be positive. For most combinations of foaming agents and particle material,
this condition is not fulfilled, but for proteins at the A–W surface (not a very
smallgvalue) and small-molecule surfactants in the particle, spreading may
occur. Mechanism (d) may occur more readily when the film is stretched, as
during beating, because thengawis higher. Incidentally, extensive stretching
of a film will always cause its rupture.
Another condition for all mechanisms in Figure 13.19 to occur is that
the thin water film between the particle and the A–W surface has to break
for the particle to be ‘‘wetted’’ by the air. Colloidal repulsion will counteract
this. However, if the particles are oil droplets containing fat crystals, such
wetting may readily occur (see Section 13.5 for the mechanism), making the
emulsion a very efficientfoam-breaking agent.
It is well known that lipids (e.g., from lipstick) can readily break beer
foam. It is also known that skimmed milk, which contains very little fat,
does foam copiously on beating, but that adding a little whole milk strongly
impairs foaming, provided that the fat globules in the milk contain fat
FIGURE13.19 Possible mechanisms involved in the rupture of aqueous foam films
of thicknessdinduced by hydrophobic particles; 1, 2 and 3 indicate subsequent
stages. (a) Solid particle; (b) and (c) oil droplet; (d) oil droplet or composite particle.
Thick arrows indicate spreading of oil or surfactant. A is air, W is water;yis contact
angle as measured in the water phase;Zis viscosity of the water phase (subscript W)
or the oil phase (subscript O);PSis spreading pressure.