Yeast respiration increases, and therefore rises more quickly and voluminously, with an increase in temperature up until 35 degrees Celsius, at which point respiration will gradually slow. At 50 degrees Celsius, the yeast will begin to die due heat damage to the cells. Below 20 degrees Celsius, the yeast will not ferment.
Respiration is a crucial part of the fermentation process that occurs when yeast breaks down carbohydrates into carbon dioxide and alcohols. When yeast cells are able to respire more efficiently, they can in turn break down carbohydrates more efficiently, leading to more fermentation taking place. By knowing the precise temperature at which a specific yeast ferments most efficiently, less yeast is wasted and better products are created.
Some of the oldest known human-produced nourishment, including most types of bread and all alcoholic beverages, depend on the yeast fermentation process, so humans have extensively studied the best methods for maximizing the process. In addition, yeast is used by the scientific and medical communities in many applications. Even some other species have been found purposefully seeking out fermenting fruit in order to get drunk off of the naturally occurring alcohols, albeit at a much lower level than most of human society today.