Article

Whey protein soluble aggregates from heating with NaCl: Physicochemical, interfacial, and foaming properties

Department of Food Science, Nestlé Research Center, Vers-chez-les-Blanc, CH-1000 Lausanne 26, Switzerland.
Langmuir (Impact Factor: 4.46). 04/2007; 23(8):4155-66. DOI: 10.1021/la0632575
Source: PubMed

ABSTRACT Whey protein isolate was heat-treated at 85 degrees C for 15 min at pH ranging from 6.0 to 7.0 in the presence of NaCl in order to generate the highest possible amount of soluble aggregates before insolubility occurred. These whey protein soluble aggregates were characterized for composition, hydrodynamic diameter, apparent molecular weight, zeta-potential, surface hydrophobicity index, activated thiol group content, and microstructure. The adsorption kinetics and rheological properties (E', etad) of these soluble aggregates were probed at the air/water interface. In addition, the gas permeability of a single bubble stabilized by the whey protein soluble aggregates was determined. Finally, the foaming and foam-stabilizing properties of these aggregates were measured. The amount of whey protein soluble aggregates after heat treatment was increased from 75% to 95% from pH 6.0 to pH 7.0 by addition of 5 mM to 120 mM NaCl, respectively. These soluble aggregates involved major whey protein fractions and exhibited a maximum of activated thiol group content at pH > 6.6. The hydrodynamic radius and the surface hydrophobicity index of the soluble aggregates increased from pH 6.0 to 7.0, but the molecular weight and zeta-potential decreased. This loss of apparent density was clearly confirmed by microscopy as the soluble aggregates shifted from a spherical/compact structure at pH 6.0 to a more fibrillar/elongated structure at pH 7.0. Surface adsorption was faster for soluble aggregates formed at pH 6.8 and 7.0 in the presence of 100 and 120 mM NaCl, respectively. However, interfacial elasticity and viscosity measured at 0.01 Hz were similar from pH 6.0 to 7.0. Single bubble gas permeability significantly decreased for aggregates generated at pH > 6.6. Furthermore, these aggregates exhibited the highest foamability and foam liquid stability. Air bubble size within the foam was the lowest at pH 7.0. The coarsening exponent, alpha, fell within predicted values of 1/3 and 1/2, except for very dry foams where it was 1/5.

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    • "Schmitt et al. [11] reported that in the absence of added salt stable suspensions of spherical particles with a radius of about 60 nm formed in solutions of 10 g/L WPI during heating at pH 6.0, whereas at pH 6.6 and pH 7.0 small curved strands were formed. The ␨potential of the two types of aggregates was similar, but the surface hydrophobicity was lower for the spherical particles. "
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    • "The third is an ion-induced conformational change, which leads to altered hydrophobic interactions and aggregation at elevated temperatures (Kinsella and Whitehead, 1989; Wang and Damodaran, 1991). The latter indicates that calcium acts principally on BLG aggregation (Mulvihill and Donovan, 1987; Petit et al., 2011) by both increasing the size of aggregates (Allen and Smith, 2001; Schmitt et al., 2007) and lowering the BLG denaturation temperature, which in turn, favours aggregate formation (De Wit, 1990; Simmons et al., 2007). Its role in BLG unfolding is limited to the reinforcement of the native BLG tertiary structure (Petit et al., 2011). "
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    • "The mixtures of WPI and –SH reagents were prepared by mixing equal amounts (by volume) of WPI stock solution and freshly prepared solutions of DHLA, DTNB, NEM or GSH to produce a final protein concentration of 1% (w/w) and a molar ratio of these reagents of 1:1 relative to the b-Lg monomer. Assuming that $50% of the total WPI protein is b-Lg and 10–15% is a-La (Schmitt et al., 2007), 1% WPI is equivalent to $0.5% (w/w) b-Lg. Therefore, the b-Lg concentration in 1% WPI is similar to that used in our previous study with pure b-Lg (5 mg/ml) (Wijayanti et al., 2013). "
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