The Fresh Loaf

News & Information for Amateur Bakers and Artisan Bread Enthusiasts

Technical Yeast Inquiry

RandMan's picture

Technical Yeast Inquiry

Hello everyone,

As a disclaimer, I know that there are a handful of past yeast questions on these forums. I have used the search feature and spent a while last night reading through some threads, but I still can't seem to find my information of desire.

What I am wondering about is how well commercial yeast thrives after incorporated into a dough or pre-ferment. On this site, I have read about people claiming that once a poolish or biga has doubled and is no longer growing, it has peaked and should be used at that point. The average time span being around 6-12 hours from my readings. I have read quite a bit of Peter Reinhart's books, Crust and Crumb and BBA in particular. He talks about making your pre-ferment and after it doubles, punching it down and refrigerating it overnight, or up to 72 hours. So, what happens after 72 hours? Or in the case of some of the posts I've read here, after 12 hours? Does the yeast die and start producing off-flavors?

I know that with a natural starter, it can be kept almost indefinitely by feeding it. I guess something I don't understand is why this can't be done with commercial yeast. Does commercial eventually die even if being fed, while natural doesn't?

All of this leads me to wonder why true pate fermentee, or "old dough" is possible. If on day 1 you make a final dough and reserve a piece for the next day, then obviously a portion of day 1 is incorporated into day 2. If you take a piece of day 2 and do the same thing for day 3, then day 3's dough would have elements of not only day 2's dough in it, but day 1's as well. What happens on day 200, when there are still minute quantities of day 1, 2, 3, 4, 5, 6 etc. doughs still being recycled day-after-day? Would this not imply that the commercial yeast is constantly being refreshed and rejuvinated over long periods of time? If so then I wonder why you can't just make a biga or poolish, put it in the fridge, and feed it just like a sourdough or levain over an extended period of time?

This post is the by-product of an immense amount of strong, black coffee and culinary curiosity. If any part of it too "all over the place" just ask me to try to reiterate and I will do so in a less caffeinated state of being.



sphealey's picture

=== I know that with a natural starter, it can be kept almost indefinitely by feeding it. I guess something I don't understand is why this can't be done with commercial yeast. Does commercial eventually die even if being fed, while natural doesn't? ===

The inestimable Ms. Wink will probably provide the full technical answer, but I believe the answer to this question is that to a certain extent you can (bakers yeast came down through the ages to us somehow), but eventually sour-forming bacteria will also start to grow in the refreshed starter.  Bakers yeast doesn't do very well in the presence of acid, so eventually it will die out and be replaced by more acid-tolerent yeasts.  Voila - a sourdough starter. 


richawatt's picture

I would suggest buying "bread" by jeffery hammalman.  He has a lot of info about yeast and what not.  Or just google yeast, and you will learn about commercial and wild strains.  But from what I know, you can keep refreashing commercial yeast.  The yeast itself will continue to reproduce and thrive as long as there is food available.  The yeast will not give you off flavors ever, it is the other bacteria that are in the culture that will.  Most of them die in the environment that yeast thrive in so they are not a worry if you have a strong yeast culture. However, lactobactillus will live with yeast, and they produce lactic acid.  This will eventually kill off commercial yeast for they don't like acid environments.  And lactobactillus are on every grain of flour.  So if you continue to refresh a commercially yeasted starter, it will die and the bacteria, both good and bad will take over.  However, there are also wild strains of yeast on every grain of flour too, so there is a chance that they will take over and your starter will once again be alive with yeast and you can refresh it forever.  You should try and make a sourdough starter.  It is not as hard as some poeple make it out to be.  Just takes time and patience.  And if you do try, try making two at a time, incase one does not take. 

proth5's picture

There is more in our bread dough to consider than yeast.

A poolish or a biga is made to give the benefits of a very long, slow fermentaion to some of the flour.  But if that fermentation goes on too long, not only has the yeast exhausted its food supply (and, of course, we are introducing more food and more yeast in the final dough) but the gluten will begin to degrade.  Use a totally exhausted (collapsed) pre ferment and you are asking for trouble not just because the yeast isn't viable (because you are adding more...) but because you have taken the benefit to a detriment.

