Physical Chemistry of Foods

caused by denaturation, tends to decrease overrun, simply becauseG?then
is much higher, implying that more protein is needed to cover a given
surface area. Heat denaturation of whey proteins tends to decrease overrun
by a factor of about four.
Beating conditions have considerable effect on overrun, but few
systematic studies have been done. During beating, large bubbles are
formed, and these can be broken up into smaller ones (Section 11.3).
Especially at highj-values,coalescence of bubblescan occur. Coalescence is
due to rupture of the film between bubbles, and during beating these films
are frequently stretched, i.e., increased in area by tensile forces acting on the
film. This also means that the film becomes thinner.
The cause of stretching may simply be that a rod of the beater draws
an aqueous film through a large bubble. Furthermore, films can be stretched
because bubbles are pressed toward each other by pressure fluctuations.
These can, again, be caused by the beater. According to the Bernoulli
equation (5.4), we havepþ(1/2)rv^2 ¼constant in a liquid; herepis local
pressure andvlocal velocity. At high velocity, the local pressure will thus be
small. Assuming that a beater rod moves through the liquid at 5 m?s
^1 ,a
pressure change of
ð 1 = 2 Þ 6103652 Þ¼
12 ;500 Pa will result. A moment
later the rod has passed the bubble(s) and the pressure increases again. The
bubbles thus periodically decrease and increase in volume (according to
Boyle’s law:p 6 V¼constant) and are frequently pressed together. Hence
frequent and possibly considerable stretching of films will occur.
As mentioned in Section 10.7, a fairly high value of theGibbs elasticity
EGis needed to keep a film stable by means of the Gibbs mechanism,
depicted in Figure 10.29c. Figure 10.35 shows examples ofEGas a function
of surfactant concentration c in the liquid. There is an optimum
concentrationcoptforEG. If a film is stretched, its thickness will decrease,
leading to a higher value ofEG. More important, however, is that the
magnitude ofcwill decrease on stretching, because additional surfactant
will become adsorbed onto the enlarged film surfaces (replenishing of
surfactant from the bulk of the liquid is negligible in a thin film). As long as
the local concentration is abovecopt,EGwill increase on stretching, but ifc
is or becomes<copt,EGwill rapidly decrease, and the film will probably
break. Hence there is a critical concentration for breaking the film, probably
somewhat belowcopt. This has been confirmed by stretching of isolated
films.
Figure 11.4b shows some examples of the effect ofbeater speedon
foam volume obtained, and the results are in qualitative agreement with the
reasoning just given. When increasing the beater velocity, the volume of
foam formed at first increases, presumably because more bubbles are
formed. Still higher velocities cause a decrease, presumably because pressure