From: delwiche@sunflower.bio.indiana.edu (Charles Delwiche)
Newsgroups: rec.food.sourdough
Subject: sourdough starter ecology (was Re: more on the biology of sourdough)
Date: 26 Aug 92 00:09:49 GMT
Article-I.D.: usenet.BtKDsD.7yL
Organization: Biology, Indiana University - Bloomington

Gee... thanks for all the nice comments on my slightly scattered essay
on the biology of sourdough.  I'm trying to learn more about the nitty
gritty details of the process from a biological point of view, and as
I get things figured out I'll try to share with you all.  It may be a
while before I have time to write anything else though, so here's a
real long one to test your patience:

In response to my previous essay on the biology of sourdough, 
fritz@hpfcbig.sde.hp.com (Gary Fritz (tmp)) wrote:

>: Sourdough is like most other
>: microbiologically produced foods in that the fermentation is
>complex,
>: with each batch going through an ecological succession of different
>: taxa.  In other words some critters start early, but get wiped out
>: later on by other more slowly growing critters.  (There is a lot of
>: literature which is aimed at getting a good sourdough starter into
>: continuous culture, which is what you want if you are going to
>: mass-produce sourdough).

>Hm.  Do I read this correctly to mean "you can't cycle your starter
>continuously"?  I.e. you have to refrigerate or otherwise "rest" it
>between batches?    

No! Not at all.  In fact I strongly suspect that one gets the _best_
sourdough if one cycles the culture on a regular basis.  Continuous
culture is when you have a vat that you are constantly adding starting
ingredients (e.g. flour and water) to, and constantly removing
"mature" starter from.  What you are describing is a "batch" culture,
where you start a new batch each day.  That ought to work beautifully,
as long as your starter is healthy and happy enough (and warm enough)
to "mature" in one day, which it should be able to do.

What happens when you "feed" your starter is that you introduce a
whole bunch of new growth substrate (food), and the critters go on
about digesting that.  Unfortunately for the critters, but fortunately
for us, each species is somewhat limited in terms of what they can
eat.  In order to digest the food in the flour, mostly starch and
other complex polysaccharides, the things living in the starter have
to secrete enzymes that can break down the polysaccharides into the
sugars they are made up of.  Enzymes are highly specific in terms of
what kinds of molecules they can break up, and because each one of the
several organisms you have living in the starter has a limited
selection of enzymes, they have a limited variety of molecules they
can use for food.

This is delightful for us, because as the organisms digest the flour
they release various materials that are waste products to them, but
important flavor components to us.  For example, my own favorite
_Saccharomyces exiguus_ (a yeast, which is a unicellular fungus) is able
to digest polysaccharides and some sugars, but is completely unable to
digest the sugar maltose.  That means that if _S. exiguus_ is living
all by itself, it will eventually build up a lot of maltose, which is
just a waste product so far as it is concerned.  (Incidentally, for
those of you who don't know this, fungi and bacteria do most of their
"digestion" externally; they secrete enzymes into their environment,
the enzymes break down whatever it is they are floating around in, and
then they absorb the smaller broken-down components like sugars and
vitamins, which they then break down internally for energy)  But along
with _S. exiguus_, one also finds _Lactobacillus sanfrancisco_, which
is a bacterium which is *only* able to digest maltose.  _L.
sanfrancisco_ breaks down maltose, and turns it into (primarily)
lactic acid.  Lactic acid (along with some acetic acid) makes the
dough sour, which makes it taste good to us.  The acid also drops the
pH of the starter, which helps out _S. exiguus_, because it, unlike
many yeast, doesn't mind living in an acidic environment.

Now if _S. exiguus_ and _L. sanfrancisco_ were the only beasties in
the world capable of living in sourdough starter it would be a
relatively simple thing to make and maintain starter.  Sourdough would
also be a much less varied and interesting food than it is.  One of
the things that I, for one, love about sourdough is that one is having
an adventure each time one eats sourdough, especially if it is
home-made or from a small bakery.  There are actually at least a half
a dozen distinct yeast and maybe 20 or 30 different kinds of bacteria
(lactobacilli and related organisms) that can show up in sourdough,
and each one of these probably adds a subtly different flavor to the
dough.

How does disturbance (like the addition of fresh flour and water)
affect the microbial community?  The best analogy I can think of is to
the prairie:  Prairies are plant/animal communities that used to exist
on all of the continents, close to the middle of the continent, where
there are moderate amounts of rainfall, and the weather is fairly
harsh, with cold winters and hot summers.  

