Natural bread is based on a sour dough starter or pre-ferment. In this fermented dough grow bacteria and lactic acid bacteria that produce carbon dioxide and alcohol, thus encourage sourdough.
It is made from strong flour like Manitoba and water, but to accelerate the fermentation times usually is used a starter, such as not pasteurized honey, yogurt, the soaking water of raisin or ripe fruit. Better is to use organic flour and ingredients, as they are rich in micro-organisms which help the fermentation. To create a sourdough from scratch I suggest to use weak flour, so that the higher quantity of carbohydrates in it feeds the bacteria more strongly. Once the sourdough is alive and rising you can use also strong flour as Manitoba for refreshments.
Making sourdough is not easy: it takes patience at first, to make it active and acid at the right point, and after a lot of care, as it should be kept alive with continuous refreshments of flour and water. Only in this way bacteria and enzymes contained in yeast may be kept alive, to get a strong yeast.
Step 1: Put the flour in a bowl and add two cups of low-fat yogurt or other starters. Mix well with a spoon until the dough consistency is soft. The dough should be liquid, but if you want it to be more solid, just add flour. In this case put the dough on a floured surface and knead it to get a smooth ball that you will put in a large glass bowl.
Step 2: Practice on the dough a cross with a smooth bladed knife, cover the bowl with plastic wrap and drill holes with a toothpick so that it passes the air and micro-organisms can develop carbon dioxide to make sour dough starter leavening. Let it stand for 48 hours at an ambient temperature between 78° F and 82° F.
Step 3: After 48 hours, the yeast will be moist and soft, its volume will be increased, with a whitish color and smell slightly acid and alcoholic but not stinging. So sour dough must be refreshed: take 7 oz of it, taking care to discard the most superficial part, which is more in contact with the air but leaving the inner part, the heart of the yeast.
Step 4: Place the 7 oz (throw the rest) in a bowl on a scale and add the same weight in manitoba flour, thus 7 oz, for a total weight of 14 oz, and the half of the weight in water (80% in water if you prefer the liquid one), then 3.5 oz for a total weight of 17.5 oz (you'll always put 3 oz of water at the beginning because the mixture may absorb more or less).
Step 5: Knead for a few minutes with your hands to reach a soft texture, but not sticky. Store it in a tall glass container, so that it has space to grow, covered with plastic wrap pierced with a toothpick. Let it leaven at room temperature for 24 hours.
Step 6: You'll have to refresh sour dough starter with another 7 oz of it together with 7 oz of flour and 3.5 oz of water, repeating the same operation daily for about 14 days.
Put the yeast refreshed in a glass jar and flatten it on the bottom with the hands, so it will uniformly grow. On the 15th day your yeast will be grown, showing big holes.