Levain aka wild yeast or sourdough starter is a key ingredient in making bread.
It is a type of preferment which matures slowly prior to its addition to bread dough. Typically prepared from a flour and water mixture, it is really simple to make on your own. Not to mention that all you need are some simple equipment such as a 4-cup glass canning jar, a clean wooden or plastic spoon and plastic wrap.
Unlike commercial baker's yeast that can be bought off a shelf, levain is superior because its natural, using biological leavening. It also makes the bread richer in nutrients and more healthy. The resulting sour taste is from the lactic acid produced by the lactobacilli, giving it a unique taste, flavor and texture.
At the same time, since the fermentation process is natural, no two levain preparations are exactly the same. For more experienced bakers, the compostion of the ingredients can be adjusted to achieve the desired result. Which explains why some sourdough loaves are more sour while some are less so.
The levain consistency can be either that of batter (ie wet) or dough-like (ie. dry). This is a result of their hydration level which is defined as the weight of water used divided by the weight of flour used.
It is also worth noting that levain can be used to make all kinds of baked goods, including pastries and desserts.
Making levain is somewhat labour intensive as the process usually takes a minimum of 5 days to capture the natural yeasts in the air. Once the fermentation starts, you will need to add flour and water to feed the yeast so that it can grow and develop into levain. These are referred to as feedings and are made once to twice daily.
The end point of the fermentation is often determined based on a few observations, namely the formation of dense foamy structure, the rise is complete with minor recession in the center and the development of a pleasant aroma.
Having your own levain can also become a long-term relationship if you are able to commit to the regular feedings required.