Electrostatic Interactions between Lactoferrin and b-Lactoglobulin in Oil-in-Water Emulsions - Food Colloids: Self-Assembly and Material Science - page 172

In the absence of b-lactoglobulin, the z -potential of the emulsion droplets
was around +50 mV, because the lactoferrin used to stabilize the drop-
lets has a net positive charge at pH 7.0, as shown above. The z -potential
became less positive, and eventually changed from positive to negative, as the
b-lactoglobulin concentration in the emulsion was increased. This change
suggests that b-lactoglobulin, which has a net negative charge at pH 7.0,
adsorbed to the surface of the lactoferrin-coated emulsion droplets. The z -
potential was very close to zero (i.e., no net charge on the droplet surface) at
0.42 wt.% addition of b-lactoglobulin. Further increases in the b-lactoglobulin
concentration resulted in negative values of the z -potential. The reversal of
charge upon the adsorption of charged polymers on the surface of oppositely
charged colloidal particles has been shown by many workers. 10,11 It presumably
occurs because only a fraction of the charged groups on the polymer is required
to neutralize the oppositely charged groups on the particle surface, and the
remaining charged groups remain on the particle surface or may protrude into
the aqueous phase.
The changes in z -potential were also reflected in measurements of the surface
protein concentration, which showed an increase in adsorbed protein up to
0.42 wt.% added b-lactoglobulin, but no further change above this concentra-
tion (Figure 4). Further evidence of b-lactoglobulin adsorption onto the
lactoferrin-stabilized droplets was obtained by SDS-PAGE analysis of the
emulsion droplets; it was shown that the intensity of the b-lactoglobulin band
at the droplet surface increased with increasing b-lactoglobulin concentration
in the emulsion, whereas the intensity of the lactoferrin band remained essen-
tially constant (see inset to Figure 4). The adsorption of b-lactoglobulin

3

2.5

2

1.5

Lactoferrin

1

β -Lactoglobulin

0.5

1 2 4 5 678

0

1

0 0.2 0.4 0.6 0.8 1.2 1.4 1.6

β -Lactoglobulin concentration (%)

Figure 4 Changes in protein surface coverage of emulsion droplets made with 30 wt.%
soya oil and 1 wt.% lactoferrin (pH 7.0) as a function of b -lactoglobulin added
into the emulsion. The inset shows SDS-PAGE patterns of adsorbed protein;
lane 1 represents lactoferrin alone, and lanes 2-7 show increasing additions of
b -lactoglobulin