Effects of extrusion on the polyphenols, vitamins and antioxidant activity of foods
Introduction
Extrusion cooking is one of the most important food processing technologies which has been used since the mid 1930s for the production of breakfast cereals, ready to eat snack foods, and other textured foods. Over the years, extrusion cooking has become the major processing method for food and feed industries, and it is rapidly evolving from an art into a science (Riaz, Asif, & Ali, 2009). In the past decade extrusion cooking has been studied extensively to produce variety of speciality foods including pasta products and ready to eat breakfast cereals, baby foods, snack foods, texturised vegetable protein, pet foods, dried soups and dry beverage mixes, as it not only improves digestibility (Singh, Dartois, & Kaur, 2010) but also improves of nutrients bioavailability (Gu, House, Rooney, & Prior, 2008) compared to conventional cooking. In addition to these properties, extrusion cooking is preferred over conventional cooking/processing techniques because of its ability to develop range of products with distinct textural advantages including expansion, crispiness and general mouthfeel; being versatile, high productivity, low operating costs, energy efficiency and shorter cooking times.
Consumers have developed a growing understanding of how the composition of food products can impact on the nutritional quality of foods (Brennan, 2005). Bioactive compounds in food and food products play a vital role in human providing protection against many chronic and degenerative diseases (Van Dokkum, Frølich, Saltmarsh, & Gee, 2008). Apart from health benefits, natural phenolic compounds in food and food products also act as free radical terminators, chelators of metal catalysts, and singlet oxygen quenchers abetting lipid oxidation to improve shelf life and consumer acceptance of extruded snacks. For example, Viscidi, Dougherty, Briggs, and Camire (2004) observed that the addition of ferulic acid and benzoin at levels of 1.0 g/kg or higher generally resulted in delayed onset of oxidation in oats based extrudates. Fruits and vegetables are considered as major source of bioactive compounds however grains are also potential source of bioactive compounds particularly phenolic acids. The majority of these compounds are lost during processing due to their sensitivity towards processing conditions such as temperature (Riaz et al., 2009). Research do date indicates that majority of extruded food products are prepared from food grains with few studies focussing on the incorporation of fruit and vegetables to obtain bioactive compound enriched snacks products of acceptable quality (Dehghan-Shoar, Hardacre, & Brennan, 2010).
This review aims to explore the effect of extrusion processing on polyphenols, vitamins and antioxidant activity of extruded food products which did not receive much importance in recent years.
Several extrusion process variables can influence the composition of finished products. These include raw material characteristics, mixing and conditioning of raw material, barrel temperature, pressure, screw speed, moisture content, flow rate, energy input, residence type, screw configuration etc influences physicochemical properties of extrudates Critical extrusion process variables such as temperature, screw speed, and moisture content may induce desirable modifications, thus improving palatability and technological properties of extruded products (Brennan, Monro, & Brennan, 2008). These conditions have the ability to produce both positive or negative influences on the bioactive compounds of the extrudates. Several studies have shown that extrusion processing significantly reduces measureable bioactive compounds in food products. Table 1 lists some examples showing effect of extrusion on bioactive compounds. Korus et al. (2007) investigated the effect of extrusion on polyphenol content and antioxidant activity of common bean they observed a significant decrease in polyphenol content and antioxidant activity. Similarly, Delgado-Licon et al. (2009) observed a significant decrease in the total polyphenols and antioxidant activity during extrusion of bean/corn mixture. They observed that the decrease in bioactive compounds was dependent on process condition. Shih, Kuo, and Chiang (2009) observed a significant decrease in β-carotene and anthocyanin for both yellow and orange sweet potatoes after extrusion. Some studies show a decrease up to 80.3% in the level of total phenolic acid after extrusion of kiwicha (Amaranthus caudatus) (Repo-Carrasco-Valencia et al., 2009a, Repo-Carrasco-Valencia et al., 2009a). This decrease may be due to decarboxylation of phenolic acids during extrusion. Yagci and Gogus (2010) also observed that both feed moisture content and barrel temperature causes significant decrease in total phenolic content. Phenolic compounds during extrusion may undergo decarboxylation due to high barrel temperature and high moisture content may promote polymerisation of phenols and tannins leading to reduced extractability and antioxidant activity (Repo-Carrasco-Valencia et al., 2009a, Repo-Carrasco-Valencia et al., 2009b, Dlamini et al., 2007). However, in some cases the level of bioactive compounds in extruded products may increase in extrudates for example ferulic acid content is reported to increase by three times in extruded cereal grains (Zieliński, Kozlowska, & Lewczuk, 2001). A similar increase in total phenolic compounds is reported after extrusion cooking of sweet potato (Shih et al., 2009); cereals in combination with vegetables (Stojceska, Ainsworth, Plunkett, Ibanoglu, & Ibanoglu, 2008). The increase in the levels of certain phenolic acids in extruded products is generally due to the release from the cell wall matrix.
