Phospholipids oxidation and foaming enhancement of egg albumen

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Wang, Guang
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Tong Wang
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The essential theme of this dissertation focuses on better utilization of egg products and exploration and resolution of problems associated with functionalities of egg lecithin and egg protein in food. Egg provides both high quality protein and lipids, especially lecithin. Egg lecithin is the second major source for commercial lecithin, and its application covers foods, cosmetics, and pharmaceutical fields. One of the main functionalities of egg albumen protein in food preparation is its excellent foaming property as seen in various foamy food products, such as meringues and angel food cakes.

A major concern in utilizing egg lecithin as an emulsifier in food emulsions is lipid oxidation. Lipid oxidation causes off-flavor and losses in nutritional quality. In emulsion systems, multiple factors could affect oxidative stability of lecithin of both soy and egg sources. In our study, the role of transition metal ions, cupric and ferric ions, in oxidative stability of egg and soy lecithin was evaluated in emulsions. The effect of pH on lipid oxidation was also examined under two concentrations for each ion (50 yM and 500 yM). For the source of EL and SL used in the studies presented in Chapter 2, it was found that egg lecithin (EL) had a similar peroxide value (PV) development pattern as soy lecithin (SL) when treated with cupric ion under both acidic and neutral pH. Acidic pH of 3 accelerated oxidation of both EL and SL, especially under high concentration of cupric ion. When treated with ferric ion, EL oxidized much faster than SL. EL had higher secondary oxidation products (TBARS) than SL. Acidic pH accelerated TBARS development for both EL and SL, but EL had more significantly increased values. Cupric ion was more powerful than ferric ion in catalyzing oxidation of both EL and SL under both acidic and neutral pH conditions as measured by PV and TBARS. Overall, SL showed better oxidative stability than EL under our experimental conditions. This study also suggests that using multiple methods is necessary in properly evaluating lipid oxidative stability, especially in the situation where lipid hydroperoxides tend to be easily decomposed.

As a minor component in egg lecithin, ethanolamine plasmalogen attracted our attention because of the increased interest in studying its antioxidant effect in an in vivo and in vitro. The reported antioxidant effect might have contributed to the better oxidative stability of EL than SL as we previously found in bulk oil. To better understand the role of ethanolamine plasmalogen in maintaining oxidative stability in in vitro system, an EthPm extracted from bovine brain (BBEP) was used in our oxidation study. It was found that in the purified soybean oil (PSO) system, the addition of 200 and 1000 ppm BBEP promoted lipid oxidation, whereas SL added at the same amount showed a similar oxidation trend to the PSO control. The more BBEP is added, the faster the lipid oxidation. Lipid oxidation of BBEP treatments was even faster in the presence of cupric ion than its non-cupric counterparts. In commercial soybean oil (CSO) with the presence of tocopherols, SL at 1000 ppm as an antioxidant acted synergistically with the natural tocopherols, but the addition of BBEP accelerated lipid oxidation as evidenced by the oxidative stability index (OSI). In the model liposome system, BBEP-added egg PL (EPL) liposome tended to have a fast breakdown of the lipid hydroperoxides, consequently promoted more secondary oxidation products. The oxidation of liposome lipid in the presence of copper was not affected by the presence of BBEP, which indicates that the hypothesis of ethanolamine plasmalogens chelating cupric ion as the antioxidant mechanism was not supported. The addition of cumene hydroperoxide to the EPL liposome promoted lipid oxidation as reflected by a very fast development of PV and TBARS. However, our study failed to differentiate the effect of BBEP and SL and their concentration on oxidation. It can be concluded that EthPm is not an antioxidant but rather a pro-oxidant in bulk lipid systems, whereas addition of EthPm has no significant effect on oxidation in liposome PL.

While egg lipid is of a concern due to its oxidative stability when used in emulsified food, yolk lipid contamination to egg white is another issue involved in egg protein foaming. Our study revealed that a concentration as low as 0.022% (as-is basis) of yolk contamination caused significant reductions in foaming capacity and foaming speed. Further test confirmed that it is the neutral lipid fraction of egg yolk causing the major detrimental effect on foaming, and the phospholipids fraction did not give significant foaming reduction at a concentration as high as 0.1%. High-speed and short-time shearing caused no apparent damage but longer shearing time significantly impaired foaming. Foaming was significantly reduced at a temperature of 55 yC for 10 min, whereas it did not change up to 3 min at a heating temperature of 62 to 64 yC. Industrial processing steps (pumping, pipe transfer, and storage) did not produce negative effects on foaming of the final products and the controlled pasteurization was actually beneficial for good foaming performance. Therefore, it is concluded that yolk contamination of the egg white was the major factor in reducing foaming properties of the white protein.

Foaming properties of egg white protein are affected by yolk lipid contamination, but proteins of basic nature may restore the foaming properties of the yolk-contaminated egg white protein. In this study, we also esterified the acidic groups on soy protein isolate (SPI) and studied the potential of such modified protein in improving foaming. It was shown that the modification changed the isoelectric point of soy protein isolate (SPI) from 4.5 to about 10. Sonication was proven to be a very effective means to re-disperse the methanol-denatured soy protein during the reaction condition, as shown by the improved solubility profile. The sonicated-modified SPI (SMSPI), when added to the yolk-contaminated (at 0.4% level, as-is basis) egg white, gave significantly improved foaming properties. Addition of SMSPI (16%, based on dry egg white, and 1.6% based on liquid egg white) increased the foaming performance of both pure egg white and yolk-contaminated egg white. SMSPI consistently performed better than the unmodified SPI for improving foaming. Addition of SMSPI fully restored foam expansion and foam liquid stability of 0.4% yolk-contaminated egg white, and it even out-performed the foaming of pure white protein. Therefore, a feasible solution to restore the foaming properties of yolk-contaminated egg white has been identified.

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Thu Jan 01 00:00:00 UTC 2009