Saliva-Induced Emulsion Flocculation: Role of Droplet Charge - Food Colloids: Self-Assembly and Material Science - page 464

Therefore investigations of the dynamic processes occurring in the mouth
which affect food structure are needed.
Recently, the physical/chemical effect of saliva on protein-stabilized food
emulsions was reported. 10,11 Saliva was shown to induce flocculation in emul-
sions stabilized by several emulsifiers, including sodium caseinate and whey
protein isolate. Depletion flocculation has been indicated to be responsible for
droplet flocculation in the emulsion + saliva mixtures. The salivary mucins,
among other proteins and protein complexes present in saliva, are thought to
be responsible for this behaviour. Mucins are a family of polydispersed mol-
ecules which are the main constituents of the slime layers that cover the mucous
epithelia throughout the body. They are composed of disulfide-linked mon-
omers that contain heavily glycosylated domains carrying a net negative charge
due to the presence of sialic acid residues and sulfate groups.
In this chapter, we provide an overview of the role of emulsion droplet
charge on saliva-induced flocculation, meant as a first clue to explain the oral
behaviour of emulsions. The results are described in more detail elsewhere. 12
The second part of this chapter aims to gain insight into the flocculation
mechanism of positively charged emulsions. Lysozyme was chosen as a model
for studying the interaction between saliva and positively charged emulsifiers
both in solution and adsorbed on the emulsions droplets. We suggest that
complex formation between lysozyme and saliva plays a major role in the
flocculation of positively charged emulsions upon mixing with saliva. 13
32.2 Materials and Methods
32.2.1 Materials
Freeze-dried b-lactoglobulin (b-lg) purified according to the previously
described method 14 was provided by Wageningen Centre for Food Sciences
(WCFS, Wageningen, the Netherlands). Lysozyme from chicken egg white
(L6876) was obtained from Sigma-Aldrich Chemie (Zwijndrecht, the Nether-
lands) and used without further purification. Polyoxyethylene sorbitan mono-
laurate (Tween 20) was purchased from Quest International (Zwijndrecht, the
Netherlands), sodium dodecylsulfate (SDS) from Merck (Schuchardt, Ger-
many), and cetyltrimethylammonium bromide (CTAB) from BDH Chemicals
(Poole, England). Sunflower oil (Reddy, Vandemoortele, the Netherlands) was
purchased from a local retailer. The BCA t Protein Assay Kit was from Pierce
Biotechnology (Rockford, IL, USA) and sodium azide was from Merck
(Schuchardt, Germany).
32.2.2 Saliva Collection and Handling
Unstimulated whole human saliva was collected for 30 min each from 11
healthy non-medicated volunteers from 8.30 to 10.30 a.m. The collection
procedure is extensively described elsewhere. 12 Briefly, after optionally having