Yeast becomes more active when warm, but it dies at high temperatures, such as when it is in baking bread in the oven. According to Fleishman's, active dry yeast dissolves and activates best in water that is 100 to 110 degrees Fahrenheit. Once the baker mixes the yeast into dough, he should place it in a warm, draft free area to rise.
Yeast is a microscopic single-cell fungus that requires food, moisture and warmth to survive. It is commercially available in an active dry form and as cakes that consist of yeast cells mixed in a starchy material. The dry form does not become active until the user supplies moisture and food. Yeast cakes have moisture and food, but refrigerating them slows the organism's growth. Once they reach the ideal temperature, they begin to convert sugars and starches into alcohol.
Many breads use yeast as a rising agent. As a byproduct of the sugar conversion, yeast produces carbon dioxide gas. This trapped gas in the dough causes it to rise. Brewers use yeast of the genus Saccharomyces in the conversion of sugar into alcohol. It is essential for making beer, wine and distilled beverages as well as alcohol for industrial purposes.