Purpose

• Yeast works by eating sugar and releasing alcohol and carbon dioxide as byproducts. In breads, this results in the dough filling with little pockets of carbon dioxide as it ferments, rises and bakes. If the fermenting and baking are done correctly, the bread will be fluffy and soft. In alcohol, the yeast releases carbon dioxide, ethanol and other alcohols into a liquid, whether it is a fruit juice, as in wine, or water-based beer.

Temperature

• If yeast is kept at an optimal temperature, the fermentation process keeps working to produce tasty bread or refreshing beer. Yeast fermentation generally occurs at warmer temperatures, but the perfect temperature varies depending on the kind of yeast used. Watch the thermometer and fermentation carefully, because too-warm temperatures kill yeast.

Too Cold

• Whether yeast is "too cold" for fermentation depends on what you are making and what sort of yeast you are using. For example, yeast fermentation for bread dough occurs from 80 to 90 degrees Fahrenheit, but lager beers ferment below 55 degrees Fahrenheit. Some yeasts will stop fermentation below 60 degrees, but others will continue to work. If fermentation stops, chances are the mixture is too cold, and the yeast is dormant.

Restarting Fermentation

• If the temperature is too cold and fermentation has ceased, restarting the process may be as easy as warming your mixture. Do so slowly, and carefully monitor the temperature change so that it does not warm too quickly and kill the yeast. If you are brewing beer, agitating the liquid may also help, especially if the yeast has sunk to the bottom. When the mixture becomes warm enough, the yeast will wake up and go to work again.