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played in relation to the stability of the bubbles. Ip and co-workers
4
investi-
gated the dependence of the stability of metallic aluminium foams on silica
particle size, wettability and concentration. They concluded
4
that only silica
particles with correct wettability could stabilize foams and that stability
increased with increasing particle concentration and with decreasing particle
size. On the other hand, Hudales and Stein
5
stated that only 'large' (1-10 mm)
glass particles in the presence of a surfactant (CTAB) could inhibit thin film
rupture and delay film drainage. In addition, theoretical work from Kaptay
6
postulated that solid particles can stabilize bubbles only when the bubbles are
not larger than 3-30 mm. In addition, monodisperse hydrophilic silica particles
were investigated by Sethumadhavan et al.;
7,8
they also found that foam
stability was dependent inter alia on the size of the particles.
Hydrophobic particles that can adsorb at the air-water (A-W) interface are
naturally present in foods, i.e., fat crystals. However, the size, shape and
adsorption properties of fat crystals are difficult to control, making it not
straightforward to do model experiments with such systems. In contrast, silica
particles form good model systems, as their surface-active properties can be
easily changed by chemical modification of the surfaces (as demonstrated by
Dickinson et al.
2
). On the other hand, unmodified hydrophilic silica particles
are the only silica particles that could be allowed for use in food systems; but
they do not have any tendency to adsorb at an A-W interface. Therefore, a
physical method of surface modification must be developed to render such
particles hydrophobic enough to adsorb. In addition, any such particles in
foods must be of a small size (e.g.,
o
0.1 mm) to avoid complications during
digestion and effects on perceived texture and mouthfeel. Such inorganic
particles of small size are in fact naturally present in many food stuffs, although
in rather small quantities.
In this study, the effect of different types of hydrophilic silica particles on the
stability of gas bubbles is described. The particles were composed of either
fumed or colloidal silica, meaning that they were in a powder or liquid form,
respectively. The fumed silica particles had a nominal size of 20 nm, although
the actual size depends on the state of dispersion of the powder, whereas the
colloidal silica samples have a well-defined particle size. The big advantage of
colloidal silica as compared to fumed silica is the smaller particle size of the
former, as well as the fact that it is highly monodisperse. In this study, however,
we focus on a sample available with the smallest mean particle diameter, i.e.,
5.5 nm. Adsorption of low-molecular-weight surfactant or protein was used to
modify the surface-active properties of the particles. Such systems may act as
more realistic models of hydrophobic particles in foods, where both protein and
surfactant will compete for adsorption at the A-W interface.
24.2 Materials and Methods
Pure fumed hydrophilic silica particles of nominal diameter 20 nm were
specially made by Wacker-Chemie (Munich, Germany). The colloidal silica
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