**6. Influence on microbiological quality**

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

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., 2007).

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 be involved.

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 mesophilic *Bacillus cereus* group.

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 therefore high for humans, and industrial processing methods that are effective in reducing mycotoxin contents in processed samples have received increased attention. Among them, extrusion cooking may be one of the most effective ways to reduce mycotoxin levels in processed products, especially if glucose or other additives such as ammonia or sodium bisulphite are included as ingredients. This is especially important since extruded products are highly popular in the food and feed market (Castells et al., 2005).
