**7. Products**

280 Thermoplastic Elastomers

reduce the damage to nutritional properties, but still adequately inactivate the enzymes

Minerals are heat stable and unlikely to become lost in the steam flash-off at the die. Extrusion can improve the absorption of minerals by reducing other factors that inhibit absorption, like phytates and condensed tannins. In addition, extrusion cooking usually increases the amount of iron available for absorption. For foods fortified with minerals prior to extrusion, some problems can be verified, like the formation of iron complexes with phenolic compounds that are dark in colour and detract from the appearance of foods; added calcium hydroxide can contribute to decrease expansion and increase lightness in

One of the most important consumer requirements is the microbiological safety of food products. Most conventional extruded products such as snack foods and breakfast cereals are safe to eat because the raw materials are subjected to high temperatures (higher than 130°C) and the water activity of the product is low because the product is dried to a moisture content of less than five per cent. Although it is well known that most vegetative organisms, yeast and moulds are destroyed under typical extrusion conditions, the operating conditions under

The reduction of antinutritional factors, the increase of product microbiological safety and much better consumer acceptability are also related to extrusion cooking (Sumathy et al.,

The extrusion process, as it is carried out at high temperatures, even with short residence times, eliminates a high amount of microorganisms (Baik et al., 2004). The extrusion process also allows obtaining lower water activity values in the final product, with values between 0.1 and 0.4. Therefore, it is possible to extend the shelf life of products (Fellows, 2000).

Fraiha et al. (2011) emphasize studies with heat-resistant microorganisms showing that shear stress may be involved in microbial load reduction during the extrusion process, predicting that mechanical forces might cause cell rupture. These authors studied a pretreatment of *Bacillus stearothermophilus* spores in a 99% CO2 modified atmosphere and checked that it did not affect cell viability during food extrusion. For them, heat was not the sole phenomena to explain cell death during extrusion, a mechanical damage of cells might

Okelo et al. (2006), studying the optimization of extrusion conditions for elimination of mesophilic bacteria during thermal processing of animal feed recipes, pointed out that, in general, thermal processing is designed to eliminate mesophilic organisms and not thermophilic organisms such as *Bacillus stearothermophilus*. It was predicted that most pathogenic organisms in feed would be inactivated by extrusion cooking through selecting extruder conditions within the experimental variable ranges that maximized spore destruction of thermophilic bacteria. This reduction would also likely include members of

Mycotoxins are a risk to human health mainly via the intake of contaminated foods of plant origin, such as corn and wheat, which are consumed worldwide. The risk of exposure is

responsible for the development of the undesirable off-flavour.

which spores are inactivated are not well understood (Guy, 2001).

colour of some products (Singh et al., 2007).

**6. Influence on microbiological quality** 

2007).

be involved.

mesophilic *Bacillus cereus* group.

#### **7.1 Second and third generation snacks**

The evolution of snacks occurred rapidly and can be divided into three generations. In the first, the raw material, such as whole grains, is processed through the combination of moisture, cooking temperature and drying. Only second and third generation snacks are produced by thermoplastic extrusion, and a flow diagram is shown in Figure 3.

Fig. 3. Flow diagram for the production of second and third generation snacks

Thermoplastic Extrusion in Food Processing 283

designed to allow expansion after leaving the extruder because the moisture in the formula (whether natural or added) is released from a zone of elevated temperature and pressure to ambient conditions. The die holes control the shape of the finished cereal pieces, once they are cut with a rotating knife on the outer face of the die. The extruded products can be sugar-coated or coloured and flavoured to produce a variety of products for various tastes (Fast, 2000; Eastman et al., 2001). In expanded extruded products two important characteristics are expansion and texture. Extruded products are characterized for their expansion and usually maximum expansion is desired for expanded extruded snack products. For expanded extruded breakfast cereals a different structure is desired. It is necessary to obtain products with higher apparent density, lower porosity and thicker cell walls, as these products will be immersed in an aqueous medium, such as milk, and must maintain their texture during the longest time possible, absorbing the lowest quantity of

BCs can be considered protein sources (even though cereal proteins are incomplete due to limiting essential amino acids such as lysine), as they are often formulated with various different types of cereals and consumed with milk. When produced with whole grain flours, they can be considered sources of fatty acids and fibres. Breakfast cereals are sources of vitamins and minerals, as grains contain significant amounts of B-group vitamins, tocopherols, and minerals such as iron, zinc and copper, apart from being normally consumed with milk or yoghurt, considered important sources of calcium (Jones, 2001).

Nowadays, the development of new products in the BC segment aims at the production of breakfast cereals with a functional appeal. Recent research reports the use of antioxidants such as tocopherol and lycopene (Paradiso et al., 2008; Dehghan-Shoar et al., 2010) and fibres, such as β-glucans, gums and oat, wheat and passion fruit brans (Holguín-Acuña et

To human diets, especially those of vegetarians, the ingestion of high protein content products has been incorporated, including, for example, meat extenders and meat analogues, obtained through the extrusion process of vegetable proteins, resulting in a product with appearance and texture similar to the fibrilar structure of meat (Strahm, 2006; Macdonald et al., 2009). Meat extenders are obtained by thermoplastic extrusion at low moisture contents (20-35%) and meat analogues are obtained by thermoplastic extrusion at high moisture contents (50-70%). The raw materials commonly used to produce meat extenders are defatted soy flour and soy protein concentrate (SPC), whereas for the production of meat analogues, soy protein

Amongst the main vegetable proteins used to produce meat analogues are proteins from legumes such as soybeans, common beans and peas, and from cereals, especially wheat proteins responsible for the formation of the gluten network (Riaz, 2000; Strahm, 2006).

The use of raw materials with high protein contents in extrusion began around the 1970s, with the use of soy for the production of texturized soy protein and meat analogues. Although researchers agree that, during extrusion of high protein content raw materials, denaturation, melting and alignment during mass flow occur, there is still a need to understand the physicochemical and rheological changes involved, once the phenomena that lead to the

al., 2008; Vernaza et al., 2010; Yao et al., 2011; Ryan et al., 2011).

concentrate (SPC) and soy protein isolate (SPI) are used.

moisture possible (Collona et al., 1998).

**7.3 Meat analogues** 

Second generation snacks or expanded snacks, where most extruded snacks are classified, are usually low in bulk density and are often marketed as high-fibre, low-calorie, highprotein and nutritional products (Lazou & Krokida, 2011). Different raw materials used to produce these kinds of snacks (i.e. flours and/or cereals and tubers starches and proteins) are processed in a extruder resulting in a continuous mass, that is cut into pieces of uniform size, being afterwards dried, flavoured and stored (Booth, 1990).

Third generation snacks or pellets are normally called "half products". These snacks are produced almost the same way as second generation snacks, however, when the product exits the extruder, it has the form of the die, that is, it is not expanded, being dried in this form. The expansion of the product occurs afterwards through frying, heating by hot air or in a microwave oven (Riaz, 2000; Carvalho et al., 2009). This kind of product presents a low moisture content (between 7 and 10%), high density and stability to be stored for a long time without microbial damage (Carvalho et al., 2009).
