The Fresh Loaf

News & Information for Amateur Bakers and Artisan Bread Enthusiasts

leavening versus fermenting

Lumpynose's picture

leavening versus fermenting

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; 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.

Mini Oven's picture
Mini Oven

yeast ferments food too.  It is possible to ferment without Lactobacilli and just use yeast.

Lumpynose's picture

Even if you don't add lactobacillus to a bread recipe (via a starter) the lactobacillus is there and starts fermentation, although not as quickly.  Using a starter gives the lactobacillus a head start in the dough.

Mini Oven's picture
Mini Oven

Yes, lactobacilli are in the flour, but with a short commercial yeast fermentation of the dough, the lactobacilli (plural) don't really have a chance to do anything. Even an 8 hour rise at 25°C will not show much lactobacilli activity, yet, the dough ferments! The yeasts are digesting flour and burping CO2 gas.

Yeast is an aggressive fermentor. Take two mixtures of 100% hydration flour & water, now add just a pinch of yeast to one of them. Let them stand covered overnight (23°C) and then inspect the differences. The one containing yeast is fermenting and beginning to show signs of deterioration with by-products like gas and alcohol. The other? well... the flavor has improved but signs of fermentation are absent.

Just ask someone who's grown a wild starter. The bacteria have to wake up and start increasing in numbers before the natural yeast can wake up and join the party. Nothing is notable in the first 24 hours.

Chuck's picture

The idea that only the lactobacilli in a starter are really of any importance gained a lot of favor way back in the 80's. For that reason, for a while the standard advice (I got it from Sunset Magazine!) was to feed your starter with milk rather than water. (That was fine  ...until something got contaminated or spoiled, then almost always the starter was a complete loss and had to be entirely thrown away.)

One reason that shooting for a pure lactobacillus culture at home may not be such a good idea is that such a culture could easily be "invaded" by other bugs  ...some of which are toxic. A balanced yeast/bacteria culture is very stable (highly resistant to invasion). It's more or less the case that each one sets up a prime environment for the other  ...and only the other.

The "sour" flavor is already available in a dried form like yeast. I know of two different ways to access the concept:

One is "sour salt" (citric acid). You have to be careful with sour salt though, because the line between "enough" and "too much" is rather hazy, and "too much" results in bread that tastes like lemons.

The other is a real "instant sourdough flavor". It's available at least from King Arthur Flour, and probably elsewhere too. My experience is it's very easy to use and tastes great  ...but you have to be careful not to tell your eaters about it, as it's quite likely to be thought "not PC". (The other downside is it's pretty expensive.)

Lumpynose's picture

"feed your starter with milk rather than water"

The problem with that, as I understand it, is that there are different lactobacillus (subspecies, or whatever it's called), and the ones that ferment milk into yogurt aren't necessarily the same ones that ferment grains.  This same misconception pops up regularly with vegetable fermenting; somewhere along the line some people got the idea that you can jump start a vegetable ferment with the clear runoff from yogurt (the whey), and the ones that know better keep pointing out that that doesn't help.

The other factor, which I left out, unintentionally, is the enzyme activity during a long rest.  That could well be what Hammelman was talking about with the "flavor components."

I'm not proposing that the starter not have any yeast; as you rightfully point out, it's needed for the proper environment for both the lactobacillus and the yeast.  I think I'm more just proposing that relying on the wild yeast is more problematic than it's worth; just ignore it and use commercial yeast, since both types of yeast provide leavening, and probably the same flavor advantages.  But commercial yeast is more reliable and predictable.

Chuck's picture

... feed your starter with milk rather than water ... the problem with that...

I thought I was pretty clear this was a very bad idea that I heard about thirty years ago. Sometimes what seems clear to me in the writing only comes through rather garbled to others in the reading; so let me re-state it quite baldly: do not even think about doing this!

placebo's picture

I think I'm more just proposing that relying on the wild yeast is more problematic than it's worth; just ignore it and use commercial yeast, since both types of yeast provide leavening, and probably the same flavor advantages.

I don't find relying on wild yeast problematic. It's been pretty predictable here. It definitely takes a lot longer, however, and I can certainly understand the desire to crank out a loaf in a few hours instead of spending the better part of a day or two to produce one.

To me, the disadvantage of using commercial yeast is that it leavens too quickly. It's the long, slow rise of sourdough that allows the flavor to develop. I've made bread that relies on only the starter for leavening as well as those that use commercial yeast in conjunction with the starter. Both methods produce loaves that taste good, but I definitely prefer the taste resulting from a slow rise.

