I've talked to so many ladies who are completely intimidated at the thought of working with yeast! "What if I kill it?" is always the question! The goal is for the yeast to feed on the sugars in your dough and produce specific gases which lift and raise your dough into the high light loaf that we recognize as a "good loaf of bread"! It truly is possible to kill yeast. High heat will do it every time! But let's just think for a minute of how glad we are that yeast dies. Imagine if it didn't! (You slice off a few pieces of bread for breakfast, but by lunch the loaf is again twice it's size! The next morning you find the bread bag burst and bread dough rolling off the counter and onto the floor!) Aren't you glad that yeast can be killed! The alternative might be down right scary!
The goal in working with yeast is to keep it warm. Be Goldilocks and aim for not too hot and not too cold, but just right! If you can bathe your baby, or yourself, without scalding either of you, you can successfully work with yeast. Too hot of temperatures will kill the yeast. Eventually, after your loaf has risen to the size you wish, you will bake it in the oven and basically "kill" the yeast and stop the rising process. It will not actively grow any longer. Cool temperatures will not kill the yeast but it will not grow until it is brought into warmer temperatures. Some recipes call for placing your dough in the refrigerator over night to add more depth of flavor. This doesn't kill the yeast but does make it go dormant and halt it's activity.
I use the SAF active dry instant yeast. It is much faster working then regular yeast. You can find small packets at the grocery store but it is far cheaper to buy yeast in bulk. I get mine at Martin's Farm Market. Look for it in the refrigerator. It comes vacuumed packed but after opening the package, it is best to keep your yeast in the refrigerator or freezer. It will stay fresh for months when kept cold.
Many bread recipes call for mixing yeast, warm water and a small amount of sugar in a bowl first. Allow this to sit for a few minutes. The yeast should start to feed on the sugar and become bubbly. This is the way our grandmothers tested their yeast to see if it was active and alive before adding their precious flour.
The water used in bread baking should always be lukewarm, not hot. I find 100 to 110 degrees is about right but it isn't necessary to check the temperature. When I first was married, I thought our tap water didn't get hot enough. So I heated my water up in the microwave. Bad move! After ruining two batches of bread (I truly understand now the fear of killing your yeast!) I decided that the tap water is warm enough. It would be better to use room temperature water then water that is too hot! If you can comfortably put your wrist in your water, it should be good!
All bread is basically a mixture of flour, liquid, sweetener, and yeast. A great loaf of bread can be made with only those four things. Salt, eggs, milk, fats, spices and all the other various ingredients found in recipes, are added for flavor, nutrition, or richness.