Saliva-Induced Emulsion Flocculation: Role of Droplet Charge - Food Colloids: Self-Assembly and Material Science - page 466

was conducted by laser diffraction as indicated above. The rheological behav-
iour of emulsions upon addition of saliva was characterized by measuring the
shear-rate dependent viscosity. Shear viscosity measurements were carried out
in duplicate at 201C with 10 min recovery time using a Paar Physica MCR 300
rheometer with cone-and-plate geometry CP75-1 of angle 11 (0.0175 rad) and
gap-width 0.05 mm (at the tip). The shear-rate was initially increased from 1 to
1500 s 1 and then decreased again to 1 s 1 .
32.2.5 Characterization of Complex Formation between
Lysozyme and Saliva
For the detection of complex formation between saliva and the positively
charged protein emulsifier, a 10 mg mL 1 lysozyme protein stock solution was
prepared in 10 mM NaCl. The mixture with saliva contained 5 mg mL 1 of
lysozyme stock solution and 0.6 mg mL 1 of salivary proteins. Confocal
scanning laser microscopy (CSLM) was performed with Leica TCS SP micro-
scope used in the single-photon fluorescence mode. The set-up was configured
with an inverted microscope (model Leica DM IRBE) and an Ar/Kr laser. The
objective lens used was a 63X/UV/1.20NA/water immersion/PL APO. Oregon
Green (OG) was used to covalently stain the lysozyme. With stirring for 2 h at
room temperature, 2.5 mg Oregon Green t 488 was dissolved in 249.5 mg of
dimethylformamide (DMF), and then 4 mg of the OG-DMF was added to 2 g
of the sample to be labelled. The sample solution was gently stirred overnight at
41C avoiding exposure to light. After mixing with saliva, the sample was excited
at 488 nm and the emission fluorescence was detected between 500 and 560 nm.
32.3 Results and Discussion
One of the key parameters influencing the saliva-induced emulsion flocculation,
which is expected to play a role in the in-mouth perception of the emulsions, is
the charge on the emulsion droplets. Therefore the saliva-induced flocculation
of emulsions with different droplet charges was examined. 12 A summary of the
flocculation behaviour in relation to the z -potentials on the emulsions droplets
is shown in Table 1.
Microscopy and laser diffraction analysis were carried out to characterize the
effect of charge on the morphology and size of the flocs upon mixing an
emulsion with saliva. The effect of droplet charge on the surface-weighted mean
diameter d 32 of the emulsions before and after mixing with saliva is shown in
Figure 1. While the d 32 values of the original emulsions varied minimally
(between 0.6 and 1 mm), a large variation (from 0.6 to 45 mm) was found for the
mixtures with saliva. For negatively charged and neutral emulsions stabilized
by SDS, b-lg (pH ¼ 6.7) and Tween 20, the d 32 value remained unchanged
before and after mixing with saliva. Microscopy pictures of the mixtures reveal
that for the SDS-stabilized emulsion, with z ¼ 90 mV, no flocculation had
occurred. But flocculation was observed for the b-lg and Tween 20 emulsions