The bottles are typically in my kitchen for at least a few hours beforehand, where the temperature is 68-72 F depending on time of year. While I agree the bottle reduces the temperature of the water a bit, I have doubts that it’s a lot. Next time I make a starter I’ll check the water temperature 5 minutes after putting it in the bottle.

If the temperature drops to 90 F, it doesn’t appear to matter, as I get a vigorous starter every time.

There’s a thread currently going on WMT where folks are posting their starter methods, all of which work. I suspect that it doesn’t really matter which method is used, as all methods posted are reported to produce good results.

You got me thinking — next starter I think I’ll check the temperature every 30 minutes. Since it’s only 1 cup water, I suspect that the temperature reduces to room temperature faster than anyone might expect.

That said, it works the way you do it, so why change it?

I have been warming my water in a metal cup on the stove, and inoculating the yeast into that.

Cover the bottle loosely to keep “stuff” out.

Note: I just published an update with more details, addressing your question, plus making other points that may help.