- Yeast must be activated by dissolving in warm water. The liquid should be from 105°F to 115°F. If water is too warm, the yeast will be killed and dough will not rise. Use candy thermometer to determine if water is at safe temperature to dissolve yeast. To “proof” yeast, dissolve yeast with 1 teaspoon sugar in 1/4 cup water and allowing it to stand for a few minutes. The mixture should become foamy in 5 minutes, confirming that the yeast is still active. Do not use packages of yeast which are older than their expiration date.