**1. Introduction**

264 Thermoplastic Elastomers

[9] Dahiya A.; Kamath, M. G. & Hegde, R. R. (2004). Meltblown Technology, In: *The Official* 

http://web.utk.edu/~mse/Textiles/Melt%20Blown%20Technology.htm

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*Textile Research Journal*, Vol. 73, No.1, January 2003, pp. 15-21, DOI:

*Website of the University of Tennessee*, 21.11.2011, Available from:

Extrusion cooking was first introduced in food and feed processing in the late 1950s. Since then, the systems involved have grown in popularity, efficiency and flexibility. Extrusion cooking technology is most used for cereal and protein processing in the food industry and is closely related to the pet food and feed sectors. In the last decade, the development of extruders has evolved to yield sophisticated products, new flavour generation, encapsulation and sterilisation.

Thermoplastic extrusion is considered a HTST (High-Temperature, Short-Time) process in the food industry, and it permits, with little or no modification of the basic equipments and appropriate process control, the production of a great variety of food and feed products (Camire et al., 1990; Chang et al., 2001; El-Dash, 1981). This technique has been widely used with raw materials such as corn, wheat, rice and, especially in recent years, with soy (Chang et al., 2001; Kadan & Pepperman, 2002).

Depending on the raw materials and of the characteristics desired for the final product, extruders operate with low, medium or high shear; however, thermoplastic extruders are used for high shear. As examples, pasta and processed meat products are produced with low shear (cold extrusion); meat analogues and some pet foods are produced with medium shear; and expanded snack products, breakfast cereals and textured vegetable proteins are produced with high shear (thermoplastic extrusion).

For Fellows (2000), the two main factors that influence the characteristics of extruded products are: raw material characteristics and operational conditions of the extruder. As main characteristics of the raw material, the following can be highlighted: type of material, moisture content, physical state, chemical composition (quantity and type of starch, proteins, fats and sugars) and pH of the material. The operational parameters that can be pointed out as important are: temperature, pressure, die diameter and shear force, with the latter being influenced by the internal design of the extruder and by its length; as well as screw geometry and rotation speed.

Guy (2001) and Stanley (1986) relate the following advantages to the thermoplastic extrusion process: versatility, low costs, high production yields, good quality products and no effluents.

Thermoplastic Extrusion in Food Processing 267

Pre-conditioning with steam or water has always been an important part of the extrusion process. Recent research has shown that efficient throughput of the extruder is almost doubled if the starting material is pre-conditioned with steam or water (Guy, 2001). There are many applications of extruded cooked food products where pre-conditioning plays a key role in the overall extrusion process. These products include direct expanded and flaked breakfast cereals, pre-cooked pasta, textured vegetable proteins, meat analogues, extruded

Pre-conditioning is not applied to all extrusion processes. In general, this step is applied when moisture contents around 20 to 30% and long residence times of the material are used. Pre-conditioning favours uniform particle hydration, reduces retention times within the extruder and increases throughput, increasing the life of the equipment, due to a reduction in the wearing of barrel and screw components, also reducing the costs of energy involved in the process (Huber & Rokey, 1990). Depending on screw configuration, the residence time of the material inside the extruder can vary from 5 seconds to more than 2 minutes, with the average residence time of the material in the pre-conditioner being 3 minutes. Preconditioning occurs with the addition of hot water (80-90°C) or steam, through spray nozzles, with the use of steam reducing energy consumption of the equipment up to 60% during the process. The most commonly used pre-conditioners have 2 axis of different diameters and rotation speeds, guaranteeing a residence time between 2 to 4 minutes and a production capacity between 300 and 18,000 kg.h-1. When it is necessary to add melted fats or oils during pre-conditioning, it is best to do it at the end of the equipment, because if addition is done at the beginning, a coating may be formed over the particles, making water penetration more difficult. The main aim of pre-conditioning is to uniformly hydrate the

Most raw materials used in food extrusion are solid. The feeding system is normally composed of a holding bin where the material is loaded and the discharge of the material can occur through a vertical feeding screw, a horizontal feeding screw, a horizontal vibrating trough system, a disk feeder or a volumetric belt feeder. It is necessary to guarantee a constant and non-interrupted feeding of the raw materials into the extruder for an efficient and uniform functioning of the extrusion process (El-Dash, 1981). When liquids are added, they can be dosed using a rotameter, orifice and Venturi meters, positive displacement meter, magnetic flow meter or metering pumps (Chessari & Sellahewa, 2000).

The screw of the extruder is certainly its most important component, not only to determine cooking degree, gelatinization and dextrinization of starch and protein denaturation, but also to ensure final product quality. Screws can be mono-piece (composed of a unique piece) or multi-piece (composed of various elements) (El-Dash, 1981). Screw elements can vary in number and shapes, each segment is designed for a specific purpose. Some elements only convey raw or pre-conditioned material into the extruder barrel, while other segments compress and degas the feedstock. Others must promote kneading, backflow and shear.

**2.1 Pre-conditioning** 

**2.2 Feeding system** 

**2.3 Screw** 

bread crumbs, and third-generation snacks.

raw material in order to eliminate any dry core (Strahm, 2000).
