As mentioned in Section 10.7,a fair

As mentioned in Section 10.7,a fairy high value of the Gibbs elasticity EG is a needed to keep a film stable by means of the Gibbs mechanism,depicted in Figure 10.29c.Figure 10.35 shows examples of EG as a function of surfactant concentration c in the liquid.There is an optimum concentration copt for EG.If a film is stretched,its thickness will decrease,leading to a high value of EG.More important,however,is that the magnitude of c will 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 above copt ,EG will increase on stretching,but if c is or becomes < copt,EG will rapidly decrease,and the film will probably break .Hence there is a critical concentration for breaking the film,probably somewhat below copt.This has been comfirmed by stretching of isolated films.
Figure 11.4b shows some examples of the effect of beater speed on 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 fist increase,presumably because more bubbles are formed.Still higher velocities cause a decrease,presumably because pressure fluctuations then become large enuogh 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.Sine 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 concerns polyhedral foams, which are formed at volume fractions above that for a close packing of spheres. This critical value of φ equals about 0.7. The average value of φ is 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 epecially 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%, for φ = 0.95, it is even 1900%; such a foam would make a ver fluffy food.Most aerated foods are different. They are dilute foams in 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 a gelling polymer. Gelatin makes a viscous liquid at temperatures above 30°C, and the liquid can then be beaten to form bubbles, which cream very sluggishly. Cooling the system to below 20°C then causes gelling, and the foam is mechanically stabilized.
The continuous phase can be solidified, which occurs in variuos ice cream – like products. Air is beaten in the liquid while it is frozen, whereby most of the water is converted into ice; this make a “solid foam.”
Particles that adhere to the A—W interface (see Section 10.6 ) can make bridges between air bubbles,thereby forming a bubble network.The prime example is whipped cream,where fat globules (partially solid emulsion droplets) cover the air bubbles and also make a continuous network in the aqueous phase; generally, φ ≈ 0.5. Something similar occurs in(high fat) ice cream:see Figure 9.1.
When beating egg white, which contains about 10% protein, part of the protein becomes denatured at the A-W interface, thereby aggregat-ing,and the particles so formed also cover the air bubbles and make a continuous network. The air can be much higher than in ice cream.
A heat treatment can convert some more or less liquid foam systems into a solid foam.A prime examples is, again, foam based on egg white –such as foam omelettes and meringues-since at high temperature protein denaturation occurs, which causes gelation.Another well-known case is provided by bread, where baking causes(a) the gas cells in the dough to grow and (b) a fairly stiff gel to be formed of the continuous phase(the dough).Moreover, the foam is converted into a sponge,because most of the thin films between the gas cells break as they become brittle at high temperature.(Question: Why is it necessary that the films break to obtain a good loaf of bread?)
All of these systems can be made so as to have a high yield stress and hence good stand-up properties. A high yield stress aloso prevents leakage of liquid from the foam.
The most important foam property may be stability against various physical changes.Something on this has been mentioned already, and more will be given in Chapter 13.