Article

Association of denatured whey proteins with casein micelles in heated reconstituted skim milk and its effect on casein micelle size

Food Science Section, Fonterra Research Centre (formerly NZDRI Ltd.), Private Bag 11029, Palmerston North, New Zealand.
J Dairy Res (Impact Factor: 1.6). 03/2003; 70(1):73-83. DOI: 10.1017/S0022029902005903
Source: PubMed

ABSTRACT

When skim milk at pH 6·55 was heated (75 to 100 °C for up to 60 min), the casein micelle size, as monitored by photon correlation spectroscopy, was found to increase during the initial stages of heating and tended to plateau on prolonged heating. At any particular temperature, the casein micelle size increased with longer holding times, and, at any particular holding time, the casein micelle size increased with increasing temperature. The maximum increase in casein micelle size was about 30-lactoglobulin to the whey-protein-depleted milk caused the casein micelle size to increase markedly on heat treatment. The changes in casein micelle size induced by the heat treatment of skim milk may be a consequence of the whey proteins associating with the casein micelles. However, these associated whey proteins would need to occlude a large amount of serum to account for the particle size changes. Separate experiments showed that the viscosity changes of heated milk and the estimated volume fraction changes were consistent with the particle size changes observed. Further studies are needed to determine whether the changes in size are due to the specific association of whey proteins with the micelles or whether a low level of aggregation of the casein micelles accompanies this association behaviour. Preliminary studies indicated lower levels of denatured whey proteins associated with the casein micelles and smaller changes in casein micelle size occurred as the pH of the milk was increased from pH 6·5 to pH 6·7.

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    • "Dynamic light scattering (DLS) experiments were performed in manual mode and at a back scattering angle of 173 using a Malvern Zetasizer Nano ZS instrument (Malvern Instruments, Malvern, Worcestershire, U.K.) and disposable plastic cuvettes. The details and methodology of this technique have been described previously (Anema & Li, 2003). "
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    ABSTRACT: The bovine milk casein proteins (alpha(s)-casein (ACN), beta-casein (BCN) and kappa-casein (KCN)) form complexes with lactoferrin (LF), an anionic protein also present in bovine milk that has an important bacteriostatic function especially in infant food. LF and each casein form complexes at near neutral pH, which is in between the respective pI's (approximate to 8.3 and approximate to 4.6). The stoichiometry of the complexes is determined by the net charge of the proteins. Optimum complexation occurs at charge neutrality and is characterized by a maximum in turbidity. Mixing LF with each casein at a ratio where charge neutrality is obtained leads to a new complex coacervate phase. The kinetics of complex formation for LF/BCN and LF/KCN is rapid and appears to occur through a nucleation and coalescence process. However, the kinetics of complex formation between LF and ACN is much slower and therefore a nucleation and growth process is proposed, based on model calculations of the turbidity. The composition of the complexes is the same as the experimental mixing ratio if mixing ratio leads to a neutral complex. On standing and light centrifugation a complex-coacervate phase is formed, which is a viscous liquid.
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    • "DRAFT values, denatured whey proteins become mostly associated with the casein micelles (bound), whereas at high pH values, mostly soluble complexes (whey protein/κ- CN) are formed (Singh and Fox, 1985; Anema and Li, 2003a,b; Vasbinder and de Kruif, 2003; Anema et al., 2004; Renan et al., 2006; Anema, 2007; Lakemond and van Vliet, 2008). When milk is heated at pH values 6.2, 6.5, and 7.1, about 90, 70, and <15%, respectively, of the denatured whey proteins are associated with the casein micelles (Anema and Li, 2003a; Anema et al., 2004; Lakemond and van Vliet, 2008). "
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    ABSTRACT: Milk for yogurt manufacture is subjected to high heat treatment to denature whey proteins. Low milk pH values (≤6.5) at heating result in most denatured whey proteins becoming associated with casein micelles, whereas high milk pH values (≥7.0) at heating result in the formation of mostly soluble (nonmicellar) denatured whey protein complexes. There are conflicting reports on the relative importance of soluble and casein-bound whey protein aggregates on the properties of acid gels. Prior studies investigating the effect of pH of milk at heating used model gels in which milk was acidified by glucono-δ-lactone; in this study, we prepared yogurt gels using commercial starter cultures. Model acid gels can have very different texture and physical properties from those made by fermentation with starter cultures. In this study, we investigated the effects of different pH values of milk at heating on the rheological, light backscatter, and microstructural properties of yogurt gels. Reconstituted skim milk was adjusted to pH values 6.2, 6.7, and 7.2 and heated at 85°C for 30 min. A portion of the heated milk samples was readjusted back to pH 6.7 after heating. Milks were inoculated with 3% (wt/wt) yogurt starter culture and incubated at 40°C until pH 4.6. Gel formation was monitored using dynamic oscillatory rheology, and parameters measured included the storage modulus (G') and loss tangent (LT) values. Light-backscattering properties, such as the backscatter ratio (R) and the first derivative of light backscatter ratio (R'), were also monitored during fermentation. Fluorescence microscopy was used to observe gel microstructure. The G' values at pH 4.6 were highest in gels made from milk heated at pH 6.7 and lowest in milk heated at pH 6.2, with or without pH adjustment after heating. The G' values at pH 4.6 were lower in samples after adjustment back to pH 6.7 after heating. No maximum in the LT parameter was observed during gelation for yogurts made from milk heated at pH 6.2; a maximum in LT was observed at pH ∼4.8 for samples heated at pH 6.7 or 7.2, with or without pH adjustment after heating. Higher R-values were observed with an increase in pH of heating, with or without pH adjustment after heating. The sample heated at pH 6.2 had only one major peak in its R' profile during acidification, whereas samples heated at pH 6.7 and 7.2 had 2 large peaks. The lack of a maximum in LT parameter and the presence of a single peak in the R' profile for the samples heated at pH 6.2 were likely due to the partial solubilization of insoluble calcium phosphate when milk was acidified to this lower pH value. No clear differences were observed in the microstructures of gels between the different treatments. This study indicates that heating milk at the natural pH (∼6.7) created an optimum balance of casein-bound and soluble denatured whey proteins, which resulted in yogurt with the highest gel stiffness. Copyright © 2015 American Dairy Science Association. Published by Elsevier Inc. All rights reserved.
    No preview · Article · Jul 2015 · Journal of Dairy Science
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    • "Therefore, these global microstructural parameters could not explain the different diffusion behaviors of solutes observed between these suspensions. The protein particles in heated milk may have a larger effective diameter than in unheated milk, due to association of denatured whey protein complexes with the micellar surface (Anema & Li, 2003; Jeurnink & de Kruif, 1993). Indeed, the UF-suspension presented a mean particle size (or obstacle size) of about 10 nm larger than in the MF-suspension. "

    Full-text · Dataset · Jul 2015
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