how can I tell the starter is at its peak?
I've made my first starter as of two days ago. Right now it's sitting out at ca 68, and I feed it twice a day. It's nice and bubbly. After I fed it at 9am today, it doubled within 3-4 hours. I decided to watch what will develop, since I'm totally new to this. It's been sitting there, motionless, since 1pm. It's still motionless, at 5:22pm. If I understand this correctly, there will be a point when it peaks in vigor, and then slowly lose power; so one wants to catch that point in time when it's at its strongest. How exactly do I know this?? Clearly there are a good few hours wiggle room, but roughly? Does the smell change? Right now it smells to die for good, but not sour (I take it it will take a week or more to develop its fullest sour potential. I intend to keep it sitting out for now because it's so much fun to watch.
Any tips on when the time has come for the starter to lose its power?
you don't really know the peak until it has passed, then you can say with the clarity of hindsight "that was a peak". You need to learn the dynamics of your starter by experience. You will not achieve any precision in estimating the peak, but you'll know (for your particular feeding schedule and ratios) what rise is normal (typically double, but ymmv).
It's pretty forgiving, if it seems to be starting to fall, you can still use it, and it's only slightly sub-optimal.
This photo by Dan DiMuzio from an older thread on the same topic might be helpful.
www.thefreshloaf.com/comment/95326#comment-95326
Dan DiMuzio is a professional baker and author of Bread Baking, an Artisan's Perspective.
When it has risen to the point where it will float on water it is ready and at its peak.
BTW ... the starter does not insert the sour into the bread, the starter working very slowly allows the sour bacteria, which take longer, to do their thing in the initial rise (bulk ferment) or in the final proof. The reason one wants to catch a starter at its peak is to do the two rises quickly so you don't get a very sour loaf.