Yeast

yeast, microscopic, single-celled fungi, having round to oval cells which reproduce by forming buds, are vital to the alcoholic fermentation process, which, starved of oxygen, transforms grape juice to wine. sugars are used as an energy source by yeast, with ethanol and carbon dioxide as major by-products of the reactions.

The word yeast (which may be singular or plural unless it encompasses yeasts from more than one species) is an old one whose meaning has changed significantly with the flowering of microbiological science. It originally derived from an ancient word meaning ‘to boil’, ‘to seethe’, or ‘to be troubled’. In 16th-century English, it referred to the froth on the top of a brewing tank and to the semi-solid material that could be collected both from that froth and from the bottom of the tank. From the mid 17th century, the meaning of the word yeast changed to that of a single-celled plant, a thallophyte and one of the lowest members of the plant kingdom along with algae, lichens, and fungi.