Electrostatic Interactions between Lactoferrin and b-Lactoglobulin in Oil-in-Water Emulsions - Food Colloids: Self-Assembly and Material Science - page 169

concentration of the emulsion droplets (milligrams of protein per square metre
of oil surface) was determined using the method described by Ye and Singh. 8
Sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE), as
described by Srinivasan et al., 9 was used to determine the composition of the
protein adsorbed at the surface of the emulsion droplets. The SDS gels were
prepared on a Miniprotean II system (Bio-Rad Laboratories, Richmond, CA,
USA). After destaining, the gels were scanned on a laser densitometer (LKB
Ultroscan XL, LKB Produkter AB, Bromma, Sweden). The percentage com-
position of each sample was determined by scanning the areas for lactoferrin
and b-lactoglobulin and expressing the individual whey protein peaks as
fractions of the total.
The electrophoretic mobilities, and hence the calculated z -potentials of the
emulsion droplets, were determined using a Malvern Zetasizer 4 instrument, the
associated Malvern Multi-8 64 channel correlator and the AZ4 standard
electrophoresis cell, incorporating a 4-mm diameter quartz capillary. The
temperature of the electrophoresis cell was maintained at 25 0.51C using a
water jacket that was temperature controlled by the Peltier system. An applied
voltage of 80 V (corresponding to a sensed voltage of approximately 60 V
across the capillary, i.e., approximately 12 V cm 1 ) and a modulation frequency
of 250 Hz were used in all experiments.
11.3 Results and Discussion
11.3.1 Emulsions formed with Mixture of Lactoferrin +
b -Lactoglobulin
The values of the average droplet diameters d 32 of the emulsions formed with
various concentrations of pure b-lactoglobulin, pure lactoferrin or a mixture of
lactoferrin + b-lactoglobulin are shown in Figure 1. For all the emulsions, the
droplet sizes were almost independent of the protein concentration in the range
1-3 wt.%, with the b-lactoglobulin emulsions showing slightly smaller droplets.
At protein concentrations below 1 wt.%, the average droplet size of the
emulsions formed with a mixture of lactoferrin + b-lactoglobulin was much
larger than that of the emulsions formed with either single protein. The
emulsions formed with a mixture of lactoferrin + b-lactoglobulin at o 1wt.%
total protein concentration at pH 7.0 gave bimodal size distributions, whereas all
other emulsions had monomodal distributions.
Figure 2 shows values of the z -potential of the emulsion droplets stabilized
with b-lactoglobulin, lactoferrin or lactoferrin + b-lactoglobulin as a function
of protein concentration. The z -potential of the emulsion droplets made with
b-lactoglobulin alone was negative, and it became more negative with increase
in protein concentration from 0.3 to 1 wt.%; but it did not change at higher
protein concentrations. This indicates that the amount of adsorbed lactoferrin
increased with increasing protein concentration. The emulsion droplets stabi-
lized with lactoferrin had a positive z -potential value, which gradually became