hans fugal

There have been some discussions of yeast and oxygen of late, and I got
thoroughly confused. So I went to the great source of confusion itself
(the internet) and did some research.

oxygen to reproduce. Without oxygen comes fermentation, which means the
yeast gets less energy and produces alcohol, but that doesn't seem to
imply that it doesn't reproduce.

At least one apparently reputable site [1] with references seems to say
directly that yeast does in fact continue to reproduce in the absense of
oxygen:

"Many higher animals share this property of oxygen balance with yeasts.
When given nutrient (sugar) and oxygen, they will burn fuel quickly like
a stoked fire, but when deprived of oxygen, they will reproduce by cell
multiplication and division (rather than metabolize). This kind of
behavior--burn fuel or divide--is common to many biochemistries and
these kinds of organisms are classified as facultative anaerobes; they
essentially scrounge a meager living out of whatever particular
cir***stances are handed to them."

and

"As little as two pounds of yeast starter can raise 500 pounds of bread
dough." By my calculations that is 0.4% inoculation. If that's enough to
raise that much dough, then to me at least it implies that there's
reproduction going on.

If you have evidence to the contrary of these statements I'm happy to
entertain them as well. I just thought I'd share my findings.

1. http://science.nasa.gov/NEWHOME/headlines/msad16mar99_1b.htm

dick adams

With regard to yeast and simple organisms, that statement is not
very true. Yeast, for instance, has no other job than to procreate.
If it is metabolizing, it is dividing. Starved, some strains may
attempt ***ual reproduction and spore formation. Otherwise,
in yeast, procreation is via simple cell division (budding for for
bread yeasts).

With humans, procreation is is big. While they are eating, they
are procreating, always ***ually. When they are starving, they
procreate even more, and the United Nations plus faith-based
NGOs step in with many romantic solutions, rarely addressing
procreation, or any means to limit it.

Yeasts and humans have several symbiotic interfaces. Whereas
yeast can utilize atmospheric oxygen when it is available, humans
can utilize alcohol, a well-known metabolic product of yeast, when
things get tough.

This curve shows the behavior of human- and yeast populations
when food it getting short: http://en.wikipedia.org/wiki/Logistic_growth

--
****y

P.S. It should say logarithmic growth.

hans fugal

Thanks, I was wondering if there was a difference between yeast
metabolising and yeast reproducing.

dick adams

Shit, I dunno!

doc

Hans,
I guess you can find anything on the internet. I found the following,
from somebody who does not regularly post to this group.

dept. Microbiology
Univ. Hawaii at Manoa

"Fellow yeasties--just a follow up note on the recent discussion of
providing yeast with ergosterol when growing them anaerobically. Yeast
won't grow under strictly (very strictly) anaerobic conditions because
the conversion of squalene to the first sterol, lanosterol, requires
molecular oxygen to form the beta-hydroxyl group at the number 3
position in ring A, nothing will substitute. Yeast are amazing oxygen
scavengers and it takes quite a bit of effort to achieve anaerobic
conditions sufficient to prevent growth on defined medium--see
Andreason, A.A., and T. J. B. Stier. 1953, J. Cell. Compar. Physiology
41:23-26 (ancient history). Besides the usual methods, one good way to
provide an anaeroic environment is to bubble your misture, nitrogen gas
for e.g., through a culture of yeast--the yeast scavenge whatever
oxygen is left to make sterols."

Not enough to do a quantitative assessment which is what **** wants,
but an indication that yeast is pretty good at using any O2 available,
and a specific reason why O2 (or at least O) is necessary for yeast
growth.

Doc

hans fugal

Neat, thanks. I wonder if there's an experiment that could be done by
non-microbioligist bread bakers to determine whether or how well yeast
is able to multiply in dough. Put another way, just how anaerobic does
it get in there? Maybe someone can stick his head in a big vat of dough
and try to breathe.

doc

Hans,

**** posed a closely related question a while back, for which no answer
has been offered. Perhaps the process of figuring out the experiment
converts you into an amateur microbiologist bread baker. I think you
will need a way to either measure directly (or infer indirectly) the
yeast population density at a point in time. And I am inclined to think
that this means a good microscope and sample prep skills, or a
quantitative chemistry approach that requires more exotic equipment and
technique. It is stuff like this that has kept baking in the hands of
empirisists and out of the hands of closed-loop process control freaks
(for good or for bad).

Cheers,
Doc

will

I've read the stirring theories and agree with the notion that one
needs to redistribute the critters and starch periodically. Sort of
like moving the cows to new grass.

