Saliva-Induced Emulsion Flocculation: Role of Droplet Charge - Food Colloids: Self-Assembly and Material Science - page 469

lysozyme and CTAB, the width of the distribution increased after mixing with
saliva. The increased polydispersity of the above mentioned emulsion + saliva
mixtures also affects the d 32 values. This, together with the previously men-
tioned considerations, might explain why the measured d 32 values (e.g.,45mm
for the CTAB-stabilized emulsion + saliva mixture) are far from the sizes
observed by microscopy (4 200 mm).
Flocculation affects the viscosity of emulsions, typically leading to shear-
thinning behaviour. 19,20 Viscosity measurements can therefore provide useful
information about the state of flocculation and hence about the flocculation
mechanism. Higher viscosities, even at high shear-rates, are obtained in samples
where droplets are strongly and irreversibly bound to each other. Conversely,
aggregates in emulsions flocculated by depletion fall apart at high shear-rates,
resulting in similar viscosities to non-flocculated emulsions. A convenient way
to evaluate different flocculating emulsion + saliva mixtures is to calculate the
ratio Z mix / Z emul in which the viscosity of the mixture ( Z mix ) is normalized by the
viscosity of the corresponding emulsion ( Z emul ). In this way, the viscosity
increase due to flocculation caused by saliva is independent of the original
emulsion viscosity. As expected, emulsion + saliva mixtures exhibited shear-
thinning behaviour (Figure 3). Higher positive z -potential values of the emul-
sions resulted in a stronger increase in the viscosity after saliva addition, as
indicated by the greater values of Z mix / Z emul . The results indicate that the flocs
of positively charged emulsions in the presence of saliva are not completely
disrupted by shear over the applied shear-rate range, which is in line with the
irreversibility of the flocculation towards dilution (Figures 1 and 2).

14

12

12

10

8

6.3

6.2

6

3.6

4

3.1

2.9

2.8

2.2

1.9

1.7

1.6

2

1.2

0

-44 21 30 74

ζ (mV)

Figure 3 Viscosity of the emulsion + saliva mixtures normalized by the viscosity of the
emulsions calculated at 10 s 1 (white columns), 100 s 1 (black columns) and
900 s 1 (shaded columns) for different z -potentials: 44 mV for b -lg (pH ¼
6.7); +21 mV for b -lg (pH ¼ 4.3) (as in the mixture of saliva); +30 mV for
lysozyme; and +74 mV for CTAB