The Fresh Loaf

News & Information for Amateur Bakers and Artisan Bread Enthusiasts

More than you want to know about Labs and Yeast

dabrownman's picture

More than you want to know about Labs and Yeast

Yerffej's picture

It is not more than I want to know but it is far more than I am willing to decipher.  If anyone is inclined to make a summary of this in English, it is a summary I would look forward to reading.


FlourChild's picture

I was all set to find a new and interesting article full of insights... then realized I already had this one bookmarked.  My science background isn't good enough to express the limitations, etc. of this paper, but I've learned a great deal just from looking at the pictures.  :)

For instance, figure 2 shows two graphs, the one on the left shows how sourdough yeast don't care a whit about pH (straight line), while lactic-acid producing bacteria change their behavior dramatically depending on how much acid is already present (u-shaped curve).  The graph next to it shows how both react to salt- both are retarded, but LABs more so than yeast (steeper decline).

And figure 1 shows that the LABs experience peak activity at about 32C/90F, while the sourdough yeast peak at a lower temp, 27C/81F.

Figure 3 shows that sourdough yeast are inhibited more than LABs by some of the by-products of sourdough fermentation, especially acetic acid and ethanol.

Cool stuff.

dabrownman's picture

has it right.  Yeast is effected more by high and low temperatures,  ethanol and acids than the Labs.  Salt and and low ph effects the Labs more than the yeast.  If the Labs run out of the sugars they need (that the yeast produces for them) they start producing ethanol instead of acids.  Oddly, research suggests than Labs produce a little less than half the CO2 in a SD culture and the yeast a little more than half - who knew Lbs produced CO2 and ethanol if they run out of food?  Higher temperature effect yeast more than Labs where Labs peak at 32-33 C and yeast at 27 C.

For those that wanted to know why baking soda was in SD recipes it might be to counteract a low possible low ph condition that effects Lab growth rates adversely, especially in long retards where the yeast isn't producing the food that labs need and the Labs are reproducing 3 times faster than the yeast.  Baking soda would also help at high temperatures where labs thrive also reproducing at 3 or even 10 times faster than yeast which don't like the heat and a low ph condition could result.

The results show that for non white flour breads, the labs and yeast should have plenty enough to feed on and not run out of food  to eat.   White flour is not so nutritious for Labs and yeast to feed on and ph, temperature and other factors are more important.  

Since yeast  produce much if the food that Labs eat you want them active enough to provide it at the end of the proofing cycle - suggesting and 85 F final proof temperature but the labs will have plenty to eat at the beginning so long cold retards 36-40F will allow the Labs to reproduce 3 times faster than the yeast and have enough food to eat without having to rely on the yeast to provide it.  The higher Labs to yeast ratio you can provide in the cold beginning retards and the hot end proof will give you the most sour at the time the dough is risen to bake properly.

When yeast has eaten all the food and gone dormant - not producing food for the Labs, Labs start producing ethanol.  So now we know where some of the top floating ethanol hooch comes from in an unfed starter. 


Fred Rickson's picture
Fred Rickson

Lactobacillus sanfranciscensis (bacteria) and Candida milleri (yeast).

That was an interesting research paper, and it was published in Science so it should have been well reviewed before publication.  Here are a few of my take-aways.
C. milleri optimal growth is at 27C/80.6FL. sanfranciscensis optimal growth is at 33C/91FC. milleri and L. sanfranciscensis can grow within pH 3.5. - 7.0And, a couple of quotes...."Our data are furthermore in agreement with the “baker’s rule” that low temperatures during sourdough fermentations (20C/68Fto 26°C/78.8F) are better for yeast growth than higher temperatures.""In agreement with this assumption, it was observed that the addition of NaCl (salt) to wheat doughs inhibited the growth of lactic acid bacteria while it exerted a stimulating effect on yeast."One thing I am not sure of is how much the bacteria and yeast, individually contribute to flavor.  The simple idea has been that yeast are the major flavor providers, and the bacteria produce only the "tang" of acid.  But, I doubt this because many bacteria are famous flavor producers....yogurt, cheese, sauerkraut, kimchee, spoiled milk for just a few.  While good bacterial growth should produce acid and its tang, I sort of doubt that simply encouraging yeast growth gives all the flavor we enjoy.And, despite consistent random comments that cooler dough temperatures promote bacterial growth, and therefor more acid and tang, the opposite is true although the optimum growth temperature for both organisms is high for a kitchen.  Look up an earlier piece by Debra Wink on lactic acid fermentation for a nice summation of bacteria and yeast growth.Hope this helps.
dabrownman's picture

