If you have ever boiled potatoes and left the water sitting in the pan overnight, you know perfectly well how to grow yeast. Or if you have leftover porridge still sitting in the breakfast dishes the next morning (am I giving myself away?), you might have at least noticed that it smells a bit like beer?
The first yeasts were undoubtedly discovered that same way (the Sumerians didn't always do their dishes either), and eventually leavened bread and alcoholic beverages were the result. Yeast, of course, is now available packaged (and sometimes even preserved -- read your label) and convenient in our local markets, and it would be pretty inefficient to return to growing your own yeast before baking a batch of bread.
But you might find it fun. The tastes vary from batch to batch and you might find the results quite worthwhile, as well as educational.
The earliest yeasts came from various sources. Since wild yeast generally produced unpredictable results, peasants learned to use yeasts from fermented wine and freshly brewed beer. Porridge was sometimes mixed with wine, and the soured combination added to dough. Sliced new potatoes, scalded and set out overnight, could be used also as a medium on which to grow a yeast. The liquid extracted from the potatoes was added to bread dough. Once a fermented dough was obtained, bakers would keep part of it out each day to use as a "starter" for the next batch, a practice which is maintained even today in the preparation of renowned San Francisco sourdough.
The way to grow yeast most predictably is to start with a small quantity of dry yeast and go from there. Homegrown yeast can be used as a starter (save out one cup each time to start the next batch) for years if kept cool and "freshened" frequently (either used to start a new batch or given new food to live on).
Potatoes are a popular place to grow yeasts. My favorite way is to chop up 3 or 4 medium sized potatoes (scrub the skins really well, you don't need to peel them) and cook until they fall apart. Drain and mash them up, adding enough potato water back to make 3 cups of mashed potatoes. Add 1/4 cup of honey, stir it in well and cool until just warm to touch (if they are too hot to touch they will kill the yeast seeds). Add 1 cup of starter saved out and stored (covered) in the refrigerator from the last time you made yeast, or 1 tablespoon of dried yeast dissolved in a cup of lukewarm water. Mix well and leave at room temperature, covered with a wet cloth, for at least 8 hours. Take out your 1 cup of starter and use the rest of the yeast mixture to make a batch of four loaves of bread.
You can grow yeast from sprouted grain, too. Take 2 cups of mixed cereal grains (corn, barley, rice, rye, oats, wheat) and soak them overnight. Drain, and spread on wet paper towels in a shallow baking dish, sprinkling occasionally with water for a day or two, until they begin to sprout. Put them through a food mill, mix with water to cover, and boil them up until thick. (At this point you have a delicious breakfast cereal by the way.) Add honey (about 1/4 cup) and treat just like the mashed potatoes, above.
The yeast grown in yogart, kefir and cultured buttermilk can also be used to raise bread. To a cup of cultured milk (heated to boiling and cooled to lukewarm if you prefer) and 1 tablespoon honey, 2 tablespoon dried yeast and enough flour to make a fairly stiff batter. Let this dough stand at warm room temperature 4 or 5 hours, then add 2 cups lukewarm water, stir thoroughly, and leave overnight. In the morning add the remaining ingredients necessary to make your bread and proceed as usual.
Other mediums can be used to grow yeasts -- hops, malt syrup, fruit and milk. You might find it interesting to experiment with flavors and fragrances of growing yeast. One of the classic early American yeast growing methods is used to make salt-rising bread, which has been described during its incubation period as smelling like dirty socks. Education and experimentation is always interesting, not always aesthetic! GERMAN SCHARZBROT (SOUR RYE BREAD) (Makes 2 medium or 3 small loaves) Starter: 1 quart water in which potatoes have been cooked, cooled to lukewarm 2 tablespoons yeast 3 cups rye flour
Combine in a large bowl, cover and leave for 8 to 12 hours. When ready to bake, sitr in: 2 cups mashed potatoes 1 tablespoon salt 1/4 cup malt syrup (also called malted barley syrup -- available in most health food stores) slightly heated 7 to 8 cups rye flour
Knead in the last of the flour on a floured board, incorporating enough to have a smooth, workable dough. Knead well, until dough is smooth and elastic. hCover with a damp cloth and let rise in a warm place for 1 hour.
Make into 2 medium or 3 small loaves, cover with a damp cloth, and let rise 30 minutes. Bake at 350 degrees for 1 hour with a pan of water in the bottom of the oven. Turn out on a rack to cool. When thoroughly cool slice very thin before serving.