Yeast completely dies at 140 degrees Fahrenheit, although the dying process starts at about 120 degrees. This information is critical to bread recipes that require activation of yeast with hot water.
If the water is too cool, the dough may not rise. Extremely hot water either kills the yeast or causes the dough to rise inconsistently. To ensure that the water is within the ideal temperature range of 120 to 130 degrees, check with a thermometer prior to adding the yeast. Proper activation is indicated by the appearance of tiny bubbles on the surface 5 to 6 minutes after the yeast is added to the water.