Unfortunately for the prairies, humans find these areas excellent for
the kinds of things that humans do, particularly for growing grain, so
most of the several millions of acres of prairie that used to exist in
the continental US have been converted to farmland.  But an
interesting thing happens to a little piece of prairie that is stuck
between to fields for fifty years or so; it turns into a woods.  If
trees can grow in the prairie "zone", why were there grasslands there
prior to the intervention of european agricultural practices, rather
than forests?  It turns out that the prairie only remains a grassland
if it burns periodically.  What happened in the central US before the
invention of the steel board plough is that every couple of years, on
the average, a thunderstorm (or a native american, or whatever) would
start a fire that would burn a good big chunk of the prairie.  Trees
would be killed or set back in their growth, but the grasses and
wildflowers that lived in the prairies would just reseed themselves,
or grow back from underground parts.  Fires also liberated nutrients
that were trapped in undecayed organic matter.  When the periodic
fires were stopped, trees were able to take over because they can grow
up and over the prairie plants and shade them out.

So to keep a prairie healthy it requires periodic disturbance, either
burning or mowing.  The first year after a fire a group of fast
growing, durable plants show up.  In the next year or two slighly less
fire tolerant prairie plants appear.  A few years after that the trees
start to show up.  If the area burns too often only the most fire
tolerant species survive.  If it never burns at all, trees take over.
But if disturbance occurs with just the right frequency then you have
a nice diverse prairie with lots of interesting wildflowers and the
like.

Sourdough is going to work in more or less the same way, but on a tiny
scale, all in a jar on your kitchen counter.  When you add new flour
you disturb the system, and provide a lot of raw, unexploited food.  A
succession of organisms will then increase in population size and
begin to exploit the resources.  Different organisms will reach peak
population sizes at different times.  Several factors affect this, for
example intrinsic growth rate (how fast the cells can divide), and
availability of essential nutrients (so _L. sanfrancisco_ has to wait
until _S. exiguus_ has at least gotten a good start before it has any
maltose to digest).  After a while the community will be
at the stage that you decide is just right for sourdough, you use some
of the starter and add flour and water to the rest, and start the
whole thing over again.  

Different people like to "harvest" their starter at diffent stages.  A
while ago the sourdough newsgroup was talking about whether or not it
was ok to use a starter that was so old it had developed a black scum
on the surface (that takes a few weeks in the refrigerator).  One of
the best cooks from my childhood would *always* wait until that stage
before she made bread.  If you do that you get a pretty intense loaf
of bread -- it isn't for beginners.  Other people use a very young
starter.  In fact I have been told that one of the secrets that
sourdough chefs hold dear is the exact timing of their fermentation.
It can make a big difference, or so they say.

The problem that the commercial continuous culture people have is
twofold:  they want to be continually pumping nutrients (flour) in,
and constantly tapping mature starter off.  That means that for a vat
of a given size there is probably a pretty narrow range of flow rates
through the vat that will give a starter with an acceptable flavor.
But the second problem is more subtle, and I would guess harder to
solve; a continuous flow process is fundamentally different from a
batch process in that there is a constant dribble of fresh flour into
the continuous flow vat.  By contrast when you make sourdough at home,
by the traditional process, you put in a big shlog of flour, then wait
a day or so until the starter is nicely bubbling along.  During that
time a succession of organisms have gone through and digested
different things at (slightly) different times and rates.  So a
continuous flow system and a batch system, started with identical
microfloras won't necessarily produce identical starters.  That means
that a wonderful starter in the home (or small bakery) kitchen may or
may not make good bread in a large scale operation.

How much flour+water in proportion to the amount of starter, how long
you let your starter sit, what temperature it is at, and what your
starting microflora is will all have significant effects on your
finished products.  If you refrigerate your starter (like most people
do), exactly how you do this will also have an effect.  My advice is
to experiment, talk to other people who bake sourdough, and forgive
yourself for the occasional french brick.

One last hard core technical note:  Nout and Creemers-Molenaar
(Chemie-Mikrobiologie-Technologie-der-Lebensmittel 10:162. 1987.)
tried adding baker's yeast (_S. cerevisiae_) to dutch commercial
sourdough starters.  Basically what they found was that if the baker's
yeast was added in small quantities it would just get wiped out by the
culture in the starter.  If it was added in really large quantities it
would be detectable after fermentation, but even then wasn't doing
well.  This is just another bit of evidence that says not to try
mixing your baking techniques; if you are making sourdough, make
sourdough and don't add bakers yeast.  Incidentally, that same paper
also discusses the change in community composition over a 10 week
development time for a starter.  Bottom line is, the community
changes, with some taxa slowly being eliminated.

chuck
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-- 
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Charles F. Delwiche, Dept. of Biology        |"We admit no more causes...
Indiana University, Bloomington, IN 47405    |than such as are both true
                                             |and sufficient." I. Newton
Newsgroups: rec.food.sourdough