You've got the hang of this. Today, do it again: feed your starter 1 ounce each of flour and water, and give it a stir when you think of it. Bubbles are looking good!
Not all starters are the same, so if you aren't seeing the same bubbling I am, don't worry too much about it. I've seen starters that have a slow start, but suddenly burst into action rapidly and vigorously. That's part of the charm of sourdoughs—they're quirky.
Part of what happens in a sourdough is that the bacteria converts the sugars into lactic and acetic acid, which lowers the pH level to a point where a lot of the nasty microbes won't be happy. However, the yeast in sourdough likes the acidic environment just fine.
The good yeast can live in the acid environment and the bad critters pack their bags and go away. And unlike regular yeasts, sourdough yeasts don't eat maltose (a type of sugar) so they leave that for the bacteria to munch on. The bacteria add to our starter's flavor, and the yeast add the bubbles. Meanwhile, everyone else gets out of the swimming pool.
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