Effects of urea addition on the structural and material properties of caseinate solutions and emulsions : a thesis presented in partial fulfilment of the requirements for the degree of Master of Food Science and Technology at Massey University, Manawatū, New Zealand
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Date
2014
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Massey University
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Abstract
The effects of urea addition on the self-assembly of caseinate sub-micelles and
subsequent impact on the structural and material properties of caseinate solutions and
caseinate stabilised oil-in-water emulsions (30 vol% oil) were studied.
Sub-micellar particle size and distribution were measured using dynamic light
scattering. Sodium caseinate in solution showed a bimodal particle size distribution
centred at 20 and 120 nm (radius). The increasing addition of urea (1.1 to 6.6 M) was
seen to cause a reduction in the size of caseinate sub-micelle, which was attributed to a
dissociation of the sub-micellar fraction arising from a reduction in hydrophobic
interactions.
For sodium caseinate solutions above estimated sub-micellar close-packing phase
volumes (＞10 wt% protein concentration), increasing addition of urea was seen to
significantly lower solution viscosity with more pronounced changes observed with
increasing protein concentration. All the urea-treated solutions were Newtonian.
For sodium caseinate stabilised emulsions, where protein concentrations were above
the critical threshold for onset of depletion flocculation, the addition of urea was seen to
result in a reduction in depletion free energy of the emulsions. The effects of reduced
depletion interaction on emulsion stability, structural and rheological properties were
found to be dependent not only on the concentration of added urea, but also on the
protein concentration in the emulsions. For 2 and 4 wt% caseinate concentrations,
depleted droplets reduced with increasing urea concentration and the emulsion stability
increased as well. For 6 wt% protein concentration, the stability decreased with the
addition of 1.1 and 3.3 M urea, and it was re-established when the urea concentration
was up to 6.6 M. This was attributed to the synergic effects on the depletion potential
and continuous phase viscosity which were induced by urea, and it was supported by the
results of creaming profiles, rheological properties and microstructure. Likewise, urea
had similar effects on the emulsions stabilised by potassium caseinate.
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Keywords
Urea, Casein, Caseinate solutions, Emulsions