concentration was equal and all of the proteins are well soluble. The most
likely explanation is the rate of (partial) unfolding of the protein after
becoming adsorbed; these time scales roughly are 100 s for lysozyme, 0.25 s
forb-lactoglobulin and 0.1 s forb-casein. Comparing this with the results in
Figure 11.5, it follows that the characteristic time scale during bubble
formation would be roughly 0.5 s.
To make a foam with slowly unfolding proteins, it will be
advantageous to use a method that involves longer time scales. This is the
case for injection (Method 2 in Section 11.1), and it is indeed observed that
most globular proteins, even those that are known to have a high
conformational stability, give copious foams by injection (although the
bubble size tends to be large). Foam can also be made by beating if the
protein concentration is much higher, and ovalbumin (the main protein of
egg white) provides a good example. Altogether, a substantial excess of
protein over the amount needed for A–W surface coverage has to be present
in nearly all cases.
Another variable is the effectivemolar massof the surfactant. Smaller
molecules tend to give a higher molar surface load at the same mass
concentration in solution, and thereby higher P-values. Protein hydro-
lysates often have superior foaming properties, where faster unfolding of
smaller peptides may also play a part. (On the other hand, the foam tends to
be less stable against Ostwald ripening.) Aggregation of proteins, e.g.,FIGURE11.5 Foaming properties of some proteins (solution of 0.25 mg per ml) in
relation to the dynamic surface tension.gis surface tension; d lnA/dtis the surface
expansion rate;dis the approximate average bubble diameter. (After results by H.
van Kalsbeek, A. Prins. In: E. Dickinson, J. M. Rodrı ́guez Patino, eds. Food
Emulsions and Foams. Roy. Soc. Chem., Cambridge, 1999, pp. 91–103.)