Scientific Data

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May 2, 1998, 3:00:00 AM5/2/98

For some reason, neither of my two servers is showing
this post after a few days, so I am re-posting. Sorry
for the duplication...




I did a little Internet snooping and came up
with a few things I thought were relevant/

I have taken the following quotes from a sub-page
-- an interesting site on yeast genetics. Their
data concerns standard bakers' yeast, but
I think much of it is applicable.

> The growth behavior of yeast cultures is similar
> to that of bacteria. When a growth medium is
> inoculated, the cells require a period of
> preparation before they start dividing. Following
> this lag period which may be up to several hours
> they rapidly enter the exponential phase
> during which their number and mass double at
> equal time intervals. After a period of growth
> at a relatively constant exponential rate, some
> environmental condition becomes growth limiting
> so that the rate of increase diminishes and growth
> eventually stops. The population and mass become
> constant. The culture remains stationary and the
> cells remain viable for several hours; if the
> culture is refrigerated the cells remain
> viable for months. Eventually the cells die, and
> at room temperature or warmer they will undergo
> autolysis: their own digestive enzymes become
> active and they literally digest themselves,
> reducing their proteins and nucleic acids to their
> simpler components; they produce a particularly
> unpleasant stench in the process.

This tells us a lot of things. For me, one of the
most interesting was that the yeast (in our case we
could reasonably include bacteria as well) does,
in fact, reach a stable population. It is not
simply an either/or case where the population is
either growing or dying off. In fact, their text
seems to indicate that the mircrooganisms in a
refrigerated starter remain at high concentration
for quite a while (months) after maximum population
density is reached.

The second thing, that I am kicking myself for not
remembering earlier, is autolysis. This phenomenon
somewhat supports Dick's idea that proteolytic
enzymes are released primarily after fermentation
slows down. However, this assumes that that the
yeast have reached the point of starvation, and I
doubt that this happens during the fermentation of
a dough.

Further, as mentioned above, autolysis
produces a distinctive and very unpleasant odor
and flavor. I am familiar with this from my brewing
and it's hard to miss. So, I imagine that autolysis
isn't a major cause of protein degradation during
a sourdough rise. On the other hand, this might
be of concern in a thin unrefrigerated starter.

They also say:

> Normal yeast can grow either aerobically,
> in the presence of oxygen or anaerobically,
> in the absence of oxygen. Under aerobic growth
> conditions they can support growth by oxidizing
> simple carbon sources, such as ethanol, acetate
> or glycerol. If they have adequate oxygen, they
> will completely oxidize their carbon sources,
> usually sugars, to carbon dioxide and water.
> However, under anaerobic conditions, deprived of
> oxygen, yeast can convert sugars only to carbon
> dioxide and ethanol, recovering less of the
> energy. In either case, growth will be limited by
> some essential nutrient or the accumulation of
> the toxin.
> Yeast grow equally well in liquid media or on
> a nutrient surface such as an agar plate or an
> exposed surface of some kind of food. In liquid they
> must be stirred or shaken if they are to remain
> aerobic; otherwise, they settle to the bottom of
> the container, consume the dissolved oxygen,
> and grow anaerobically.

More data later...

Sam Kinsey

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May 5, 1998, 3:00:00 AM5/5/98

>Further, as mentioned above, autolysis
>produces a distinctive and very unpleasant odor
>and flavor. I am familiar with this from my brewing
>and it's hard to miss. So, I imagine that autolysis
>isn't a major cause of protein degradation during
>a sourdough rise. On the other hand, this might
>be of concern in a thin unrefrigerated starter.

I know this unpleasant oder. Sometimes during the summer when the bakery
reaches temperature of +95, the levain will at times ferment too fast and
will collapse in on itself. About two hours or so after this the levain
begins to get an unpleasant smell to it. Just a hint. It is still usable at
this point, but not for much longer. Before that point we have made another
levain, so that the bread dough will not be effected. Once in a while the
clean up crew forgets to dispose of the old levain and, let me tell you,
when you find it, take the cover off you know what it is and you want to get
it out of the bake room fast.


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