**5. Influence on nutritional quality**

The effects of extrusion cooking on nutritional quality are ambiguous. Benefits include destruction of antinutritional factors, gelatinization of starch, increased soluble dietary fibre and reduction of lipid oxidation. On the other hand, Maillard reactions between protein and sugars reduce the nutritional value of the protein, depending on the raw material types, their composition and process conditions. Besides, heat-labile vitamins may be lost to varying extents (Singh et al., 2007).

Starch digestibility is largely dependent on complete gelatinization. High starch digestibility is essential for specialized nutritional foods such as infant and weaning foods. Creation of resistant starch by extrusion may have value in reduced calorie products (Guy, 2001; Riaz, 2000).

The nutritional value of vegetable proteins is generally enhanced by mild extrusion cooking conditions due to the increase in digestibility (Asp and Björck 1989; Arêas, 1992), probably a result of protein denaturation and the inactivation of enzyme inhibitors present in raw materials, by the exposure of new active sites for enzyme attack (Colonna et al., 1989).

Processing nutritional food products at moisture levels below 20% has been proven to be uneconomical and nutritionally undesirable. Low-moisture extrusion results in production of certain undesirable dextrins as a result of increased shear energy inputs. Losses of vitamins and reduced amino acid availability are greatly accelerated as extrusion moistures are decreased. For this reason, vitamins and heat-sensitive nutrients are usually added post extrusion when processing at low moisture conditions (Huber, 2001).

Mild extrusion conditions (high moisture content, low residence time, low temperature) improve nutritional quality, while high extrusion temperatures (higher than 200°C), low moisture contents (lower than 15%) and/or improper formulation (e.g. presence of highreactive sugars) can affect nutritional quality adversely. Also, to obtain a nutritionally balanced extruded product, careful control of process parameters is essential (Singh et al., 2007).

A benefit derived from extrusion-cooking is the partial or total destruction of potentially antinutritional factors, especially protease inhibitors, haemagglutinins, tannins and phytates, which limit utilization of nutrients in legume seeds. However, chemical alteration produced by thermal treatment could also result in decreased nutrient assimilation, including lower apparent absorption of certain minerals (Alonso et al., 2001).

Vitamin losses in extruded foods vary according to the type of food, moisture content, processing temperature and retention time. Generally, losses are minimal in cold extrusion. The HTST conditions in extrusion cooking, the short residence time of the extrudate and the rapid cooling as the product emerges from the die, cause relatively small losses of vitamins and essential amino acids (Fellows, 2000).

Extrusion cooking was reported by Saalia & Phillips (2011) as an efficient process to destroy or inactivate aflatoxins, if special conditions (high shear, high temperature, and adequate pH) are used.

Zhu et al. (1996) emphasized that by extrusion cooking, soybeans can be converted into high quality food ingredients. The short residence time and high temperature in an extruder

Thermoplastic Extrusion in Food Processing 281

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

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

are highly popular in the food and feed market (Castells et al., 2005).

**7.1 Second and third generation snacks** 

**7. Products** 

reduce the damage to nutritional properties, but still adequately inactivate the enzymes responsible for the development of the undesirable off-flavour.

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