I've been thinking about leavening and fermenting with bread making. The books I've been reading are Jeffrey Hamelman's Bread, Peter Reinhart's Whole grain breads and Artisan breads every day, and Chad Robertson's Tartine bread.
Both Peter Reinhart and Chad Robertson state that the sour flavor for a sourdough comes from the Lactobacillus sanfranciscensis bacteria, not from the yeast. The Lactobacillus produce lactic acid which makes the bread taste sour. This is what I call fermenting.
In a sourdough starter the wild yeast produces gas, and this is what I call leavening. Likewise, commercial yeasts provide leavening, except that they're faster than wild yeast and predictable.
When reading about bread making and pizza making, people use the word fermenting to describe part of the bread making process when I think what they really mean is a combination of both leavening and fermenting. It seems to me that fermenting is a catch-all word for a long rest period for the dough; for example, "bulk fermentation." Coming from the fermented vegetables background (for example, sour kraut, kimchi, and fermented pickles, where the vegetables are put in a brine solution for several weeks) I think of fermenting as the souring process from the Lactobacillus bacteria.
As an example of the terminology problem, in Hammelman's Bread, starting on page 13 he describes bulk fermentation but he mixes together the actions of leavening from the yeast producing gas, and fermentation saying that fermentation produces the superior flavors. He talks about the "production of organic acids during fermentation" without explaining how they're produced. He goes on to say that organic acids develop slowly and take hours before there are enough to benefit the bread's flavor. Nothing incorrect there, but things could be more carefully delineated and explained.
The same is also true for The Yeast Treatise at theartisan.net; fermentation and leavening are being conflated.
When describing bulk fermentation and the role of the temperature of the dough, one of the interesting things Hammelman says is that "the flavor components in the dough prefer temperatures lower than that required for maximum gas production." By "flavor components" I'm assuming he's talking about the Lactobacillus bacteria's activity. This no doubt explains how these no knead recipes work where you put the dough in the refrigerator for several days; the yeast activity is greatly slowed down while the Lactobacillus activity is slowed down to a lesser degree.
Back to the leavening side, if you're using a no-knead recipe where the bread sits for several hours and you do a stretch and fold periodically, you should do the stretch and fold gently, so that you don't squeeze out the gas that's in the dough from the yeast. This shows that leavening is occurring during the inaptly named bulk fermentation step.
For some people this may be hair splitting terminology. Before I retired I was a computer programmer and systems administrator and in that field it is crucial to always use the correct words (and not mash things together) when describing things. So I think this hair splitting is helpful for understanding the different things that are going on in the bread dough.
One new thing that I learned from Robertson's book is that for him a starter isn't just a starter; there are desirable starters and undesirable starters. An undesirable starter is one that's excessively sour. A desirable starter is one where the wild yeast is very active and the Lactobacillus is just getting up to speed, although he doesn't explain it that way and instead uses visual and olfactory clues (very bubbly and doesn't smell a lot).
Because the Lactobacillus are doing the fermenting and improving the bread's flavor and not the wild yeast, I think this is why bakers (for example, Peter Reinhart) get good results by using commercial yeast in addition to a sourdough starter. The starter is mainly seeding the dough with Lactobacillus bacteria for the fermentation and the commercial yeast provides the leavening. The starter may or may not have a good population of wild yeast, but in any event the commercial yeast produces a quicker and more predictable rise.
After thinking about this, one idea that I've had is that it should be possible to redesign the starter so that its recipe favors the Lactobacillus bacteria; the only yeast it needs is whatever is necessary to keep the Lactobacillus happy. Then, in the bread recipe, use commercial yeast for the leavening and use the starter for seeding the dough with Lactobacillus. I'm speculating that with the correct amounts of starter, yeast, and fermentation time that a good bread can be made. And probably without the long three day period that's currently necessary.
Rising times with commercial yeasts are undoubtedly well known and documented; for example, a percentage of yeast (using baker's percentages), a hydration range, and a temperature range will yield an appropriate rise in so many hours and minutes. Then, all that's needed is knowing how long of a fermentation period is needed for the Lactobacillus, how much Lactobacillus, at what temperature, etc. Matching the correct amount of yeast with the correct amount of Lactobacillus for a particular temperature, hydration, and period should yield a good loaf of bread.
All that's needed is for some enterprising food scientist to culture and dry Lactobacillus so that in addition to buying instant dry yeast we can also buy instant dry fermentation.