If you mix flour and water and leave it to sit for several days it will grow a community of micro-organisms and become sour from the lactobacillus present in the air, water and flour multiplying. If you take a spoonful of the fermented dough and feed it some more water and flour(¼ cup of each) it will ferment again. This is called propagation and it is essential to making sourdough bread. In fact the quality and vitality of your culture is the most important determining factor when making sourdough. Your wild yeast populations need to be very strong and healthy as the bacteria won’t levain your bread well and you need plenty of active yeasts to create enough gas for your bread to rise sufficiently. A sourdough culture will perform best when it is fed at least once a day but sometimes cold storage is necessary when you are not baking for more than a few days at a time. It will keep fairly well in the fridge but will need 2-3 days of feeding before it will make high quality bread again. It’s best to keep your starter culture about 68-70 degrees. If it is colder than that the yeast will be very sluggish and as it gets over 80 degrees it will begin to favor the lactic acid bacteria too much. The warmer it gets the faster it will multiply(and run out of food).
When starting a sourdough culture(levain) from scratch it will take about 2 weeks to build up enough healthy yeasts provided it is warm enough and fed daily(accept the first 2-days). Once it is active enough(it will be frothing with bubbles and smell yeasty) you can use it every day if you want. Typically though it will get used once or twice a week. In this case I recommend feeding it each day for best results. If you are only going to bake with it every other week or once a month you will want to store it in the fridge and then take it out several day before baking to feed it back to health. 1 teaspoon of active levain can be fed ¼ water and ¼ cup flour each day. A seedling heat mat with a thermostatic probe is a nice way to maintain proper temperature during the coldest months when you house is likely to be under 65 degrees.
The most important factor in maintaining a healthy levain culture is your ability to determine how “ripe” it is at any given time. Each time you feed the starter culture it will begin it’s process of feeding and multiplying. When it has consumed most of the available food from a feeding it will be “ripe” and ready to either make bread or be fed again(or put into storage). If you keep feeding your culture at it’s peak ripeness it will reward you with a frothing, lively culture that will make very good bread. So how do you tell when it is at peak ripeness? When you are first building a starter culture it will be hard to tell since the microbe communities will be small so be patient at first. As you feed your culture each day smell it, taste and notice it’s texture. Eventually you will know from a quick whiff and a look but at first pay very close attention.
When you first feed your culture it will look and taste just like a water and flour slurry. After a few hours(if your culture is fully active) you will begin to see bubbles forming in the slurry(it’s helpful to have a clear container) and after 5-10 hours it will be filled with bubbles. It will smell yeasty with a touch of alcohol and acidity. Somewhere between 10 and 24 hours the microbes will have consumed most of the available nutrients and will begin to pass it’s peak ripeness. As this happens the slurry will begin to break down and separate as large bubbles turn to small ones and gradually the liquid will separate and become increasingly acidic and alcoholic in taste and smell. The more sour your starter becomes the less suitable for bread making it will be.
It’s important to become familiar and comfortable with your levain before making bread with it as it is very discouraging to have a bread failure due to poor starter. If you’re not sure if your starter is ready feed it another day or two. It will be frothing with bubbles when it is ready and not before. Those bubbles are proof of the gas it will generate to raise your bread. If it is not there before you mix your dough it won’t be there in the dough.
All you need is whole wheat flour and water. Yeast are everywhere in our environment and can be gathered from almost any food like grapes, or apples but it is of course present on wheat berries and in wheat flour in plenty of quantity