**3.1 Ready-to-eat cereals and snacks**

They are produced with cereal flour and starch ingredients, the extruded products are highly expanded and have several shapes and textures.

Ready-to-eat cereals are manufactured from mixtures of cereal flour and starch combined with small amounts of malt, fat, sugars, emulsifiers and salt. The extrusion process requires moisture of 20% and temperature >150ºC, after the extrusion, they are dried and toasted.

Much research has been made to obtain ready-to-eat cereals, as the control of the extrusion process is rather complex, due to the large number of variables affecting it (Ostergard & Bjorck, 1989, Stojceska et al., 2009).

Some early works, such as that of Lawton et al. (1972) considered extrusion temperature and initial moisture in the raw material as the variables with greater effect on starch gelatinization, and the maximum degree of gelatinization and shear occur when these

El-Dash (1982) defined the process of thermoplastic extrusion as being a continuous process in which mechanical friction is combined with thermal heating in order to continuously mix, plasticize and gelatinize the starch, denature protein materials, restructuring them for the

Single-screw cooking extruders were developed in the 1940's to make puffed snacks from cereal flours or grits. An expanding demand for precooked cereals and starches required machines with larger capacity, so extruders with a nominal capacity of 5 ton per hour were developed in the 1960's, with numerous news applications: snacks, infant feeding, pet foods, etc. In the 1970's products containing more than one component were developed, such as egg rolls and ravioli for coextrusion. Then, the use of two extruders in series, the first for cooking and the second one for forming and structuring, resulted in several products. At the end of the 1970's, the use of twin-screw extruders for food processing that expanded the range of application began (Mercier & Feillet, 1975; Linko et al., 1981; Harper, 1979). Finally, the extruders are meant for specific markets, as it will be seen for extruders intended for the productions of biodegradable packagings (Flores et al., 2010; Mandrogón et al., 2009).

Food extruders are generally available with segmented screws and barrel section, facilitating total control over the configuration of the machine to get a combination of various process parameters. Extrusion cooking is a high-pressure operation that provides sudden expansion of the processed products. The physical characteristics of the extrudate reflect the

Beneficial effects include destruction of antinutritional factors, gelatinization of starch, protein denaturation/texturization, increased soluble dietary fibre and reduction of lipid oxidation. But Maillard reactions between protein and sugars can reduce the nutritional value of the protein, depending on the raw material types, their composition and process conditions. Heat-

Some of the use applications of the extrusion process to physically and/or chemically

They are produced with cereal flour and starch ingredients, the extruded products are

Ready-to-eat cereals are manufactured from mixtures of cereal flour and starch combined with small amounts of malt, fat, sugars, emulsifiers and salt. The extrusion process requires moisture of 20% and temperature >150ºC, after the extrusion, they are dried and toasted.

Much research has been made to obtain ready-to-eat cereals, as the control of the extrusion process is rather complex, due to the large number of variables affecting it (Ostergard &

Some early works, such as that of Lawton et al. (1972) considered extrusion temperature and initial moisture in the raw material as the variables with greater effect on starch gelatinization, and the maximum degree of gelatinization and shear occur when these

effectiveness of the process and suitability of ingredients (Patil et al., 2005).

labile vitamins may be lost to varying extents (Singh et al., 2007, Patil et al., 2005).

modify starch both in the areas of food and biopolymers will be presented below.

**3. Thermoplastic extrusion process** 

**3.1 Ready-to-eat cereals and snacks** 

Bjorck, 1989, Stojceska et al., 2009).

highly expanded and have several shapes and textures.

obtaining of products with new textures and shapes.

variables act with opposite end values and when both values are high or low, low degrees of gelatinization occur. Other works showed that the extrusion process destroys the organized crystalline structure of the starch granule, with different degrees of intensity which depend on the ratio of amylose to amylopectin and on the independent variables used such as humidity and shear (Charbonnieri et al., 1973). Thus, starch may be gelatinized, which is what occurs at the extruder with humidity lower than 20% (Linko et al.,1981), dextrinized, which happens in more severe conditions and with low humidity content (Gomez & Aguillera, 1983, 1984). The process may also lead to starch liquefaction and partial hydrolysis of starch molecules (Faubion et al., 1982).