I have never been able to read Mr Reinhart's books (we apparently have such different teaching/learning styles that I  gave away the only book of his that I purchased) and I have not, in my experience, made a pre ferment, punched it down, and refrigerated it.  This might be a way a simulating a retarded ferment on a pate fermentee.  I don't know.  I would not do that for my normal commercially yeasted pre ferments.

But the same principle applies with retarding doughs.  After a point, the dough itself degrades. Not just the yeast, the dough itself.

With a pate fermentee, yes a bit of the old dough (and the old yeast) is perpetuated.  The old yeast might be dead, the old dough degraded, but this is small as a percentage of the final mix.  And again, we add more food and more yeast.

I tend to think in practical terms.  Commercial yeast is there to make quick work of making bread.  It comes in convenient form.  We don't need to feed the stuff every day - it is happy dried and stored in a cold place. While it is theoretically possible to perpertuate a  - I'll need to call it  - "a culture" as some sort of yeast supply this is what the yeast companies do for us.  They maintain a strain of yeast under very controlled conditions so that it stays consistent over time.  So, of course, you can just keep feeding commercial yeast and it will continue (like the fungus it is) to grow and reproduce.  Some of the fungi will reach the end of their lives; they will die and need to be dealt with.  Waste products will accumulate in the culture and need to be removed if everyone is to be happy. Conditions must be just so, or the particular variety of yeast that was to be cultivated will be unable to live. (Ok, this is sounding live keeping an aquarium vs keeping a pond - so if you have ever done those two things you know what I mean...). But the fast activity and easy handling of commercial yeast comes at a price.  It must be fed carefully and it needs special conditions to thrive.  It is not as robust as wild yeasts.  (If you are a gardening sort, think tea rose vs rosa rugosa - both are roses and both will grow and reproduce, but the rugosa will do it with much less care. Also see the aquarium/pond comparison.) In the environment of the average kitchen, the introduction of wild yeasts from flour, air, hands, etc will most likely lead to some kind of "sourdough."

If you want sourdough, start and maintin a mother culture.  If you want commercial yeast (which behaves quite differently in your baking, by the way) best to buy it.

Hope this helps.

davidg618's picture

...that was originally made with commercial yeast, ala Greenstein's  "Secrets of a Jewish Baker", that I feed weekly with rye flour. It's as viable now as when I first made it. Admittedly, the commercial yeast may be replaced by wild yeast(s), but I doubt it. However, I've no way to prove it. A microbiologist likely could identify the dominant yeast and bacteria strains present now, but I certainly don't have the wherewithal to do so. I only have the benefit of my senses, and I see no apparent difference in its performance, smell, or texture from its original conditions.

I don't use the Rye Sour as my primary levain (I add Instant Dry Yeast to my rye doughs that contain the Rye Sour) but I build 25 oz. of fresh Rye Sour, from approximately 4 oz. of stored Rye sour. It expands, and develops flavors, now three months old, subjectively indistinct from those produced when first developed. 

And recently I baked baguettes made with a poolish pre-fermented for 12 hours at room temperature, incorporated into a dough chill-retarded and bulk fermented for 17 hours at 55°F, pre-shaped, and rested for 1 hour at room temperatured, shaped and final proofed for one hour, and finally baked revealing excellent oven spring. All this yeast activity started with 1/8 tsp. of Instant Dry Yeast in the poolish. 

The point I'd make with these two examples is, given the right environment (flour, water, proper PH range, and temperature) commercial yeast will continue to multiply. I consciously rely on it doing so in the baguette formula and techniques I use. 

I wouldn't push dough development beyond 24 to 36 hours. My retarded commercial yeast doughs stay strong during that time frame, 0% to 30% of their flour pre-fermented. However, I've had sourdough's made with 40% preferment I left unfed for 36 hours, in an attempt to develop sourness, that yielded a very slack dough that no amount of manipulation strengthened.

