33. What temperature should my starter be for best results?

Typical sourdough actually may contain three different types of microorganisms.

We all know about yeast and those bacilli that produce lactic acid.
There may also be different bacilli in your dough, namely ones that
produce ordinary vinegar or acetic acid.

(There is also the possibility that there are still different
microorganisms in there, but you usually don't want that to happen.
Worst example are the bacilli that produce a kind of acid that also
makes very old butter stink.)

Each microorganism has its own favorite temperature.

The bacilli that produce lactic acid like rather high temperatures of
37-40 degrees C or 99-104 degrees F.

The bacilli that produce vinegar are active only if there is yeast
that has already produced alcohol. (Yeast always does that, it never
produces gas without producing alcohol, so the word "alcohol" should
not alarm anyone.) Those bacilli like rather low temperatures, 20-25
degrees C or 68-77 degrees F.

Personally, I want lactic acid and not vinegar in my sourdough. You
can tell the two apart by the fact that lactic acid tastes sour, but
does not smell sour. Also, vinegar escapes as a gas during the baking
process as well as during storage of the bread, whereas lactic acid
stays.

Yeast will grow (multiply) fastest at 24-27 degrees C or 75-81
degrees F. (Yeast also needs oxygen to multiply.) Yeast will produce
gas fastest at a somewhat higher temperature, namely 30-32 degrees C
or 86-90 degrees F.

So, my own conclusion from all this is: the temperature which you use
to maintain the starter will, in the long run, affect the kind of
microorganisms you have in there.

If you want lots of lactobacilli, use higher temperatures when
refreshing the starter. If you refresh your starter at comparatively
low temperatures, you may get a dough that smells sour and contains a
lot of vinegar, but the resulting bread isn't all that sour.

-Andreas