Particle Tracking as a Probe of Microrheology in Food Colloids - Food Colloids: Self-Assembly and Material Science - page 308

hence the physical shelf life, of the emulsion with respect to gravity-induced
creaming.

20.2 Experimental
The probe particles were carboxylate-modified polystyrene (COOH-PS) micro-
spheres of three different nominal mean diameters (0.21, 0.5 and 0.89 mm).
Spray-dried sodium caseinate (4 82 wt.% dry protein, o 6 wt.%moisture, o 6
wt.% fat and ash, 0.05 wt.% calcium) was obtained from DeMelkindustrie
(Veghel, Netherlands). Food-grade xanthan gum powder (Kelzan 'S' F850414)
was obtained from Kelco (San Diego, CA). Some of this xanthan had been
labelled covalently with fluorescein-5-isothiocyanate (FITC). 16 1-Bromohexa-
decane (density 0.99 g ml 1 ) was used as the oil phase in order to minimize oil
droplet buoyancy effects in the cell of the confocal microscope. 14 The emulsion
aqueous phase was a 20 mM imidazole buffer solution, adjusted to pH 6.8 with
HCl, with sodium azide (0.01% w/v) included as antimicrobiological agent.
Further details of the materials used can be found elsewhere. 17
Oil-in-water emulsions were made with a single-pass laboratory-scale jet
homogenizer 18 operating at a pressure of 300 bar. Aqueous solutions of sodium
caseinate (2 wt.%) and xanthan were prepared by adding protein powder and
xanthan gum powder to the buffer solution (imidazole) and then gently stirring
overnight at room temperature to ensure complete dispersion. In the final
emulsion, the protein concentration was 1.4 wt.% and the amount of dispersed
oil phase was always adjusted to 30 vol.%. This protein/oil ratio was roughly the
minimum required to emulsify all the bromohexadecane without bridging
flocculation, while avoiding any significant amount of unadsorbed protein which
might cause depletion flocculation by caseinate nanoparticles. 19,20 Droplet-size
distributions of the sodium caseinate-stabilized emulsions with xanthan added
prior to emulsification, as determined using a Malvern Mastersizer 2000, were
monomodal with 3 mm. The

presence of xanthan in the range 0-0.15 wt.% did not affect the droplet-size
distribution: the average diameters remained constant at d 32 ¼ 0.35 0.03 mm
and d 43 ¼ 0.95 0.05 mm.
Steady-state shear viscosities of emulsions and xanthan solutions (0.02-0.07
wt.%) were measured at 201C using a Bohlin CVO-R rheometer with a double-
gap geometry (DG 24/27). Samples were covered with a thin layer of silicone oil
and a solvent trap was employed. Freshly prepared emulsion samples were
initially subjected to a high shear rate (150 s 1 ) to break up any existing
aggregates or phase-separated regions; half an hour after this treatment, the
zero shear rate viscosity was determined for each sample.
Confocal microscopy was carried out with a Leica TCS SP2 confocal laser
scanning microscope (CLSM) operated in fluorescence mode with a 40 times
oil-immersion objective. The oil phase was stained with Nile Red dye, and
fluorescence from the sample was excited with the 488 nm Ar laser line. A 5 ml
sample of the freshly made emulsion was transferred to a small beaker. A 20 ml

95% of droplets in the size range from 0.1 to

B B