low temperatures promote lab or yeast growth - they don't   At 36 F labs reproduce at a rate 34 times less than do at 90 F and and yeast reproduce at 39 times less.  The point is that the lab to yeast reproduction ratio is 3.78 at 36 F and 3.26 at 90 F.  At high and low temperatures, the labs are far outproducing yeast by a factor of at least 3 or more, while at room temperature of 68 F to 75 F the reproduction rates are about the the same with the lab to yeast ratio at about 1.

Low temperature retarding for 36 hours will give you the same amout of labs in the dough as 1 hour at 90 F.  But it would take 39 hours to to have the same amount of yeast that 1 hour at 90 F would produce.  If you want sour you want to maximize lab reproduction and minimize yeast production over time.

This points to long low temp retarding of starters, levains and dough with short high temperature final proof to maximize labs while minimizing yeast - while building the starters and levains and fermenting the dough at 85 F to 90 F.  Since the lab to yeast ratio never goes below 1, if you want the least sour bread possible just do everything at 72-75 F where yeast production is greatest compared to labs. 

The two other things I thought odd were:  Labs produce ethanol when they run out of food instead of acid and that labs produce about 50% of the CO2 too.


Fred Rickson's picture
Fred Rickson

The data in the paper are pretty straight forward; that particular yeast grows better at temps a little cooler than that bacterium seems to enjoy. How you play with your build, messing with yeast and/ bacteria ratios is up to you.  The paper only deals with growth curves by organism, not how your bread tastes.  You use these data however you see fit.  


dabrownman's picture

data from Ganzel that was not included in the original postt.  I have a chart of the data but this site won't allow charts to be posted.  The data shows that neither yeast nor Labs lke the cold at all, but yeast is more adversly affected by the cold than the Labs.  This data shows why 36 F along with time and 90 F are so effective at producing the labs that make sour in bread with a SD starter.   Hope this clears up the confusion.  This info came from doc.dough.

 growth   rate equationa(k-T)^b*exp(c(k-T))
 L. SF IL.  SF IIYeast (C-Milleri)


T(°F)T (°C)L. SF IL. SF IIYeastratio   L-SF-I/yeast
Levin bred's picture
Levin bred

But the growth curve directly affects how the bread tastes, so the paper does in fact deal with flavor.

Dabro's point is that if you want a less sour bread avoid temperatures in either extreme (30s or 90s).  By keeping in the mid 70s (judging by the chart the most even ferment looks to be about 75-77F) you will not run the risk of having a bread that tastes like curdled yogurt.  A ferment in the fridge, while producing less bacteria overall will still, over time, produce a more sour loaf.  Something fermented in, say, a proofing box, will behave the same way, only more quickly.


A belated thanks for the growth chart Dabro!  It is probably more helpful than a lot of people here realize!

dabrownman's picture

down some recent research that may change the way we think about bread fermentation.  I have heard that they have discovered that Labs may actually provide half of the CO2 in SD breads.  We think that yeast supplies the CO2 for rise and Labs supply the the acid for tang.  To make sour we want to reduce the yeast to allow a longer time before proofing is complete and the bread needs to be baked while promoting Labs to produce more acid for as long as possible.

If it turns out he Labs are producing CO2 as well as yeast do, we will have to rethink our methods on how to make bread sour.

Bigblue's picture

Such a helpful thread. Thanks for all the work here.