El-Hady and Habiba (2003) studied the effects of extrusion process variables such as barrel temperature and feed moisture on total phenols content of whole meal of peas, chickpeas, faba and kidney beans. They observed a significant decrease in total phenol in extruded products. This decrease was mainly attributed to individual effect of both temperature and moisture. However they did not observe any interaction effect of feed moisture and barrel temperature on the phenolic content. Most of the bioactive compounds are temperature sensitive and barrel temperature plays a significant role in the stability of these bioactives. Mahungu et al. (1999) observed that the profile of isoflavones during corn/soy blend is greatly influenced by the extrusion barrel temperature followed by moisture content with small changes in isoflavone content. They observed an increase in the acetyl derivatives of genistin and daidzin and proportionate decrease in malonyl analogues with an increase in barrel temperature indicating thermal decarboxylation. High temperature causes malonyl conjugates to undergo heat-induced decarboxylation and deesterification (Mahungu et al. 1999).
Cereal and cereal based products are also rich source of vitamins (Tiwari & Cummins, 2009). Like other bioactive compounds extrusion cooking have profound effect on the stability of vitamins in extruded snack food for example higher barrel temperatures and low feed moistures favour ascorbic acid degradation during extrusion (Killeit, 1994). Stability of vitamins during extrusion is extensively reviewed (Camire et al., 1990, Cheftel, 1986, Killeit, 1994, Riaz et al., 2009). Athar et al. (2006) studied the effect of extrusion processing conditions on the stability of vitamins. They observed that extrudates obtained from short barrel (90 mm) extruders had a higher retention rate of B vitamin group (44–62%) compared to 20% for long barrel extruders. Anuonye, Onuh, Egwim, and Adeyemo (2010) studied the stability of vitamins during extrusion of Acha (Digitaria exilis)/soy bean blend and observed a 6% decrease in Riboflavin (B2), a 86.36% decrease in pyridoxine (B6), andno significant change in Ascorbic acid content. Athar et al. (2006) observed that the retention of vitamins during the extrusion process is not related to initial levels of the vitamins and varies with the cereal type. For example pyridoxine was stable in maize compared to oats and the maize + pea ingredient. Retention of vitamins during extrusion is generally depends on the stability of vitamins for example thiamine and pyridoxine are more sensitive to heat compared to riboflavin. High temperature, short-time extrusion cooking is also reported to influence the stability of fat soluble vitamins such as Vitamin A and E (Tiwari and Cummins, 2010). For example Zielinski, Michalska, Piskula, and Kozlowska (2006) observed a significant decrease (about 63%) in vitamin E content of buckwheat during extrusion cooking. Similarly, Zieliński, Kozlowska, and Lewczuk (2001) observed that extrusion cooking can cause a significant decrease (63–94%) in tocopherols and tocotrienols content of extruded cereals. Further, they observed that ratio of tocotrienols to tocopherols increase after extrusion cooking indicating that tocotrienols are the main residual isomers of vitamin E. Vitamin E include tocopherols and tocotrienols and are naturally occurring antioxidants present in cereals grains and are well recognised for their bioactivity. Among biologically active forms of Vitamin E both α-tocopherol and α-tocotrienol are least resistant to temperature compared to other form (Zieliński et al., 2001). Sensitivity of various forms of vitamin E varies with extrusion process variables for example a significant decrease in α-tocopherol is reported with an increase in extrusion temperature and a significant decrease in γ-tocopherol with increase in moisture content during extrusion of grass peas (Lathyrus sativus L) (Grela, Jensen, & Jakobsen, 1999). Similarly, Harper (1988) reported a decrease in Riboflavin with increase in feed moisture content and high screw speed.
Section snippets
Phenolic acids
Extrusion cooking of cereal and pulses blends has been investigated extensively (Adamidou et al., 2011, Frias et al., 2011, Jisha et al., 2010, Lazou et al., 2010) leading to the development of low fat, high fibre and protein extruded products. Extrusion of common beans with corn starch blend is reported to increase total phenol, and antioxidant potential of extruded snacks (Anton, Fulcher, & Arntfield, 2009). They observed a significant increase in total phenol content and antioxidant activity
Antioxidant activity
Antioxidant activity and antiradical activity of extruded products is dependent not only on the level of bioactive compounds but also on the composition of bioactive compounds. Rice-Evans, Miller, and Paganga (1996) reviewed the biological properties of the flavonoids and the relationship between their antioxidant activity as hydrogen donating free radical scavengers, and their chemical structures. Korus et al. (2007) observed a lower antioxidant activity for dark-red beans compared to black
Conclusion
Nutritional and clinical studies indicate that whole grains may have advantages over refined flour due to presence of various bioactive compounds. Extrusion cooking of whole grains will ensure higher level of phytochemicals in extruded food products. Heat processing- particularly under severe conditions- may give rise to chemical and physical changes that impair the organoleptic properties and reduce the content or bioavailability of some bioactive compounds. Even if a food has a high content
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