Symbiosis between lactobacilli and yeast in the fermentation of sourdough bread


IIRC the yeast gives off CO2 as a byproduct and the LAB gives off lactic acid (the “sour” in sourdough). But byproducts of what? Are both the yeast and the LAB feeding on the sugars in the flour? And what is the chemistry that explains that the only bacteria you end up with are safe and not a harmful strain of bacteria?

In short, I’m hoping someone can explain the chemistry involved in sourdough fermentation like I’m 5.


In: Chemistry

Those byproducts are byproducts of their metabolism/life. They “exhale” CO2 like we do and “pee” lactic acid like we do. They are feeding off the sugars and starches in the flour/dough. The lactic acid that is produced is what keeps it safe(r).

LAB and common sourdough yeasts are more acid-tolerant than other bacteria/yeast species, so they can create an environment that is safe for them but hazardous to other bacteria by excreting lactic acid. This is a bit like how people can build an environment where we can live but wild animals can’t.

This doesn’t guarantee food safety forever, but it does slow down the rate of spoilage.