Suppose that you have plenty of wheat flour, but very little yeast. You can stretch the yeast by making a bread starter. This technique is commonly used to make sourdough bread, but you can use a starter for almost any recipe. It doesn’t have to be a sourdough bread.
To make the starter, all you need is a small amount of yeast, water, and flour. Combine 1 cup whole wheat flour with 1/2 cup water. Do not use chlorinated town or city water, as it will kill the yeast. Using whole grain flour provides good nutrition for the yeast to get started. Add a small amount of yeast, 1/2 teaspoon or less. Stir well and let it rest for a few hours, until it is bubbly and has doubled in size. Then refrigerate. Each day or two or three, split the dough into two parts, and add a cup of flour (it can be white flour at this point) and a half cup of water to each. Let it rise, then refrigerate again. You can use one of the starters in a bread dough, and continue the process of feeding the remaining starter every 2 or 3 days. When you split a starter, you can discard one half, or keep it as a separate starter and use it in a bread recipe.
Now suppose that you have no yeast at all, but plenty of flour. How can you make a starter? One approach is to use whole grain rye or wheat, mix with warm water, and leave the mixture at room temperature for a few days. The hope is that the whole grain flour includes some wild yeast that will eventually take over the flour to produce a starter. Here is one description of this process and yet another. But this approach did not work for me.
Another method is to obtain wild yeast from raisins. The skin of the grapes have wild yeast on them, and when dried the high sugar content supports the growth of yeast, but not bacteria. That is why raisins keep without refrigeration: lots of natural sugars and not enough water for bacteria to grow.
I soaked some raisins in warm water for a half hour, and then added it to a flour and water mixture. You can refrigerate the dough, because the yeast will grow at low temperatures. But I would leave it out for a few hours to give the yeast a head start. This worked well for me. Here is a similar approach using organic raisins. This bread baker recommends adding a little yogurt to the raisin water. The yogurt has the type of yeast that makes a sourdough type starter. Make sure the yogurt has “active cultures” (live yeast).
I suppose that, once the SHTF and prepping goes mainstream, one of the main food staples to be in short supply will be flour, but then also yeast. I’m not sure how quickly manufacturers can ramp up production of bread yeast to meet a sharp increase in demand. There are only a few commercial manufacturers of yeast.
I have about 50 lbs of flour with my stored food supply. I figure I can make 40 to 44 loaves of bread with that amount of flour. And that amount of flour will also require about 12.5 ounces of yeast:
2 teaspoons of yeast (8 grams) per loaf
3 cups of flour (18 to 20 ounces) per loaf
So 8 grams of yeast times 44 loaves is 352 grams or about 12.5 ounces. I keep a 2 lb. container of active dry yeast in the freezer (Red Star brand). That is more than enough yeast, even if I decide to double the quantity of stored flour. You can make about 100 loaves of bread from 2 pounds of yeast.