crystals. On the other hand, whipping cream contains a very high
concentration of fat globules, and it is readily whipped into a quite stable
foam. However, the concentration of fat globules in whipping cream is so
high that these particles very rapidly cover all of the air bubble surfaces,
giving rise to Pickering stabilization (Figure 13.18).
Question
When one splashes beer into a glass, a considerable head of foam is generally formed;
beer contains about 0.5%protein. However, beer devoid of alcohol shows far less
foam and the bubbles are larger. Addition of about 1%ethanol restores the normal
foaming behavior. The stability of the foam is also affected by ethanol concentration,
and has a maximum at about 0.25%ethanol. What would be likely explanations?
Answer
A look at Figure 10.4 shows that 1%ethanol added to water (i.e., about 0.22 molar)
causes a significant decrease in surface tension of about 5 mN?m^1. Half a percent
of protein is sufficient for protein-covered foam bubbles to form, but—as discussed
in Chapter 11—a small-molecule surfactant is more efficient than protein in the
formation of foam and the breaking up of large bubbles into smaller ones. This is
because the protein has a low molar concentration and needs to unfold upon
adsorption to cause considerable lowering of surface tension, which takes a relatively
long time. Hence the ethanol should indeed promote foam formation.
The effect on stability is more difficult to explain, since we do not know what
kind of instability is involved. Assuming for the moment that it is coalescence,
ethanol may well impair the ‘‘coherence’’ of the layer of adsorbed protein, thereby
promoting its rupture: at the prevailing concentration the surface load of ethanol is
significant, causing a decreased surface shear viscosity. The optimum concentration
may then be explained by the decreased bubble size—which would lower the
coalescence rate—which effect is for higher ethanol concentration offset by the
smaller coherence.
13.5 PARTIAL COALESCENCE
In most food oil-in-water emulsions, the oil is a triglyceride mixture, and
several such oils are partially crystalline at room temperature. This means
that the oil droplets can contain crystals; such droplets are better calledfat
globules. The system is not a true emulsion, as it has three phases. Fat
globules are subject to partial coalescence or clumping, as depicted in Figure