Lumpynose's picture

Does the citric acid provide the all of the same advantages of lactic acid?  In addition to the flavor there's dough conditioning and making the bread last longer.

pmccool's picture

then you may want to refer to some dictionaries or other sources, Lumpynose.  Fermentation is generally regarded as a process, often but not necessarily anaerobic, that converts one kind of molecule into one or more (usually simpler) other molecules.  For instance, sugars to alcohols or alcohols to acids.  Fermentation can be accomplished by bacteria, by yeasts, or by enzymes in the absence of biological organisms.  So, you are correct in your assumption that lactobacillus bacteria are involved in (some) fermentation processes but incorrect in your assumption that yeasts are not involved in fermentation.

Carbon dioxide is one product of yeast fermenting sugar into alcohol.  That does leaven our breads.  The leavening in yeasted breads is not a process that is separate from fermentation.  Rather, it is a by-product of fermentation; specifically fermentation caused by yeasts as they digest flour.  If you are acquainted with salt-rising bread, you are also aware that leavening can be accomplished through the activity of a bacterial fermentation.  Finally, leavening can also be accomplished with chemical agents (baking powder, baking soda) and with foams (think of angel food cake).

And, yes, there are a number of ways that one can manipulate a culture's environment to encourage or discourage different populations of bacterias and yeasts.  Quite a bit has been written here (feel free to use the Search tool at the upper left-hand corner of the page) and in much greater detail in various peer-reviewed papers.  Dive right in; there is some fascinating information available.


proth5's picture

Thanks, my friend.  I told myself to let this one go...

pmccool's picture

was better than mine, Pat.

I learned something in the process though: the bit about fermentation occurring in the presence of enzymes. That was an interesting addition to my growing collection of not-particularly-useful random facts.


ehanner's picture

The idea of being able to adjust the composition of dough with flavor additives is what got the bread industry in trouble in the first place. Regional differences in taste make it impossible to arrive at standards for what "good" bread should taste like. As others have said so eloquently above in one form or another, fermentation happens under many conditions. Learning to control it with hydration, temperature and food supply choices are the keys to "good bread", what ever that might be.

The first time I followed the recipe of Anis Boabsa of Paris was just after he won the annual "Best Baguette in Paris" contest. His procedure is to mix the dough with cold water using a very small amount of yeast and after a short time of bench time, refrigerate the dough in bulk for 21 hours (approx). I can tell you that this is wonderful bread made with only commercial fresh yeast. Apparently the judges all agreed that this is "good bread".

Since the word police are watching, I would point out that it is Hamelman, with one m in the middle.

A very interesting thread you started lumpynose. You are among friends here. Many IT types and such.


sam's picture

According to the Handbook of Dough Fermentations  (and I hope I am not violating any copyrights)...

Page 45 lists 7 types of Saccharomyces yeasts found in wild sourdough cultures, and another 8 more yeasts of different types (Candida, Hansenula, Pichia, Torulpsis).    Of course, your mileage may vary in terms of what is living in your own particular culture -- those are just the ones identified from testing.

Saccharomyces cerevisiae seems to be the most common of the yeasts.   I could be wrong but I think that is what most "baker's yeast" is also.   

Page 52 has a table called:  Yeast-Produced Flavor Compounds In Sourdoughs.   It doesn't break down which yeasts produce which compounds.  I won't type out all the individual compounds, but the general categories and flavors are:

1) Alcohols, with flavors of:   Alcoholic, Fusel-like (burning).

2) Carbonyls, with flavors of:  Malty, Butter, Fruity, Green (fatty), Aromatic (sweet), Pungent, Rancid, Rancid Butter, Sweaty, , Unpleasant (copra-like), 

3) Esters, with flavors of:  Ether (pineapple), Roasty, Fruity, Apple peel, Banana, Pear (bittersweet), Rum (pineapple), Pineapple (banana).

It also specifies that LAB produces some of those compounds as well.   I guess the point is, even if you use plain ole baker's yeast of Saccharomyces cerevisiae and no sourdough culture, while you may or may not get as many of the different flavor compounds as a sourdough culture, you should still get good solid flavor in the bread, especially if you do pre-ferments via poolish or cold-retarding.    Yeast is not just about generating CO2 to make bread rise.   Of course, your taste buds as your ultimate guide.