There are certain times in life we are told to use a liquid that is lukewarm, mainly when we are bathing or feeding a baby, and when we are baking with yeast. For those of us who are not experts at either, this direction can be stressful—how warm is lukewarm? What will happen if it is too hot? Or too cool? We know (and don't want to think about) what will happen if the liquid is too hot to bathe or feed the baby, but what about when using yeast in a recipe? Before we get into those details, it is important to understand what lukewarm really means.
Definition of Lukewarm
The word lukewarm comes from the Middle English word "lukewarme," the "luke" being a derivative of the word "lew" meaning tepid, which means only slightly warm. Lukewarm is a way to determine the temperature of a liquid, but, interestingly enough, there really isn't a scientifically established degree when water becomes lukewarm.
Depending on the source, you will find differing opinions on what temperature range constitutes lukewarm water. Some references say it is between 100 and 110 F (36.5 to 40.5 C). But others report lukewarm water falls between 98 and 105 F. However, there are schools of thought that lukewarm means room temperature (72 to 74 F), and others believe it means body temperature (98.6 F), while one definition is adding 15 degrees to room temperature water.
If you feel comfortable choosing one of these temperature ranges, use a thermometer to determine when the liquid is lukewarm. However, if you don't have a thermometer handy, or would simply like to do it by "feel," there is an easy way to discern the right temperature. Run the water over your wrist and if it feels warmer than your body temperature, but not hot, that should be just about right. Just be sure to run the water for a bit and maintain that temperature, and that it's not getting hotter.
Yeast and Lukewarm Water
When making recipes that include yeast, you need to add a lukewarm liquid (usually water or milk) to activate the yeast. It is important that you get the temperature right since cold water won't get that yeast going, and hot water will kill it. Yeast is a leavening agent—what makes the bread rise—so it needs to be alive before the dough is put in the oven (where the yeast dies due to the high temperature). An active yeast converts the sugars in the dough into carbon dioxide, which causes the dough to rise and create bubbles after the dough has risen. And lukewarm water activates the yeast.
If you are not using a thermometer, run the water until it feels comfortably warm and no longer cold, making sure the temperature is consistent. If you are uncertain, it is better that the water be cooler rather than warmer because hot water can kill the yeast.
Depending on whether you are using active dry yeast or instant yeast, you will handle the yeast activation differently. Active yeast needs to be initiated by mixing it with lukewarm water. Red Star Active Dry Yeast recommends hydrating the yeast in a liquid that is between 110 F and 115 F when using for regular baking recipes, and if using in a bread machine the liquid temperature should be 80 F.
Instant yeast, also called fast-acting, rapid-rise, and fast-rising, does not require any mixing with lukewarm water; instead, the dry yeast is added directly to the dry ingredients in the recipe. However, if you would like to give the yeast a "good start," you can add it to 1/2 cup of lukewarm water that has been mixed with 1 teaspoon of sugar. Stir until dissolved and then let it sit until the mixture begins to foam vigorously, which should take between 5 and 10 minutes. An important thing to note, however, is that if you choose to use lukewarm water with instant yeast, you need to decrease the total liquid in the recipe by 1/2 cup to account for the water you added with the yeast.