Subject: Sourdough Science 101

From: [email-masked] (Jeff Renner)
Date: Fri, 26 May 1995 11:13:30 -0400

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
-- 
Jeff Renner in Ann Arbor, Michigan c/o [email-masked]