The production of snacks through extrusion represents a great achievement for the Food Technology area as it efficiently converts crude cereal flours into products with different shapes, flavors and long shelf-life.

At first, snacks were obtained from whole grains combined with moisture content, cooking temperature and drying, considered the first generation (Huber & Rokey, 1990).

The second generation snacks presented more expansion ability, and were obtained with flours refined from cereals and cereal and tuber starches. These snacks have a large volume and require appropriate packagings to avoid humidity, light and heat, and they must protect the product against mechanical shocks, in order to avoid breaking during transport and storage. These facts boosted the development of third generation snacks (Huber & Rokey, 1990).

Third generation snacks are not expanded through the extrusion process, for this reason they are known as pellets or half-products, for they will be expanded through a process of deep-frying or hot air, or with the use of microwaves, during consumption. Although they have an additional process for expansion, these products present great advantages in transport and storage (Huber & Rokey, 1990).

With the worldwide tendency of weight gain by the population, studies showed that gelatinized starch, if excessively consumed, contributes to an increased number of diseases, such as obesity, diabetes and increased blood triglycerides, which may lead to serious heart diseases (Jenkins et al., 1980).

A ranking for starch has been made:


Studies evaluating the digestibility of starches in snacks showed that they presented RDS type starch (Singh et al., 2007; Goni et al., 1997). Thus, the tendency is to promote the return of the use of the whole grain, addition of fibers and ingredients leading to increased SDS and RS in snacks, in addition to ingredients beneficial to health, such as antioxidants, and omega oils. That is, the new generation of snacks must include benefits to the consumer's health, in addition to being nutritious.

Physical and/or Chemical Modifications of Starch by Thermoplastic Extrusion 45

Two types of products were produced: pure whole meal products and breakfast cereals made from whole meal/maize blends were processed by pilot-plant extrusion and the enzyme-resistant starch (RS) content and hydrolysis index (HI) were not correlated to the

Yanniotis et al. (2007) verified the effect of pectin alone or in combination with wheat fiber on the physical and structural properties of extruded cornstarch, under specific moisture content, barrel temperature and screw speed conditions were studied using a laboratory single-screw extruder. These authors observed that fibers reduced the size of the cells and increased their number and pectin increased porosity and reduced expansion ratio and

Stojceska et al. (2009) investigated the use of brewer's spent grain and red cabbage barley and red cabbage in wheat flour and corn starch extruded in co-rotating twin-screw extruder, under the conditions studied, the results were promising towards the increase of total dietary fibre

The blends of various formulations of durum wheat flour (8-20%), partially defatted hazelnut flour (PDHF) (5-15%), fruit waste blend (3-7%) and rice grits were extruded using single-screw extruder and when higher, PDHF and fruit waste content caused an increase in the total phenolic content and antioxidant activity of the extruded samples, whereas percentage starch gelatinization and digestibility values decreased (Yaĝci & Göĝůş, 2009).

Limsangouan et al. (2010) demonstrated the effect of extrusion processing on the functional properties of extruded snack foods developed from cereal and legumes, and the by-products from herbs and vegetables, and the extrusion process slightly decreased the antioxidant

Starch may be modified through physical or chemical methods and its use relates to

to provide functional features in food applications which native starches may not

provides economical advantages in many applications where more expensive additives

Some researchers used simultaneously a chemical reagent and the extrusion process to obtain modified starch for various purposes, such as production of expanded extruded products (Lai et al*.,* 1989); starch phosphate production (Chang & Lii, 1992); alcohol production (Chang, 1989), extruded rice flour (Clerici & El-Dash, 2006) and acidic extruded rice flour (Clerici et al., 2009) for production gluten-free bread, and lactic beverage

Table 2 shows new starches that are been used in thermoplastic extrusion researches.

According to Light (1990), modified starch is used in foods, for 3 main reasons:

and the level of total antioxidant capacity and total phenolic compounds of the snacks.

extrusion temperature, but with whole meal products (Chaunier et al., 2007).

hardness of the snacks.

**3.1.4 Antioxidant activity** 

capacity and phenolic content.

it is readily available, and

are used. For example: gums.

**3.2 Chemical modification of starches** 

improved quality and decreased cost of the products.

normally provide. For example: pudding mixture,