David G

proth5's picture

So, if the rye "sour" has exactly the same smell as the day you made it, how is it sour?  

Again, I have not read the book that you mention, but if I just took rye flour, commercial yeast, and water and mixed them together, the result would not be sour at all.  Perhaps you left it at room temperature for many days so it had the chance to become sour?

Since rye is a very rich source of wild yeasts, one must ponder if it became populated with wild yeasts at a very early age and the commercial strain faded quickly so it has behaved consistently since you first began to use it.

I don't know, but it does seem odd that the very same strain of commercial yeast that you added would survive under kitchen conditions in a rye flour mix - especially if it became sour. Years ago we used to spike our nascent sourdough starters with commercial yeast - which, of course wasn't needed - it might have given us the feeling that something was happening while the wild yeasts had time to take hold.

But it could happen.  Again, yeast is just a fungus, and if you provide the right conditions it will multiply (as it does when you take your mature poolish and give the yeast more food.) But in terms of the sour and its use with rye, it does seem like something must have changed so that you have a rye "sour" and not just a rye/water/ commercial yeast mix.

Sorry to pick at nits, but your opening paragraph was just too intriguing.

davidg618's picture

Is more a name, a label, than an attribute.

Perhaps your nit-picking supports your particular beliefs about natural levains.

I stand by what I wrote, especially that, unless we isolate, and identify the critters that are in our starters, cultures, barms, sours, sourdough, etc. etc. we have only our observations and faith that they contain the "right" yeasts and bacteria.

FYI, since you don't have the book, The Secrets of a Jewish Baker prescribes a Rye Sour as a mixture of commercial yeast, rye flour, water, and minced onion; caraway seeds are optional. 100% Hydration. Just like sourdough, a quantity of fresh Rye Sour, sufficient for the dough formula, is developed over 24 hours; and seed Rye Sour, again, just like sourdough starter, can be stored in the refrigerator or any other reasonable temperature you choose. I choose to keep mine in the refrigerator, along side my three sourdough starters. 

According to some sourdough beliefs, I'm wasting my time keeping more than one, since all of them will eventually be identical.

Hasn't happened yet.

David G

sphealey's picture

The brick-oven-building book, _The Bread Builders_, has an appendix containing an extensive interview with a scientist at the Federal Republic of Germany's Institute for the Study of Grain-based Foods Leaved with Sour-Containing Organisms (that's all one word 75-letter word in German, and I had my son translate it for me just for the amusement value!).  There's a lot of fascinating information in the interview and some overviews papers listed in footnotes about the interaction between yeast and sour-forming bacteria, and also what happens to bakers yeast in a sour environment.  Well worth getting from the library and reading!


proth5's picture

that book (and vaguely remember the appendix) and very soon - after I cross the Pacific and get enough sleep so that I can comprehend stuff - I will re read.

Thanks for the reference.

proth5's picture

"Picking nits" is just my way of trying to understand this whole mystery of wild vs commercial yeasts.  I haven't yet subscribed to a fixed set of beliefs - just trying to explore.

I've often wished that I could get an anlysis of what lives in my levain.  That would tell us all so much.

You must be particularly meticulous in your starter care.  I reconstituted a starter that originated in Okinawa when I returned to the US and although I thought I had been very careful, within four days I couldn't tell the difference between it and my "old faithful."  I am convinced that I contaminated it somehow.

Typically when we create a "sourdough" we are seeking that balance of bacteria and yeast because it brings a different set of characteristics to the bread. So, here's my next question just on a practical level.  If the original was just developed over 24 hours with commercial yeast - why hold any back and use it again?  Since it doesn't change or develop any special characteristics over time, why bother? Just to avoid using commercial yeast?

I really am curious about this - not trying to be difficult...

davidg618's picture

expecting it to change, develop more flavor or complexity, but when I thought about it further, I concluded that wouldn't happen. I take only a small amount of stored sour, and refresh it two or three times over a brief 24 hours building the volume needed for the target formula. Consequently, it's mostly new sour, and is populated with the micro-organisms dominant in the kept sour. (not necessarily to the same densities, however). Furthermore, it's likely significantly less acidic than the stored culture.

