Saliva-Induced Emulsion Flocculation: Role of Droplet Charge - Food Colloids: Self-Assembly and Material Science - page 470

The above results underline the important role of electrostatics in causing the
enhanced viscosity of the positively charged emulsion + saliva mixtures. These
combined findings seem to indicate the presence of an internal structure where
the droplets are strongly bound together. Two mechanisms, which might occur
simultaneously, are proposed to explain the droplet flocculation. Firstly, the
droplets can be connected to each other via the binding of salivary protein to
the emulsifier adsorbed on the droplet surfaces. This mechanism is known as
bridging flocculation. It has been described for other food-related systems, e.g.,
b-lg emulsions + carboxymethylcellulose at pH ¼ 3, 21 and bovine serum
albumin emulsions + i-carrageenan. 22 Secondly, a network of salivary proteins
and emulsifiers in the continuous phase could be formed, thereby entrapping
the emulsion droplets. Since the strongly negatively charged mucins constitute
up to 30% of the proteins in unstimulated saliva, a complex between these
salivary proteins and the adsorbed emulsifier layer on the droplet surface or in
the continuous phase is likely to be readily formed. This hypothesis is sup-
ported by the previously described 23 strong interaction between pig gastric
mucins and polymer solutions of cationic gelatin and chitosan at pH ¼ 5.5.
To support the hypothesis that salivary components directly interact with
positively charged polymers adsorbed to the droplet surfaces, we have studied
by CSLM the effect of saliva addition to a lysozyme protein solution. Lyso-
zyme was chosen as the model component for the study because it could easily
be covalently labelled with the fluorescent dye Oregon Green. Furthermore, the
mixing of the lysozyme protein solution with saliva did not change the pH from
the saliva's physiological value, as did occur on mixing with b-lg emulsions
made at pH ¼ 3.0.
Macroscopic pictures were taken of saliva and its mixture with the lysozyme
protein solution. Whereas saliva is a transparent liquid, the mixture was turbid
due to the formation of complexes, which were clearly visible with confocal
microscopy (Figure 4). This confirms that complex formation can indeed occur
between lysozyme and salivary components in solution. It is also feasible that
this interaction could also occur at the oil water interface between saliva and
lysozyme (as well as with other positively charged emulsifying agents).
Most food proteins form complexes with anionic hydrocolloids in the pH
region where the two macromolecules carry opposite net charge. 24 Complexes
of whey protein, b-lg and lysozyme with gum arabic and/or carrageenan, as
well as other mixed food biopolymer systems, have been extensively investi-
gated. 25-28 In these systems electrostatic attraction is considered to be the main
driving force for complexation. Our findings suggest that a similar complexat-
ion mechanism takes place involving saliva and various food components.
32.4 Conclusions
This study has aimed to improve understanding of the flocculation behaviour
of emulsions as induced by saliva. We have focused on the role of the charge on
the emulsion droplets and the contribution of electrostatics to the flocculation