Starter sourness/ripeness question
I've been going thru a lot of extremely informative old posts on sourdough starters today, and TFL is awesome! One thing is still not clear to me, though: I have read in a couple of books that "if your starter tastes sour, it is past its prime to leaven bread. Refresh the starter, wait till it is just before the point of collapse, and then it is at its prime". I believe I understand the "just before the point of collapse" part, that's the same deal as for a commercial-yeasted poolish. What I don't get is the former part: I have a starter at about 70% hydration. When I refresh this starter say every 8 hours, at 8 hours it doesn't quite look like "just before the point of collapse", it is still happily rising, but it is already plenty sour. So I'm confused: I have a starter that is, according to the book, past its prime to leaven bread, but hey it still isn't at the point of collapse anyway. (I first thought it must be the low hydration. Then I made a batch of starter at 100% hydration. That too, even after just 4-6 hours after a feed, has developed sourness but it's nowhere near collapse). What should I make out of this?