Effects of sucrose on the foaming and interfacial properties of egg white protein and whey protein isolate

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Title: Effects of sucrose on the foaming and interfacial properties of egg white protein and whey protein isolate
Author: Yang, Xin
Advisors: E. Allen Foegeding, Committee Chair
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 ½ life) of EWP, but slightly affected that of WPI and WPI-EWP combinations, which cannot be explained by solution apparent viscosity (μ). The dilational elasticity (E′) of EWP interfacial films increased with sucrose addition, while that of WPI and WPI-EWP combinations decreased. The interfacial tension (γ) 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 ½ life, regardless of protein type. Relationships between the change of bubble size over 20 min and E′/γ suggested that interfaces with E′/γ>2 can effectively slow bubble size growing in EWP foams, confirming theoretical predictions (Walstra, 2003). The drainage ½ life was proportionally correlated to μ×E′/γ, 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 E′ of individual protein components in WPI or EWP were evaluated. The major protein components of EWP (ovalbumin) and WPI (β-lactoglobulin) were separated and compared with the ovalbumin and β-lactoglobulin depleted fractions. Addition of 44.3% (w/v) sucrose decreased E′ of α-lactalbumin and the β-lactoglobulin depleted fraction, resembling the characteristics of WPI. However, sucrose showed no major effect on the E′ of β-lactoglobulin interfacial films. Sucrose increased the E′ of EWP and the ovalbumin depleted fraction but did not change that of ovalbumin. The E′ of protein mixtures suggested that the interfacial domination priority followed the order of α-lactalbumin > β-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 E′ of EWP increased with addition of sucrose while that of WPI decreased, mainly associated with α-lactalbumin, leading to different stability of protein foams.
Date: 2008-11-17
Degree: PhD
Discipline: Food Science
URI: http://www.lib.ncsu.edu/resolver/1840.16/5453


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