21. Sourdough Science 101 or How are the sourness and

Don't let the subject scare you off.  My kids tease me that since I
left teaching (biology among other subjects), I have to find other
people to listen to me.  They are the usual targets.  I'll try to be
gentle - and practical.

There have been several posts over the last little while asking the
same question in different ways - how are the sourness and leavening
of starters related?  Some starters seem too sour, or not sour
enough, or have lost their sourness, or are sluggish or too active
(not a problem for most) and folks want to know how to manipulate
this.  It also has been pointed out that lactobacilli are anaerobes,
but this needs expansion. Here are some thoughts on this.

I am a homebrewer, and have read a good bit about yeast growth.
Baking and brewing yeast are just different strains of the same
species, but wild yeasts are different species, and some are even
different genera, so this may not apply to all, but I suspect it does.

Cultured yeast needs oxygen to reproduce, so once it has depleted the
oxygen in a starter/sponge/dough, it has pretty much reached the
population it's going to have.  After this, it shifts its metabolism
to anaerobic.   Assuming that wild yeast are much the same, this
means that letting a starter or sponge sit longer is not going to
result in much more yeast, and therefore will not increase its
leavening power.  It will become more sour (see below).

Lactobacilli are facultative anaerobes (as opposed to obligatory
anaerobes), so they will continue to metabolize and reproduce *either
with or without oxygen*.  However, they only produce lactic acid once
the oxygen is depleted, resulting in a more sour starter/sponge/dough
the longer you let it sit.  I don't think you need to worry about
excluding air - the surface above the sponge or whatever is full of
CO2 from the yeast, so very little oxygen is going to diffuse into
the sponge, especially if you have it covered, and this will keep it
from drying out, too.   Of course, during this time, the gluten will
deteriorate the longer you let it sit.

What does this all mean?  If you want a maximally active culture,
whip all the air you can into it each time you build it.  I add the
water first and whip this thin batter to a froth with an electric
mixer, then mix in the flour.  This results in maximum yeast
reproduction.  Then, as soon as it has used up all this oxygen, I
build it again.  Of course, it's hard to tell just when this is, but
I generally let a sponge go until it just begins to fall.  If you
want a more sour bread, let either the starter/sponge/dough go
longer.  I find that with high protein flour such as bread or hard
whole wheat, the dough can withstand two full rises before shaping
into loaves, resulting in more flavor (not just more sourness, but
that, too).

I hope this little science lesson has practical benefits to your
bread baking.  If anyone knows more details about how wild yeasts and
lactobacilli interact, I'd welcome hearing it, especially if I'm
wrong.  I suspect the symbiosis of some cultures may change things,
but this works with my Poilane (originally) starter.

-Jeff