Sourdough bread starters are quite easy to make. They consist of two groups of micro-organisms:
The bacteria create lactic acid and substances that provide flavour, while the yeasts create bubbles of carbon dioxide to make the bread rise. It is not unusual for the consortium of microflora in a sourdough starter to change depending upon the environment in which it is use a
Add 1/4 cup of flour to a jar and mix in a source of microorganisms from one or more of the following:
Two days before use, remove the culture from the fridge, discard some of it, then feed it with a quarter of cup of flour and enough water to maintain the consistency and incubate at 25 C. After 24 hours it should be ready to use.
The starter loses its activity and does not make the bread rise as much as it used to. How to regenerate the starter? If you do not use the sourdough starter for a few days then the level of alcohol's will rise until they eventually kill of most off the micro-organisms. Without yeasts your bread will not rise.
The solution is to mix 1 teaspoon of starter with 1/2 of cup of flour and enough water to make a smooth paste. Leave it for 4-5 hours or until the culture has doubled in size then refrigerate the starter to slow down the fermentation process. The next time you take the starter from the refrigerator it will be more active. Leave it for 6 - 8 hours to ferment then take one teaspoon and add it to 1/2 cup of flour plus some water, leave to ferment for 4-5 hours then feed it another 1/2 cup of flour. By feeding the starter increasingly larger quantities of flour at regular intervals you increase the activity of the yeasts which should make your bread rise better.