Ken Aldarisio

Heat mat with a cheap thermostat has it at 25C and there's a new pellicle within a week. Good to know it wasn't a sick culture.

No fuzz, and the surface is fully covered apart from the pellicle itself. Reassuring.

Kahm returning isn't a failure. Skim and keep going. The flavour impact at this stage is minimal, though weeks of it can add a yeasty note.

The float test is the least reliable thing in sourdough. A starter full of gas floats whether or not it has the population to raise 500g of flour.

Two things to check in order. First, when are you using it? A starter that doubles in four hours is at peak around four hours, and most people use it at six or seven when it's already collapsing and much less active. Set a timer and use it just as it crests.

Second, 100g of starter into 500g of flour at 24C should not take seven hours. That's the number telling me the starter is weaker than it looks. Feed 1:5:5 twice a day for three days at a steady 25-26C before you bake again. Most "my starter is fine but my bread isn't" cases are a starter that's been kept alive rather than kept strong.

Dense tight crumb after a long rise is nearly always underfermented dough despite the clock, not overproofed.

Write the date on the bottle. Fermented sauces keep a long time in the fridge but a long time is much easier to judge with a date on it.

Yes: consistent, predictable doubling within four to six hours of a feed, three days running. That's the signal, and it beats any day count because the day count depends on your flour, your water and your kitchen.

What you're describing is textbook. Acetone early on is normal, the shift to yoghurt is the bacterial population sorting itself out, and bubbling at eight hours on day nine means it's on the way but not ready. Two things speed it up: feed twice a day rather than once, and keep it warmer, 25-26C rather than a cold worktop. Most "my starter has stalled at three weeks" posts are a jar living at 18C.

Don't bake with it early and conclude it failed. The first loaf from a not-quite-ready starter is dense, and people bin the starter when it needed another four days.