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J. Dairy Sci. 88:3784-3797
© American Dairy Science Association, 2005.

Relationship Between Physical Properties of Casein Micelles and Rheology of Skim Milk Concentrate

A. O. Karlsson1, R. Ipsen1, K. Schrader2 and Y. Ardö1

1 Dairy Technology, Department of Food Science, Centre for Advanced Food Studies, The Royal Veterinary and Agricultural University, Rolighedsvej 305, 1958 Fredriksberg C, Denmark
2 Federal Research Centre for Nutrition and Food, Location Kiel, Hermann-Weigmann-Str. 1, 24103 Kiel, Germany

Corresponding author: Anders Ola Karlsson; e-mail: aka{at}kvl.dk.

The properties of casein micelles in milk concentrates are of interest for the use of ultrafiltered (UF) skim milk concentrates in dairy products, and for the general understanding of colloidal stability and behavior of the casein micelle. The rheological behavior of UF skim milk concentrate with a casein concentration of 19.5% (wt/wt) was investigated at different pH and NaCl concentrations by analyzing flow viscometry and small amplitude oscillatory shear measurements. Viscometric flow curves were fitted to the Carreau-Yasuda model with the aim of determining values for the viscosity at infinite high shear rates and thereby estimate the voluminosity of the casein micelles ({nu}casein) in the UF concentrate. The voluminosity of the casein micelles increased with addition of NaCl and decreased when pH was decreased from 6.5 to 5.5. At pH 5.2, {nu}casein increased because of acid-induced aggregation of the casein micelles. The changes in {nu}casein could be interpreted from transmission electron microscopy of freeze-fractured samples of the UF concentrate and partly from dynamic light scattering measurements. Altered interactions between casein micelles due to different pH and NaCl concentrations are proposed to occur due to collapse of the {kappa}-casein layer, changed ionic strength, and altered distance between casein micelles.

Key Words: Carreau-Yasuda model • concentrated skim milk • rheology • ultrafiltration

Abbreviation key: {nu}casein = voluminosity of casein micelle.






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