fluctuations then become large enough to cause sufficient stretching of films
between bubbles to induce coalescence.
Generally, at a smaller surfactant concentration, the maximum
overrun is smaller and occurs at a lower beater velocity. This would be
because adsorption of surfactant at the newly created A–W interface will
cause depletion of surfactant from the bulk, and the concentration of
surfactant in the films will become critical at an earlier stage for a smaller
initial concentration.
Beating Time. Figure 11.4c illustrates that overrun at first
increases during beating—as is only to be expected—but then decreases.
Since such ‘‘overbeating’’ is typical for globular proteins as a surfactant, the
most likely explanation is surface denaturation, leading to protein
aggregation. During beating frequent expansion and compression of film
surfaces occurs, and this may readily cause strong unfolding and subsequent
aggregation of globular proteins.
11.2.4 Some Properties
The discussion given above concernspolyhedralfoams, which are formed at
volume fractions above that for a close packing of spheres. This critical
value ofjequals about 0.7. The average value ofjis rarely above 0.95 for a
food foam. A polyhedral foam has a certain rigidity and may be called a gel.
The most important rheological parameter is the yield stress (see Section
5.1.3), which should be large enough for the foam to keep its shape under
gravity. These and other gel properties are discussed in Chapter 17 (see
especially Section 17.4).
If a low-viscosity liquid is beaten to form a foam, it will inevitably
become a polyhedral foam. Liquid will always drain from it. For a volume
fraction of 0.9, the overrun is 900%, forj¼0.95, it is even 1900%; such a
foam would make a very fluffy food. Most aerated foods are different. They
aredilute foamsin the sense that the bubbles are separate from each other
and they remain spherical. To prevent the bubbles from creaming, the
continuous phase should have a yield stress. This can be achieved in several
manners:
By agelling polymer. Gelatin makes a viscous liquid at temperatures
above 30 8 C, and the liquid can then be beaten to form bubbles,
which cream very sluggishly. Cooling the system to below 20 8 C then
causes gelling, and the foam is mechanically stabilized.
The continuous phase can besolidified, which occurs in various ice
cream–like products. Air is beaten in the liquid while it is frozen,