Short communicationApparent voluminosity of casein micelles in the temperature range 35–70 °C
Introduction
Casein micelles are the natural delivery system for nutrients in bovine milk. The structure of these native casein micelles is not yet fully understood, but three main structures have been proposed: the sub-micelle model, the nanocluster model and the dual-binding model (de Kruif, Huppertz, Urban, & Petukhov, 2012). Field emission scanning electron microscopy pictures of the native casein micelle show a raspberry-like structure with an average diameter of about 50–300 nm (Dalgleish, Spagnuolo, & Goff, 2004). Constituents of the native casein micelle are proteins, namely αS1-, αS2-, β-, and κ-caseins, and minerals, mostly calcium and phosphate, likely in the form of nanoclusters (Walstra, 1990).
The hydration of bovine casein micelles mainly determines their volume fraction ϕ (−) and the steep increase in shear viscosity at increasing fractions of particles. Owing to the crowding effect both the solvent directly bound to the surface and that entrapped in the interior affects the shear viscosity. The amount of water associated with the mass concentration of dry solid β (g mL−1) is addressed by the term voluminosity v = ϕ/β (mL g−1).
In a previous study from our group a method was proposed to fit the apparent voluminosity vapp on one master curve as a function of temperature in the range from 5 °C to 35 °C (Nöbel, Weidendorfer, & Hinrichs, 2012). The voluminosity will be termed as apparent in the following because of the empirical nature of the viscosity equations used for the modelling.
Most of the rheological parameters of colloidal suspensions depend on the volume fraction of the dispersed phase. The voluminosity is necessary information to calculate the volume fraction from a mass fraction w and density ρ (g mL−1) usually predetermined in experiments
For instance, the volume fraction of native casein micelles was used in various modelling approaches (Nöbel et al., 2014, van Marle et al., 1999) or in context of the discussion of hard- versus soft-sphere behaviour (Olivares, Berli, & Zorrilla, 2013). Dense micellar casein suspensions, mainly depending on the volume fraction where sticking occurs, were focused intensively in the recent years (Andoyo et al., 2015, Bouchoux et al., 2009, Bouchoux et al., 2014, Dahbi et al., 2010, Qu et al., 2015) progressing in a discussion of dairy matrices in the context of colloidal suspensions (Silva et al., 2013, Vogt et al., 2015).
In the previous study the Krieger–Dougherty equation was used as an empirical expression in order to model the viscosity of the casein micelle suspension as a function of the volume fraction (Krieger and Dougherty, 1959, Nöbel et al., 2012). Furthermore, several other fundamental viscosity–volume fraction relations exist, taking the crowding effect with increasing packing into account, e.g., as presented by Mendoza and Santamaría-Holek (2009)where c = (1−ϕc)/ϕc (−) is constant at the critical packing ϕc (−) and [η] (−) the dimensionless intrinsic viscosity.
This paper focuses on the apparent voluminosity of casein micelles in an extended temperature range. The aim of collecting additional data is to present the apparent voluminosity as one continuous function covering all temperatures which are of interest for dairy application. For example, a precise estimate at temperatures above 50 °C is necessary in order to describe cheese softening as water migration in colloidal suspensions (Bähler et al., 2015, Vogt et al., 2015). The modelling was performed by a recently developed method (Nöbel et al., 2012).
Section snippets
Casein micelle suspensions
Casein micelle stock suspensions were obtained directly by microfiltration according to Post, Ebert, and Hinrichs (2009). Briefly, pasteurised skim milk was concentrated by means of microfiltration (0.1 μm cut-off, concentration factor i = 2, ϑ = 50 °C, Δp = 10 kPa). The concentrated suspension was diluted to i = 1 with milk permeate and filtration was repeated until concentration factor i = 6 was reached. Differently from Post et al. (2009), the suspensions were not dried. Crude protein
Temperature dependency of viscosity
The viscosity of micellar casein suspensions was measured as a function of shear stress over a large range of mass fraction (0.01 ≤ w ≤ 0.16) and different temperatures (35 °C ≤ ϑ ≤ 70 °C). The maximum achievable mass fraction was limited to w = 0.16. Higher concentrated suspensions were not liquid and pumpable during preparation by microfiltration (ϑ = 50 °C, Δp = 10 kPa). Fig. 1 shows the flow curves of suspensions at ϑ = 35 °C, 50 °C and 60 °C as an example. The viscosities were generally
Conclusion
A method previously proposed (Nöbel et al., 2012) to determine the voluminosity of casein micelles as a function of temperature was proven to be applicable in the temperature range up to 70 °C. The results were in line with literature data (Sood et al., 1976, Walstra et al., 2006, Whitnah and Rutz, 1959), where contradictory progressions at higher temperatures were deduced. Again, the decrease of the apparent voluminosity with increasing temperature found to continuously slow down within the
Acknowledgements
We sincerely thank Adrian Körzendörfer, Marc Leibl and Simon Hänßler for performing the microfiltration as part of the project Trophelia 2012 (FEI, Forschungskreis der Ernährungsindustrie e.V., Bonn). This research project was supported by the German Ministry of Economics and Technology (via AiF) and the FEI. Project AiF 17475 N.
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