The temperature of the water used for activating the yeast is an important consideration. If the water is too hot, the yeast (which is a living organism) will die. A temperature of 140ºF is usually sufficient to kill yeast cells. Cool water will not harm the yeast, but it will decrease the growth of the yeast. If the water is too cold, the yeast cells will become dormant (usually below 50ºF). The yeast will reactivate once the temperature is increased. The proper water temperature differs depending on the source of the information, but most sources agree that the water should not be cooler than 95ºF or warmer than 115ºF. The temperature can be checked with an instant read thermometer, but if a thermometer is not available, it is better to err on the side of cooler than warmer.
Quick-rising active dry yeast does not have to be activated in warm water prior to use. When mixing the ingredients for dough, it can be blended with the flour and other dry ingredients before the liquid is added.
The temperature of the kitchen or room in which the bread is prepared also affects the activation rate of the yeast. The yeast will activate at a faster rate in a very warm room and a slower rate in a cool room. On the coldest days, it is beneficial to place the yeast and warm water mixture on top of the preheated oven to keep the yeast at a more constant temperature while it activates. It is important to remember that metal bowls and utensils should not be used when activating the yeast because a reaction may occur between the metal and the yeast.