My guess is that if you look beyond yeasts you may find that oxygen is
pretty central to developing various protein structures, gluten for
instance. Perhaps Roy (AKA Chembake) has some thoughts on that.

doc

Will,
If you want to read some (relatively) new gluten theory, take a look at
J. Agric. Food Chem. 2001, vol 49, pp2627-2632. Tyrosine Cross-Links:
Molecular Basis of Gluten Structure and Function by Dr. Katherine
Tilley et al (Kansas State University) on a newly discovered structure
for gluten based on tyrosine cross-links (previously postulated to be
the result of disulfide-sulfhydryl exchange). They show that 30 ppm of
potassium bromate substantially enhances the level of tyrosine
cross-linking in baked dough (about 10X relative to the control dough
which contained 45 ppm ADA, and 3X more than a dough containing 100 ppm
ascorbic acid).

Cheers,
Doc

atty

I think we all do this experiment more or less every time we bake
sourdough bread. Surely we all have seen that when a a yeast culture is
refreshed, or made up into a dough, yeast activity is by no means a
straight line but that after a period of some activity suddenly things
take off, the dough starts to expand at much faster rate etc - and that
then after a while peaks. Of cours e most of our baking revolves around
stepping in and getting the dough in the oven at the right moment,
usually just before the peak of activity so there is oven spring.

What I understand from talking to medical/scientifc people is that
yeasts have three methods of survival, spore, division(budding?) and
(quasi ***ual) reproduction. The reproduction mode only happens when
there is optimal food (starch and oxygen?) and temperature (elsewhere,
here and other places I believe you will find this between 27 and 30
degrees Celsius) and this is the phase we see when a dough goes into
fast expansion. What I have never heard or read about is if in this
reproduction or 'colony' mode, how indivdual yeasts sense to swop from
division to reproduction (that there are enough other yeasts around to
start mating - if this is indeed what is happening)? Are the yeasts
sending signals to each other? Do they literaly move towards each other
or have they been dividing so much in good conditions that they are up
against each other already?

I think we are all fairly familiar with the division mode from storing
our cultures, by putting them in fridge to effectively force the yeasts
into just ticking over via division - though here again I have never
seen or heard any estimates of how long an individual yeast lives
before dividing.

The third method of survival of yeasts again most of us our familar
with if we have created a yeast culture for ourselves, propagation via
airborne spore. As I understand it currently, this mode a yeast goes
into as an escape mode from an unfriendly enviroment - but my
information is that however unfriendly this environment, including very
extreme temperature and lack of food conditions, a yeast will go on
emitting spores for very long periods - possibly thousands of years,
certainly decades from an environment which was once favorable (where
there was previously a big population). Thus Ed Woods Giza culture that
he captured on the balcony of the famous hotel near the Pyramids (and
near the Pyramid building period bakery that he was creating a
duplicate of) might indeed conceivably be the cutlture used in the
original bakery. In my own case my culture was captured on the
windowsill of a harvesting hut (cassita) up a Catalan mountain a few
yards from an oven built into the side of the terraced mountain - pics
in bottom gallery http://www.myplot.org/oven/. This oven we think was
built and used towards the end of the Spanish Civil War at the time of
the Battle of the Ebro when the Tivissa villagers from below escaped up
the mountain to from bombing etc and my yeast adviser friend is
convinced my culture would be descended from that used at that time
(which in turn was most likely brought up the hill form the village and
so might be of much greater vintage).

Again there has been much discussion here and elsewhere about how to
capture/create a breadmaking culture (via spore), is location
important? are the ingredients of culture capturing 'broth' important?
In my own case I used a broth of flour and fig jam from the fig trees
on the property - took about 2 and half days. Why are there so few
reports of failure to capture a culture, or of having captured a
useless culture? Is the lacto-bacilli aspect of a culture captured at
the same time as the yeast? Given that we know that there are many
thousands, even millions of yeasts about and that many of them will not
be suitable for bread baking I would suggest that as in most cases the
suggested recipes for capture 'broth' include flour that we are in fact
thus selecting for a flour loving bread baking culture. I would
illustrate the possible veracity of this by the observation that when
making cordial or 'Champagne' from the elderflower here in the UK it is
almost impossible to avoid maing a very gaseous, indeed notoriously
bottle exploding, drink/liqiud. Clearly the elderflowers have attracted
a particularly gas producing yeast to their surface.

yours
andy forbes