Extrusion is a method of food processing that utilizes several unit operations like mixing, shearing, heating, shape forming, cooling, and drying. It is defined as the process in which food material is forced to flow, under one or more varieties of conditions such as mixing, shearing, and heating, through a die which is designed to form and/or puff dry the product (Rossen and Miller 1973). The origin of extrusion technology is associated with the polymer industry (Singh and Heldman 2009). Extrusion processing was first used in food materials for making chopped meat with casings around the 1800s using piston type extruder (Ainsworth et al 2006).
Extrusion is a high-temperature short time (HTST) processing method using mechanical and thermal energy for production of products mostly like ready-to-eat cereal products, puffed snack products, pasta, textured vegetable products, pet food, and meat products. Extrusion processing is a preferred processing technique in a variety of food processes like pre convenience foods and snack products. (Harper 1979).
Advantages of Extrusion Processing
The main reason for the popularity of extruder is due to the versatility of extruder and its diverse application in the food industry. The extruder can produce extrudates of any shape or size by altering feed material and modifying operational parameters.
- Energy efficiency: It is the first parameter under consideration while the selection of any unit operation for processing. Extruders are fully automated and operate at low moisture conditions and have higher production outputs. The overall process is highly economic as it operates on low moisture, requires no labor cost; reducing the cost of water and chemical treatments and does not emit harmful effluents.
- Product quality: Due to the application of high temperature cooking for a shorter time the nutrient losses in extrusion cooking is comparatively lesser (Fellows 2009). The high-temperature treatment also aids in inactivation of enzymes, destroying antinutritional components like trypsin inhibitors and kills microorganisms (Fellows 2009, Riaz 2000).
- Adaptability: To produce the desired extrudate, the operator may alter the raw material and operational parameters of the extruder. The extruder can be efficiently used for new product development.
Processing of food extruder
A basic extruder is fitted with these components– a feeder, barrel, screws and die. Raw materials are fed into the system at a uniform rate either volumetrically or gravimetrically into the barrel and the screw kneads the product. Some extruder processes may require a preconditioning process. It is the process to enhance the extrusion of materials, it aids in imparting higher moisture and longer retention time to the product (Riaz 2000). It is reported that preconditioning for up to 4.5 minutes with water or steam increases feed temperature, moisture content, gelatinizes starch and denatures proteins; this consequently improves the efficiency of the extruder, reduces wear and tear of the equipment and lowers specific energy (Bailey et al 1995).
Extrusion is a thermomechanical process involving several unit operations which leads to a number of chemical changes taking place in the functional properties of the product. Here energy is firstly transferred through the rotation of screws and followed by the transfer of energy from heaters through barrel walls, this leads to an increase in temperature of the material and forcing it through the die. When the product leaves the die, it’s in the glassy state and it expands rapidly due to a significant drop in pressure and temperature, moisture is flashed off as steam hence, transforms into the cooked product (Alvarez-Martinez et al. 1988).F
Types of Extruders
Extruders are differentiated based on operation — Hot and Cold extrusion and construction– Single screw extruders and Twin screw extruder.
Hot extrusion is when raw materials are cooked about 100°C-180°C, in this case, frictional forces and additional heat may cause an increase in temperature of the extrudate, which is further passed through the barrel to the die, where it cools and steam evaporates due to falling in temperature and pressure.
Cold extrusion works at a temperature below 100 °C and used to shape, texturize and mix foods like dough, pasta, and meat products. In this process, the extrudate leaves the die without the addition of external thermal energy, as in the case of traditional extrusion. Many researchers have reported due to high-temperature processing of cereals using hot extrusion leads to loss of vitamins, functional proteins and flavors, perhaps cold extrusion could possibly retain the functional properties of these cereals (Boyaci et al 2012).
As the name suggests single screw extruders consists of one screw housed in the barrel, they have poor mixing ability hence raw materials those are premixed or preconditioned are employed in single screw extruders (Ainsworth et al 2006). Twin screw extruders are widely employed in the food industry, due to its versatility and flexible design. The design of twin screw is superior to the single screw extruder, they have two screws either intermeshed or non-intermeshed depending upon its application.
Twin screw extruders are categorized based on the direction of screw rotation-
- Counterrotating twin screw extruders
- Corotating twin screw extruders
Corotating twin screw extruders are widely used as they impart more mechanical energy to the material than counterrotating twin-screw extruders. Co-rotating extruders can be operated at higher speeds than counter-rotating extruders because the radial forces generated are more evenly distributed (Ainsworth et al 2006).
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- Alvarez‐Martinez, L., Kondury, K. P., & Harper, J. M. (1988). A general model for expansion of extruded products. Journal of Food Science, 53(2), 609-615.
- Bailey, L. N., Hauck, B. W., Sevatson, E. S.and Singer, R. E. (1995) Ready-to-eat breakfast cereal production. In: A. Turner (ed.) Food Technology International Europe, Sterling Publications International, London, pp. 127-132
- Boyaci, B. B., Han, J. Y., Masatcioglu, M. T., Yalcin, E., Celik, S., Ryu, G. H., & Koksel, H. (2012). Effects of cold extrusion process on thiamine and riboflavin contents of fortified corn extrudates. Food chemistry, 132(4), 2165-2170.
- Fellows, P. J. (2009). Food processing technology: principles and practice. Elsevier.
- Harper, J. M., & Clark, J. P. (1979). Food extrusion. Critical Reviews in Food Science & Nutrition, 11(2), 155-215.
- Ready-to-eat breakfast cereal production.In: A. Turner (ed.)Food Technology International Europe, Sterling Publications International, London, pp. 127–132
- Riaz, M. N. (Ed.). (2000). Extruders in food applications. CRC press.
- Rossen, J. L., & Miller, R. C. (1973). Food extrusion. Food Technology, 27(8), 46.
- Singh, R. P., & Heldman, D. R. (2009). Introduction to food engineering. Academic Press.