yeast

This are naturally occurring, single celled organisms which are found on the skins of grapes and are the primary causes of fermentation. In the fermentation process, yeast take available sugars and transform them into carbon dioxide and alcohols. When all of the sugar has been fermented, the remaining yeast die. A strong bread-like yeasty smell in a wine can be the result of poor handling, a lack of proper filtration, or incomplete fermentation.