All this process is a step to make a SOURDOUGH BREAD. If you understand the process you can manipulate the starter to your desire, to get a sour or not so sour bread.....
That means that is up to you, to bake a bread as sour as you like it... If you use this method that I described it here, the bread is almost not sour at all, but it became fluffy, crunchy and chewer because of presence of sourdough starter added to the dough.
A starter left on a counter in a very hot kitchen ( around 85F and up) will ferment like crazy and you can use it as a sour starter at a certain point, or you have to feed very frequently to prevent to became overripe: very bubbling, sharp smell.
"While sourdough starters and bread made from starters has been around for thousands of years, the term "sourdough" has a pretty short history. It is an American term that came into use during the California Gold Rush days of the late 1800's.
Before the advent of commercial bakers' yeast, the folks who traveled and settled the Western U.S. in the 19th century carried starters with them for making bread. Over time, it was discovered that starters from St Francisco area produced bread with a unique and particularly sour tang. Thus the starters and bread from that area because known as "sourdough". "
Understanding the microbiology of the “STARTER”:
The wild yeast, which grows on top of grains, fruits and vegetable, is present into the flour and this process of "STARTER" is a benefactor of co-existence of the wild yeast and Lactobacilli in the "Sourdough starter".
In a healthy sourdough starter, they both are feeding on the sugars (the starch) from the flour, with a different end result that is beneficiary to each other.
The lactobacilli create an acidic media from lactic and acetic acid end product that is very beneficiary for the wild yeast to grow and act as "an antibiotic" for other organisms. For instance the commercial BAKER YEAST can't live in such an acidic content.
The wild yeast end products are a bit of ethanol (alcohol) and some carbon dioxide (which is what causes the bread to rise).