ABSTRACT
The presence of a foam layer is very common, particularly in aerobic fermentations. Foam originates from bubbles and liquid leaving the bulk volume. At the moment that gas and liquid leave the liquid bulk volume, a process of drainage starts. This leads to a decrease in liquid content and a decrease in distance between the bubbles until mainly liquid films are present between the bubbles. Finally a honeycomb structure of air bubbles separated by very thin walls of liquid connected by Plateau borders results from this process. The gas content is very high, usually 60-90%. Foam has to be distinguished from hold-up, which indicates the bubbles in the bulk of the liquid. The gas content for hold-up is usually « 50%, much smaller than for foam. In most cases the difference between hold-up and foam is simply given as hold-up being the bubbles in the broth and foam being the bubbles on top of the broth.
