fungus — any of a diverse group of eukaryotic single-celled or multinucleate organisms that live by decomposing and absorbing the organic material in which they grow, comprising the mushrooms, molds, mildews, smuts, rusts, and yeasts, and classified in the kingdom Fungi or, in some classification systems, in the division Fungi (Thallophyta) of the kingdom Plantae.
catalyst — You can describe a person or thing that causes a change or event to happen as a catalyst.
foam — a collection of minute bubbles formed on the surface of a liquid by agitation, fermentation, etc.: foam on a glass of beer.
froth — an aggregation of bubbles, as on an agitated liquid or at the mouth of a hard-driven horse; foam; spume.
leavening — a substance, as yeast or baking powder, that causes fermentation and expansion of dough or batter.
