Good eGullet Sourdough article
I was poking around eGullet  this morning when I happened upon an excellent sourdough article  that really helped things click into place for me, at least mentally. I'm going to be travelling for a bit and so won't get back to the kitchen for sometime.
What Jim has been saying  about time, temperature and the percentage of starter in a dough made sense to me, and has helped me improve my bread (thanks!), but I still didn't understand a few things. For instance, I know that I've had a more sour bread when I retard it in the fridge, but I've also had a more sour bread when I increased the proofing temperature to about 82 degrees. Why is that?
This graph from the article, I think, might help explain it. I realize now that it's a version of the graph that Jim posted, but at the time, I didn't realize that Candida Milleri in Jim's graph was yeast.
The key factor, if I'm reading the article and Jim right, is not necessarily the level of activity, but rather the respective levels of activity between the yeast and the bacteria. At room temperature (about 70-75 degrees F), for instance, the yeast and bacteria are roughly equal in activity. However, on the edges -- lower than 70 degrees and higher than 75, bacterial activity outpaces yeast activity.
Here's what the authors say:
The right temperature is the single most critical variable. Michael Ganzle and his co-workers did some studies on this. They found the following growth rates of L. sanfranciscensis and C.milleri as function of temperature. Growth rate is ln2/generation time, i.e. a growth rate of 0.7 is a generation (doubling time) of about 1 h.
The generation times measured in laboratory media are different from that in rye / wheat / white wheat dough. If the generation time at 20 C is 1/2 of that at 30 C in my medium, the organism will also grow 1/2 as fast at 20 C compared to 30 C in dough (we checked). So, it's not the absolute numbers that matter, but the ratio of growth rate to growth rate at optimum temperature.
There are a ton of other variables, of course -- the proportion of bacteria to yeasts in the starter, the species of yeasts and bacteria in the starter, etc -- but might this explain why both retarding the dough at a low temperaure and fermenting at a somewhat elevated temperature (~82-85 F) would produce a stronger sourdough than fermenting at room temperature -- because the bacteria are more active than the yeast?
Anyway, interersting article. I'd be interested to hear what others think. Is that your take, Jim? I think I'm finally wrapping my head around this stuff ....