
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