Stopping Ostwald Ripening. This is possible by giving the A–W
surface a large and permanentESDvalue. This can be achieved by cross-
linking of protein in the adsorption layer, or by using large protein species,
be they molecules or aggregates. This is what happens when one beats egg
white, where aggregation and cross-linking occur owing to surface
denaturation of ovalbumin. Smallsolid particlesof suitable contact angle,
that adsorb to give a packed layer, will also prevent shrinkage. The
configuration is as depicted in Figure 13.18. A good example is classical
whipped cream, where the air bubbles (diameter about 50mm) are fully
covered by largely solid fat globules (about 4mm). In passing, the fat
globules will also partially coalesce during whipping, giving rise to clumps of
larger size, which also form a space filling network, lending stiffness to the
whipped cream. Some whipped toppings contain a surfactant mixture, such
as glycerol lactopalmitates, that tend to give a stiff and persistenta-gel layer
around the bubbles.
Another way to make a permanent foam is by giving the continuous
phase ayield stress. A bubble of 10mm in diameter has a Laplace pressure of
about 20 kPa (0.2 bar), and to prevent the bubble from shrinking, the yield
stress should be larger. This value is so large that the material is to be
considered a solid. There are solid foods that have permanent small bubbles,
such as some types of chocolate. If the material is solid, the system will also
be stable if the bubbles are interconnected, i.e., form a sponge rather than a
foam, as in bread.
Finally, it should be mentioned that the main instability offood foams
is generally Ostwald ripening, rather than coalescence. This is because the
bubble diameter is relatively small, mostly between 10 and 100mm. Larger
bubbles would give a too weak consistency to be acceptable in a food. For
the same reason, one generally tries to prevent strong drainage, which then
means that the films between bubbles are not very thin. It is especially large
and thin films that are sensitive to rupture, and small bubbles that are prone
to Ostwald ripening. All kinds of change in a foam, i.e., film thinning,
coalescence, and Ostwald ripening, occur fastest at the top of a foam (can
you explain these observations?).
Question
Coming back to the question at the end of Section 13.4.3, can you think of an
alternative explanation for the observation that an increased ethanol concentration
decreases beer foam stability?