The same is true for my sourdough starters. All of them were purchased, none were started from scratch. I am fairly meticulous, and disciplined feeding my starters. I feed weekly, never exceeding 10 days between feedings. I keep only a small amount of the stored starters, usually only 50g each. I feed to a 1:1:1 ratio or 1:1.5:1.5 ratio. One starter, the oldest (about 1 year) alleged to be SF, I maintain entirely on KA bread flour. I've maintain a second starter (Ischia Island; about 5 months old) in two cultures: one fed with KA bread flour, the second fed with KA First Clear flour. I clean each starter's container (glass) thoroughly each feeding using alkali cleaner I purchase to clean my beer and wine fermenters. They are rinsed thoroughly. Although we enjoy excellent, chlorine-free well water, I feed my starters with that water filtered through a 0.5 micron filter, and activated charcoal. 

I should also say I create all my formula-ready levains with a three-build approach over a 24 hour period. Here's a link to how and why I do that.

It's probably not necessary; most of the baking books prescribe a single build, in half to a quarter less time, but I have the time to do it, and it's proved 100% consistent.

The SF starter is the second most active yeast-wise, and despite its alleged origin, the least sour. I've experimented with this starter: stiff, liquid, cool, warmed, and long un-fed periods of time. There has been no noticeable change in sourness regardless of how I treat it. Since I acquired the Ischia Island starter I rarely use this one. I could discard it, but probably won't. It's almost like an old friend;-)

The First Clear flour fed Ischia Island starter is the most active yeast-wise, and I can coax increased sourness out of it by holding it at 89° to 90°F during its 3-build cycle. This is my most frequently used starter.

The Ischia Island starter fed with bread flour is the least active yeast-wise. It exhibits a lag time, greater by about a factor of 2 than the other two starters, I haven't a clue why. I've never tried to make this one more sour. I keep it for when I want to bake a nearly all-white crumb. I could discard this starter and build all-white levain from the First Clear flour fed one.

All three starters double their volume in three or four hours when I build them, and final proofing times for the sourdough formula I bake weekly, only varying which starter I use, range from 1 hour to 1.5 hours at 76°F. 

I'm satisfied with all three sourdough cultures, but will likely  discard the Rye Sour for the reasons you stated. I can build a new one in the same time it takes to create formula-ready levain from the saved sour.

Despite all the researching I've done, and the amateurish attempts I've made to alter my sourdoughs' flavor, sourdough is still a mystery to me. It's a muti-variate problem wherein the major influences, e.g., food, acidity, and temperature are, with the exception of temperature (sometimes), variables themselves. I'm still searching for the reference that gives me clear, and kitchen-applicable cause and effect guidance.

I'm reasonably aware what laboratory tests large-scale commercial brewers and vintners perform to monitor their products, toward maintaining consistency. Additionally, I own a pH meter, an acid test kit, and can purchase one-time test strips that estimate pH, sulfer-dioxide content, and malolactic acid conversion progress to aid me in my home brewing and vinting. Artisan bread-baking involves chemical processes akin to brewing and wine making. I expected to find a plethora of easily obtained information similar to that I've acquired re beverage fermentations. How wrong I was.  What I've found, at the risk of offending, is peppered with myth, legend, misinformation, and beliefs that defy Physics, Chemistry, Thermodynamics, and Biology 101. Yes, there are nuggets of truth to be found, but even they are sometimes obfuscated.

Enough ranting.

 Like yourself, I'll keep trying to understand, in part to satisfy my incurable curiosity, but mostly to bake good, healthy, flavorful bread consistently.

David G.

proth5's picture

No prob on my end.  Of course I'm enduring that day of slow brain function after crossing the Pacific.  I say I don't get jet lag, but...

Interesting stuff.  And you are particularly meticulous with your starter maintenance routine which might explain why your starters maintain their character.  I'll wash my hands and my spoons, but nothng so intense as your routine. 

