An aggregation of gas or vapour bubbles that remains as a stable suspension. The liquid is the continuous phase and exists as bubble films. Foaming is often a problem with fermentation processes where it forms on the surface of the broth. The existence of foam can be controlled by the addition of chemicals such as glycol, which interrupts the stability of the foam. A foam breaker is a mechanical device used to break up the formation of foam. Foam breakers are sometimes used in fermentation vessels in which foaming may occur and otherwise overflow. They usually consist of high-speed spinning discs and are positioned in a region above a potentially foaming liquid.