Frying, baking, boiling, steaming and microwaving are some domestic food processing methods. These processes are aimed not only to improve taste, flavour, and appearance but also to increase digestibility of foods. Choosing a suitable method for cooking food is important as the process can affect the chemical composition and bioactive compounds present in food (Ruiz-Rodriguez et al., 2007).
2.4.1 Effects of food processing on phenolic compounds and antioxidant activity
Some studies have been conducted to investigate the effect of food processing on TPCs and antioxidant activity of vegetables, fruits, grain and legumes. A significant
the increase could be attributed by an increased release of free ferulic acid and bound phenolics from the cell matrix. A similar finding has been found in durum wheat pasta prepared with debranning fractions of wheat. Boiling the pasta in water was able to enhance the release of bound phenolic compounds from food matrix during extraction, resulting in an enhancement of their antioxidant activity. However, there was a decrease in the concentration of free phenolic compounds after boiling of the sample (Fares et al., 2010). In contrast, soaking, boiling and steaming processes reduced the TPCs and antioxidant activity of lentil and chickpeas. The effect depends on the type of legumes and the processing condition (Xu and Chang, 2008).
Furthermore, a study also showed that boiling in water was able to diminish antioxidant compounds in Chenopodium quinoa seeds (Dini et al., 2010). The loss of phenolic compounds in green beans, broccoli, spinach after conventional cooking such as boiling, steaming and microwaving was due to phenolic breakdown during the cooking process (Turkmen et al., 2005).
In regards to the effects of food processing on isoflavone (phytoestrogen) content, extrusion of soy-corn mixture still retained the isoflavone content and the health benefit associated with antiproliferative activity (Singletary et al., 2000). The total isoflavones in soymilk were not reduced by autoclaving for 5 min. However, by increasing the duration of the autoclaving to 15 min, the total isoflavone concentration was decreased by 20%. The study also found that there was intra-conversion among the different forms of isoflavones to β-glycoside, acylglycoside or aglycon forms during autoclaving (Chiarello et al., 2006). Another study showed that baking and frying of cookies with soy flour did not change the total isoflavone content present in the cookies, but during the process β-glycoside form was increased (Shimoni, 2004).
Antioxidant activity of phenolic compounds, in particular the flavonoid group is affected by masking effect of proteins. The masking effect is due to binding between proteins and flavonoids resulting in polyphenol-protein complex, causing a reduction of free flavonoids. As a result, the scavenging capacity of the flavonoids was reduced. Therefore, food matrix is also an important factor that needs to be considered when processing foods containing phenolic compounds (Arts et al., 2002).
2.4.2 Effects of food processing on phytosterol stability
Another important bioactive compound in legumes is phytosterols. There is increasing interest in these compounds due to their role in the treatment of hypercholesterolemia. It has been found that intake of 2 g of sterol lowers LDL cholesterol by 10% (Ostlund, 2007). Therefore, the market of functional foods enriched with phytosterols are recently expanding. It is thus necessary to know the stability of phytosterols particularly after food processing.
Phytosterols as bioactive compounds in lipid are susceptible to oxidation during food processing and storage. The oxidation of phytosterol can occur by auto-oxidation in the presence of heat, light, sunlight and or reactive oxygen and by enzymatic oxidation. The steroid ring in pytosterol structure is subject to auto-oxidation. Whilst the side chain in the phytosterol structure is believed to be subject to enzymatic oxidation by some enzymes such as cytochrome P450 monooxygenases, dehydrogenases, hydroxylases (Hovenkamp et al., 2008). The oxidation leads to production of sterol oxidation products (SOPs) or oxidized phytosterols/oxyphytosterols (Ryan et al., 2009, Zhang et al., 2006b). Both the saturated and unsaturated, chemical structure, saturated or unsaturated, prolonged exposure to the above factors and processing medium also influence the stability of phytosterols (Soupas et al., 2004). Examples of oxidized derivatives of phytosterols (oxyphytosterols) mainly include 7-hydroperoxide, 7-keto, 5-hydroperoxide, 5,6 epoxy, and triol as shown in Figure 2-13 (Rudzinska et al., 2004, Ryan et al., 2009, Hovenkamp et al., 2008).
Figure 2-13 : Representative of chemical structure of oxiphytosterols.
R is side chain in the phytosterol structure (Hovenkamp et al., 2008)
Pan-frying is one of the cooking processes that leads to oxidation of sterols present in food and oils. During the process, a wide range of derivatives from the oxidized phytosterols such as the oxy-, epoxy- and keto- sterol derivatives were formed (Soupas et al., 2007). Oxysterols have also been identified in stored vegetable oils (Bortolomeazzi et al., 2003), enriched spreads (Louter, 2004), and dairy products (Menendez-Carreno et al., 2008).
Microwaving phytosterol-enriched milk at 900W for 2 min and electrical heating at 90°C for 15 min has been observed to reduce the total phytosterol contents by 60%.
The low percentages of phytosterols in the sample was due to degradation of their oxidized products (Menendez-Carreno et al., 2008). In contrast, the other study found that phytosterols contained in vegetables oils were stable at temperature up to their melting point of between 140 and 170°C. No oxidation and degradation was found in this temperature range. When heating was performed above 200°C for 30 min, the total phytosterols in the vegetable oils were reduced (Thanh et al., 2006).
Furthermore, boiling beans and vegetables in the water for 30 min did not change
phytosterol content. However, free phytosterol contents in some beans and vegetables were increased by this treatment (Kaloustian et al., 2008).
Cholesterol has a similar structure to phytosterols and thus can also be affected by auto-oxidation and enzymatic oxidation to produce cholesterol oxidation products (COPs). The effect of COPs on health have been well studied and was found be associated with mutagenesis, carcinogenesis and atherosclerosis (Ryan et al., 2005, Lordan et al., 2009). As a consequence, phytosterol oxidation products (POPs) are also suspected to be able to promote toxicity as COPs (Ryan et al., 2009). Such observations suggest that understanding of phytosterol stability during food processing is important in maintaining the health benefit of the bioactive compounds and in reducing the adverse effects of oxidized products possibly occured during processing.
2.4.3 Effects of food processing on stability of bioactive protein
Only a few studies have been conducted on investigating the effect of processing on bioactive peptides in lupin. Boiling, autoclaving and microwaving have been performed onwhole lupin seeds in order to study the effect of processing on allergenicity of lupin. The results of the study suggested that autoclaving at 138ºC for 30 min can minimize the allergenicity of protein with molecular weight of 23 and 29 kDa (Alvarez-Alvarez et al., 2005). Moreover, a recent study observed that, under industrial thermal, mechanical, and high pressure treatments, β and δ-conglutin of L.angustifolius were stable to the processes (Sirtori et al., 2009).
In studying the effects of food processing on health benefits of proteins in vitro, it was found that autoclaving legume proteins for 30 min resulted in a decrease in their ACE inhibitory activity. Nevertheless, autoclaving for 50 min resulted in an increase in their ACE inhibitory activity. The results suggested that autoclaving for 30 min
reported to reduce ACE inhibitory activity of protein isolate of L. albus, whitout affecting its bile acid binding property. However, an increased DPPH radical activity was observed in the study. Releasing small molecular weight protein during the heating could be the cause of a higher antioxidant activity following the treatmeant (Yoshie-Stark et al., 2006). A similar finding was found in the water-soluble protein of chickpea, where a higher free radical scavenging capacities was reported after a thermal processing at 121ºC (Arcan and Yemenicioglu, 2007).