**1. Introduction**

38 Thermoplastic Elastomers

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Starch makes up the nutritive reserves of many plants. Starch biosynthesis is a complex process, which may be summarized as during the growing season, the green leaves collect energy from the sun, this energy is transported as a sugar solution to the starch storage cells, and the sugar is converted into starch in the form of tiny granules occupying most of the cell interior. The conversion of sugar into starch takes place through enzymes (Corn Production Source, 2011; Tester et al., 2006).

Starch granules are composed of two types of alpha-glucans, amylose and amylopectin, which represent approximately 98–99% of the dry weight. The ratio of the two polysaccharides varies according to the botanical origin of the starch and classifies starch as the 'waxy' starches contain less than 15% amylose, 'normal' 20–35% and 'high' amylose starches greater than about 40% (Tester et al., 2006; Wurzburg, 1989).

Amylopectin is a much larger molecule than amylose with a molecular weight and a heavily branched structure built from about 95% (α 1- 4) and 5% (α 1- 6) linkages. Amylopectin unit chains are relatively short compared to amylose molecules with a broad distribution profile. They are typically, 18–25 units long on average (Tester et al., 2006; Wurzburg, 1989).

Nutritionally, starch is consumed as an energy source, and it is the most abundant energy source in the human diet as it is present at high amounts in cereals, roots and tubers, which products range from breads, cookies, pastes to consumption as snacks, porridges, or as processed cooked grains (white rice, corn grain) or whole grains (whole grain of rice, wheat, popcorn, etc.).

It may be technologically considered as a nutrient of great versatility, with numerous applications in the food industries and others such as in cosmetics, pharmaceutical, paints, children toys, glues and adhesives, and nowadays as a biopolymer for the production of packagings.

This versatility is related to some characteristics of starch such as being of low cost and easy to undergo physical and chemical modifications which allow changes to the physicalchemical and rheological properties of starches.

The main starch modifications obtained with the use of the thermoplastic extrusion process will be addressed in this chapter.

Physical and/or Chemical Modifications of Starch by Thermoplastic Extrusion 41

Starch must be gelatinized in the human diet in order to be digested by the amylolytic enzymes of the human digestive system. The classic model of obtaining gelatinized starches, where starch granules are slowly heated in a medium with little agitation and much water, which promotes imbibition, swelling and polymer release (Leach, 1965) for a prolonged time, such as in the obtaining of cooked rice, corn flour porridges, was replaced by other methods, such as extrusion, spray-drier and drum dryer, which promote fast starch gelatinization and followed by drying may obtain flours and/or pregelatinized starches of

Pregelatinized starches or flours are paste-forming products in the presence of cold water or (partially or totally) soluble products in cold water (Colonna et al.,1984) and present the following characteristics: they disperse more easily and absorb more water than their untreated matches, they form gel at room temperature and are less prone to deposit (Powell,

The use of gelatinized starch in food products affects their characteristics and qualities, such as bread volume and crumb (Williams & Lesselleur, 1970); elasticity and softness of pastas (pasta), digestibility and palatability, tolerance in the properties of beating and cake mixtures, ice creams, doughnuts, growth of sugar crystals in food products (Powell, 1965); texture, volume, shelf-live and stability during thawing of cakes and breads (Michael &

In the food industry, pregelatinized starches are used to achieve thickening or water retention without employing heat, for example, puddings, instant lactic mixtures and breakfast foods; to prepare ready-to-use bread mixtures, where the increased absorption and retention of water improves the quality of the product, to work as an agglutinant in the meat industry; and, as a filling for fruit pies, as they make the use of heat dispensable and increase flavor retention. They also have a non-food use, such as in the industries of textiles

Pregelatinized flours may be obtained on an industrial basis through extrusion or drum-

The use of atomizers is economically limited as starch pastes are highly viscous and require

Drum-dryers are simple and commonly used, but they present the disadvantage of highcost products due to low efficiency, low production, difficult operation, constant need for

The extrusion process presents the advantages of versatility, high productivity and low cost (Smith, quoted by Harper, 1979) and more strict control of the desired gelatinization degree, where small modifications in the equipment and/or in the raw material may lead to different final results (El-Dash, 1982). According to Lorenz & Jansen (1980), the low cost of gelatinization through extrusion is due to the fact that it efficiently converts electrical energy into thermal energy and also the manpower and space per kilogram of cooked product

The market of fast preparation products has grown and many varieties of pregelatinized flours are available in the market nowadays. The main trends in the use of thermoplastic

extrusion process applied to starchy ingredients will be approached in this chapter.

long-term stability and quick preparation.

and in drugs, paper, metallurgy, etc (Powell, 1965).

drying and through the use of atomizers (spray-drier).

drying at a low content of solids (Chiang & Johnson, 1977).

drum maintenance and adjustment (Greenwood, 1976).

required are lower than any other cooking method.

1965).

Brown, 1968).
