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I'm new to sourdough starter, so I'm a little confused regarding a statement I read recently which said that the yeast is formed AFTER you see bubbles forming in the starter. Yet, it makes sense to me that the yeast is the cause of the bubbles in the first place. Would someone please explain?
Thanks.
The answer is actually both! Yes, the bubbles are caused by the yeast, they are converting sugar to carbon dioxide, among other things. Its also an indicator that the yeast colony is multiplying. So, technically, there is (more) yeast forming.
I think the question could be referring to the idea (which comes out of some microbiology studies) that the normal yeast in sourdough often doesn't really get established for a week or more after the starter is first begun.
Healthy starters have lactic acid bacteria (which makes them sour) as well as yeast (which raises the bread). Both can make bubbles, though in a mature starter, most of the gas comes from the yeast.
When you make a new starter by just mixing flour and water together, bacteria growth is often more rapid than yeast growth for the first few days. Bacteria will also produce bubbles when they digest food, as well as producing various acids and alcohols.
After a couple days to a week, depending on your feeding schedule, all of that bacterial activity will make your starter very sour. So sour, in fact, that some of the bacteria that caused those initial bubbles can't grow anymore, since they don't like too much acid. Some of the only ones that will still grow are the lactic acid bacteria -- which is good, since they won't make you sick (and will make good flavored bread).
Essentially, those bubbles in the first few days are usually a sign that you're creating an environment that will only tend to grow yeast and the "good" bacteria. With proper feeding, the yeast now have room and food to grow in a sour starter, since the other "bad" bacteria can't.
In that sense, the first bubbles in your culture aren't necessarily (all) from yeast. They prepare the starter so that yeast can grow more freely. After the first week or two of feedings, most of the big bubbles you see when you feed your starter will then be produced from yeast.
