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|Title: ||Effects of sucrose on the foaming and interfacial properties of egg white protein and whey protein isolate|
|Authors: ||Yang, Xin|
|Advisors: ||E. Allen Foegeding, Committee Chair|
Egg White Protein
Whey Protein Isolate
|Issue Date: ||17-Nov-2008|
|Discipline: ||Food Science|
|Abstract: ||Proteins are present in aerated foods to stabilize the air/water interface by lowering the interfacial tension and forming strong interfacial films. Sugars are known to influence the functional properties of protein foams. This study investigated the mechanism responsible for sucrose effects on the foaming and interfacial properties of egg white protein (EWP) and whey protein isolate (WPI), and thereby on the functionality of their foams in food products.
In the first study, 12.8% (w/v) sucrose was added into 10% (w/v) protein solutions of EWP, WPI, or WPI-EWP combinations, with the physical properties of pre-foam solutions, foams, air/water interfaces, and angel food cakes investigated. Sucrose greatly increased the stability (longer drainage Â½ life) of EWP, but slightly affected that of WPI and WPI-EWP combinations, which cannot be explained by solution apparent viscosity (Î¼). The dilational elasticity (Eâ€²) of EWP interfacial films increased with sucrose addition, while that of WPI and WPI-EWP combinations decreased. The interfacial tension (Î³) of WPI-EWP combinations followed the temporal pattern of WPI. Angel food cake prepared from 25% WPI/75% EWP foam showed comparable cake volume as that from EWP foam but a coarse structure similar to that from WPI foam. These results suggested that WPI dominated the air/water interface in mixed systems, leading to lower stability of wet foams and angel food cake batters.
Secondly, sucrose (0 to 63.6 g/100 mL) was added into 10% (w/v) protein solutions of EWP and WPI to establish its effects on protein solutions and foams. Confocal microscopic images showed that sucrose slowed the bubble size growth over time. A linear correlation was established between the change of bubble size over 20 min and the drainage Â½ life, regardless of protein type. Relationships between the change of bubble size over 20 min and Eâ€²/Î³ suggested that interfaces with Eâ€²/Î³>2 can effectively slow bubble size growing in EWP foams, confirming theoretical predictions (Walstra, 2003). The drainage Â½ life was proportionally correlated to Î¼Ã—Eâ€²/Î³, independent of protein type, showing the foam stability can be enhanced by a viscous continuous phase and elastic interfaces. Sucrose addition altered the volume of angel food cakes prepared from WPI foams but showed no improvement on the coarse structures. In conclusion, sucrose can modify solution viscosity and interfacial elasticity, altering the foam microstructure and improving the stability of wet foams. However, the poor stability of WPI in the conversion from a wet to a dry foam (angel food cake) cannot be changed with sucrose addition.
Finally, sucrose effects on the Eâ€² of individual protein components in WPI or EWP were evaluated. The major protein components of EWP (ovalbumin) and WPI (Î²-lactoglobulin) were separated and compared with the ovalbumin and Î²-lactoglobulin depleted fractions. Addition of 44.3% (w/v) sucrose decreased Eâ€² of Î±-lactalbumin and the Î²-lactoglobulin depleted fraction, resembling the characteristics of WPI. However, sucrose showed no major effect on the Eâ€² of Î²-lactoglobulin interfacial films. Sucrose increased the Eâ€² of EWP and the ovalbumin depleted fraction but did not change that of ovalbumin. The Eâ€² of protein mixtures suggested that the interfacial domination priority followed the order of Î±-lactalbumin > Î²-lactoglobulin > egg white proteins.
Overall, sucrose can cause a general increase in protein foam stability due to increased viscosity of the continuous phase, and a protein-specific effect on stability factors associated with interfaces. The Eâ€² of EWP increased with addition of sucrose while that of WPI decreased, mainly associated with Î±-lactalbumin, leading to different stability of protein foams.|
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