Answer to Question #129415 in General Chemistry for Zania

Question

Write a detailed note on Application of extruders in food industry?

Answer 1

Extrusion is a complex food process combining several processing operations, such as mixing, cooking, kneading, heating, shaping, and forming. Extrusion is accomplished in a special screw conveyor, operated under controlled speed, pressure, temperature, and product flow. The shape ofthe extruded product is formed by forcing it through a specific die. Single- or twin-screw extruders are used. Extruders act basically as positive displacement pumps, operating at high pressures and relatively low rotational speeds. In addition to the positive action of the screws, the product is also transported by drag flow, caused by friction with the barrel surface. The net flow is the difference between the drag flow forward and the backward pressure flow. Twin-screw extruders can be operated at higher speeds than single screw extruders, providing higher flow rates, higher shear rates, and better mixing.

Extrusion is divided into low-temperature (isothermal) and high-temperature or cooking extrusion. Low-temperature (cold) extrusion is actually a forming (shaping) operation, used in the processing of pasta, cereal, and special food products, e.g., co extruded food and confectionery products. 

Extrusion cooking is more widely applied than low-temperature extrusion. It is operated at higher temperatures and pressures, producing several starch-based food products. Extrusion cooking, operated at high temperatures, can be considered, in addition to forming, as an HTST thermal process, reducing microbial contamination and inactivating spoilage enzymes.

Extrusion science and technology is based on the application of food chemistry and food process engineering to physical, chemical, and mechanical changes of food materials subjected to the flow, pressure, shearing, and temperature conditions of the continuous food extruder. The physical properties (density, texture) of the extruded products can be designed by controlling the operating conditions of a given extruder.

Twin-screw extruders are preferred over the single-screw units, because of their advantages. The material to be extrusion-cooked is usually a cereal powder (e.g., com, wheat) at a moisture content of 15-20%, which is compressed and heated above the gelatinization temperature, and then expanded through a die to a puffed (porous) product of desired shape. 

Most extruders operate with no external heating, utilizing the heat produced by mechanical dissipation of the viscous forces in the pressurized particulate food material.

Extrusion cooking technology is being increasingly applied worldwide to manufacture an ever expanding list of food products and feed materials, including snacks, breakfast cereals, pastas, texturized vegetable proteins (TVPs), pet foods, animal foods, instant beverage products and meat analogues and extenders. Examples of extruded products: baby food, weaning food, cereal bar, bread crumb, instant porridge, meat substitute, soy granules, raw material for convenience foods, enzyme-resistant starch foods, encapsulated flavours, corn/rice/wheat flakes, puffed products, chocolate, fruit gum liquorice, etc.