Foams_csm

= FOAMS =

Foam Structure
==
 * Foam is a two-phase system---the gas (air) phase is dispersed in a small amount of liquid (water) continuous phase.
 * Bubbly foam (in ice cream)---formed when the amount of gas incorporated is low enough for bubbles to retain roughly spherical shape.
 * Polyhedral foam (beer foam)---has a large gas-to-liquid ratio, bubbles are pressed against one another in a honeycomb-type structure.

Foam Formation

 * 3 process involved in foam formation : air has to be injected into the liquid (e.g. using a mixer), large air bubbles have to be broken up into smaller bubbles, the smaller bubbles have to be prevented from fusing during the formation of a foam.

__The Role of Surface Active Agent__

 * A surface active foaming agent is essential for the formation of stable foam.
 * Foaming agent---lowers the surface tension of the liquid phase, allows expansion of its surface area.
 * Surfactant forms a closely packed film around the dispersed gas bubbles.
 * 3 processes involved in the stabilization of protein foams: adsorption of the protein at the gasliquid interface, surface denaturation, coagulation of protein.

Foam Stability
Three factors affecting foam stability :
 * 1) Drainage---draining of liquid from foam.
 * 2) Dispropornation---change in foam bubble size distribution caused by gas diffusion from small to large bubbles.
 * 3) Coalescence---the fusion of foam bubbles.

1. Foam Drainage

 * Water drains from foam under gravity.
 * Foam drains along lamellae to the curved junction of thin lamellae (plateau borders), where the pressure is lower.
 * As water leaves, faces of film are brought closer together.

2. Dispropornation

 * Diffusion of gas from small bubbles into big bubbles.
 * The most important type of instability in foams.
 * Without stabilizing film, disproportionation occurs remarkably quickly.

- Egg Foams (egg white)

 * Proteins---lower the surface tension of egg white, some are denatured at the surface.
 * Coagulation of proteins at the gas-liquid interface---gives some rigidity & stability to the foam.
 * Essential in prepareation of : angel cakes, sponge cakes, meringues, souffles, etc.




 * Globulins---good foamers, producing small gas bubbles & a large volume.
 * Ovomucin---not a good foamer by itself but stabilised the foam because it is rapidly insolubilised (denatured) at the bubble surface.
 * Fat & egg yolk---even in small amounts have a detrimental effect on foam formation.




 * Fats/lipids---retards foaming because oil molecules migrate to the air-water interface before the protein molecules, inhibiting the unfolding of the protein & formation of foam.
 * Eggyolk---antifoaming behaviour is partly due to the presence of lecithin, which is believed to bind to egg white proteins, preventing them from associating at the air-water interface, competitively displacing them from the interface.
 * Adjusting egg white pH to 6.5 (adding an acid ingredient) increases foam stability to heat, because the acid makes foam less prone to over-coagulation.
 * Increase of protein concentration increases both the foamability & foam stability.
 * Problems cause by foams---vessels overflowing, processing and packaging interference, damaging materials and housekeeping issues.
 * Silicon dioxide---common chemical used in defoamers and antifoams, it can destroy and suppress processing foams.