I guess that what I was getting at with the rye sour is that in most cases we would leave it out at room temperature for an extended period of time until it did change and develop whatever characteristic we were seeking and then refrigerate it.  Under those conditions my suspicion would be that the commercial yeast would get out competed by the wild yeasts. If there were no change from the 24 hour "rye sour" in the formula - I would just make the thing up fresh when I needed it.  But I'm an engineer/baker - so really, only semi-skilled labor.

Of course, there are those who would contend that feeding one starter clear flour (with its greater ash and therefore greater utrient content) and the other bread flour is the thing that makes the difference.

I went through a long period of wondering precisely why my starter acted like it did and I still do from time to time, but not having sufficient time to devote to it as of late have settled on the "this is what I experience" approach.

There is a lot of science to be had in this area, though, and some of it on these pages. 

There is also a lot of legend, truth be told.

But even without the science, I'm a pretty consistent bread baker.  I think it is the nature of the craft that most of the important variables can be smelled, tasted, seen or felt, so although it it a complex thing practice makes better.


davidg618's picture

those that argue my two starters are different because I feed them differently. That's precisely the reason I do. I feed 1st clear flour specifically because of its elevated nutrient content. I would have been disappointed had I not seen observable differences. Fortunately, I do. 

What I don't subscribe to, although I'm open to sound arguments--haven't seen any yet--that established cultures are easily and preferentially replaced by local wild yeasts and bacteria, air borne or in the flours fed to them.

San Francisco sourdough cultures are well documented to be strong, symbiotic relationships, whose scientific basis is well understood, including their resistance to outside contamination. I haven't found scientific papers claiming other yeast/bacteria combinations form similar symbioses, but I think its a good bet they exist, and are dominant in many stable sourdough cultures throughout the world.

That's why I've purchased two of the starters, alleged to be viable cultures, and endorsed by other bakers, rather than try to create my own. I have great respect for those among us that do, but that's not my thing. First and foremost I want to bake good breads, sourdough breads especially. That's why I treat my starters as I do, and they never have failed me, nor do I expect them to.

By the way, I've followed your postings closely for months. I've learned baking tips from them regularly, and, vicariously, I enjoy too your experiences, in the foreign countries, you share. Retired now, I once traveled world-wide, and, for the most part, loved it. Frankly, traveling, especially ocean travel, is the only part of my former work I miss--every day.

David G


proth5's picture

 - about the local yeasts taking over, because I've got no data.  One would have to speculate that if the local yeasts are a particulaly vigourous variety and can survive in a number of environments, over long years, it could happen (especially for those of us not as meticulous as you), but without data I won't say it does. I'm convinced my poor old Okinawa starter "went Denver" because I contaminated it.  I do suspect that changing from Okinawan to American flour had some effect, though.  I can't speculate on the nature of the "critters" but that Okinawa flour is quite different.  I mailed some of it home so that I can send it to the lab and see what gives.

My starter is pretty reliable, but I this year my baking has taken a hit while I have been traveling.  I enjoy being in places (and have been all over the world for my job, too) but these days there is no fun in air travel. None at all...


Mini Oven's picture
Mini Oven

to create a communication loop with other yeasts?  This growth hormone seems to also stimulate other fungi, yeasts as well.   Follow this trail to the study... if you know what you're looking at.   I don't want to start a big discussion on the medical effects of this study but it is interesting that the yeasts do respond to each other producing acid.  Whether we can taste it or not is another question. 



davidg618's picture

Hi Mini,

I guess the point of my "So What?" is what does this mean when related to yeast behavior in bread dough?

I'd make two point:

1. It appears, in the reported study, a strain of Saccharomyces cerevisiae was used to model infectious yeast behavior. There are many many strains of S. cerevisiae, I suspect the strain used in this study is one frequently used in laboratory experiments. Likely its genome is mapped, and its laboratory behavior is thoroughly documented. I imagine its efficacy as a baker's yeast is unimportant.

2. I'd like to know if there is any advantage for baker's yeast strains of S. cerevisiae to have developed the same behavior.

David G.