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.
Go Back in Time...
Day Zero: What You'll Need »
Day 1: A Half-Ounce Flour and an Ounce of Water »
Day 2: No Feeding, Just Stirring »
Day 3: Feed Me More Flour! »
Day 4: 100% Hydration »
Day 5: Keep Feeding and Stirring! »
See the Future...
About the author: Donna Currie has been cooking for fun and writing for pay since the days when typewritten articles traveled by snail mail. When she combined those talents in a food column for a newspaper in her area, she realized that writing about food is almost as much fun as eating. She launched the blog Cookistry and has now joined the Serious Eats team with a weekly column about baking.