Saliva-Induced Emulsion Flocculation: Role of Droplet Charge - Food Colloids: Self-Assembly and Material Science - page 467

Table 1 The z -potential values of the different emulsions and the corresponding
flocculation behaviour of the emulsions (10 wt% oil) mixed with
salivary proteins (0.6 mg mL 1 )

Emulsifying agent pH z (mV) Type of flocculation

SDS 6.7

90

None

Reversible b b-lg (neutral) 6.7

44

Reversible b Tween 20 6.7

2

4.3 a Irreversible b b-lg (acidic) +21

Irreversible b Lysozyme 6.7 +30

Irreversible b CTAB 6.7 +74

a Measured z -potential referring to pH of mixture with saliva.
b With respect to dilution.

upon mixing with saliva. The light-scattering results indicate that this flocculat-
ion behaviour was reversible upon dilution with water, in line with depletion as
driving force for flocculation. The saliva protein composition suggests a dom-
inant role of the large negatively charged salivary mucins in the depletion
mechanism, as seen for other non-adsorbing biopolymers, e.g., methylcellulose,
dextran or exocellular polysaccharide. 10,16-18 In case of the SDS emulsion +
saliva mixture, however, the strong electrostatic repulsion between the emul-
sion droplets presumably overcomes the combined van der Waals attraction
and depletion forces. This explains the non-flocculation behaviour, as similarly
found by Blijdenstein et al. 18 in emulsions containing dextran at concentration
lower than 1 wt% and at low ionic strength (NaCl concentration r 10 mM).
For positively charged emulsion droplets, stabilized by b-lg (pH ¼ 3.0),
lysozyme and CTAB, strong flocculation, irreversible upon dilution, was
observed (Figure 1). This flocculating behaviour resulted in an increase in the
d 32 value of the mixtures with increasing z -potential. The microscopy pictures
reveal the presence of heterogeneous flocs, containing both spherical and long
thread-like structures, easily visible for emulsions with z ¼ 30 mV. The large
size of these flocs (> 200 mm) was not detected by laser diffraction. In fact, both
d 32 and the particle-size distribution (not shown) did not indicate the presence
of such large flocs. It is possible that the flocs seen by microscopy are composed
of smaller ones, which fell apart during the particle-size measurements. In
addition, both the model used for the calculation of the sizes (the particles are
considered as spheres) and the broadness of the distribution would affect the
determined size. We hypothesize that complex formation between the emulsifier
on the droplet surface and the salivary proteins - for example, the negatively
charged mucins - could be responsible for the observed flocculation behav-
iour. 12
In Figure 2 the width of the particle-size distribution is depicted as a function
of the z -potential. The width, expressed as the ratio (d 0.9 d 0.1 )/d 0.5 , where d 0.1 ,
d 0.5 and d 0.9 are the sizes for 10, 50 and 90% of the sample, is compared for the
emulsions before and after saliva addition. In line with expectations, the