How does sourdough starter not get overgrown with mold or “bad” bacteria after being left out every day, all day?


I’ve read that it’s because the “good” bacteria thrives and drives out the “bad” but why is that not the case with other foods?

Why can’t I leave my leftovers sitting out on the counter at room temp for days without making myself ill? Why doesn’t natural yeast and good bacteria take that over?

I’ve just begun my sourdough starter experience and these are the questions that are stumping me! 🙂 Thanks!

In: 5

Yeast thrives in dough/batter—it needs water and sugar. So even if we assume that ambient yeast could take over another “ecosystem” like, say, a steak, then we would need to be feeding it sugar at least.

Also, the balanced environment of sourdough starter takes many days to develop.

It can and will grow moldy, but you’re also swapping out the ingredients regularly. You don’t just leave the same wet flour to rot, you take out part of it and replace with fresh flour and water so you have turnover of ingredients. If you just let it sit without doing that (aka “feeding” it) it will turn greenish/bluish, just like bread mold when you leave bread out.

Yeast has ways of protecting itself. Also, it’s out competeing other microorganisms for resources (food). In addition, it is acidic and wet, which is protective against a lot of types of bacteria.