YEAST

I. noun Etymology: Middle English yest, from Old English gist; akin to Old High German jesen, gesen to ferment, Greek zein to boil Date: before 12th century 1. a yellowish surface froth or sediment that occurs especially in saccharine liquids (as fruit juices) in which it promotes alcoholic fermentation, consists largely of cells of a fungus (as the saccharomyces, Saccharomyces cerevisiae ), and is used especially in the making of alcoholic liquors and as a leaven in baking, a commercial product containing ~ fungi in a moist or dry medium, c. a unicellular fungus that is present and functionally active in ~, usually has little or no mycelium, and reproduces by budding, any of various similar fungi, the foam or spume of waves, something that causes ferment or activity , II. intransitive verb Date: 1819 ferment , froth