The Fresh Loaf

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Is a visual peak, really The Peak?

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davidg618's picture
davidg618

Is a visual peak, really The Peak?

Disclaimer: I am not asking the question I am about to ask, because I think common wisdom is wrong, and I have a better answer. I don't have an answer, I'm curious, and I've never seen an answer to related questions that wasn't in some way cast in The Common Wisdom.

Common Wisdom: A preferment (poolish, biga, etc., or sourdough starter) is at its peak--"peak" implying maximum yeast reproduction rate, or maximum yeast population size (I've never been entirely certain which)--is indicated by the preferment having reached its maximum expansion, and/or just beginning to collapse in its container.

My starter, at room temperature (~76*F) visually peaks in 7 hours.  Since I started baking sourdough, seriously, I've followed Common Wisdom mixing my final doughs as near to the moment my final levain build reaches a visual peak as is practical. The only exception has been when I intentionally let the final build ferment additional time, up to sixteen hours total time, trying to create the elusive sourness. Even if one or more fellow TFL contributers gives me clear, unabiguous and non-conflicting answers that defy Common Wisdom, I'll continue doing what I do now, because I'm content and happy with the consistent results I've finally obtained. This really is only a curiosity only post.

The Question: Does a visual peak, in fact, herald the yeast's maximum point of development measured directly by its rate of reproduction, or its maximum population size, or some other yeast-related paramenter?

I ask this question for two reasons. First my head is awhirl with other, physical and chemical phenomena: mass, temperature, surface tension, partial pressure, gluten tensile strength, gas aggregation (bubble size), proteolytic enzyme activity, and gravity which I imagine effect the point of collapse. (There is likely others I haven't listed.). Secondly, I've had occasion to stir-down active starter at its visual peak, and left it to rest (intentionally, and unintentionally) for a few more hours finding it again peaked at an expansion essentially identical to the first expansion. Furthermore, Except for  approximately+15 minutes of final proof time, I've not perceived appreciable variation in final proof surface tension, loaf expansion, or oven spring when I use levain that fermented up to eight hours beyond its visual peak in loaves that are within practical limits otherwise identical.

It's a great rule-of-thumb. I suspect it's been used for millennia. I intend to continue to use it. But what is it really measuring of value or a quantity associated directly with the yeast's performance.?

David G

Perhaps a better question is, "Who really cares? It works!

ananda's picture
ananda

Hi David,

In and amongst your wonderful inquisitive post I was looking for one elusive word: bacteria!

The secret of great sourdough bread is symbiotic relationship between wild yeast and bacteria.   I don't believe that can only be judged through volumetric assessment.

Best wishes

Andy

davidg618's picture
davidg618

Hi, Andy

I agree, I agree. I love and nurture my culture's bacteria--I suspect it's a lot smarter, and more sensitive than the yeast--I say good night to it every night just before tucking myself in, but isn't gas production and expansion mostly (if not all) about the yeast?

Great to hear from you,

David G

ananda's picture
ananda

Hi David,

I have always believed that both the bacteria and wild yeast in the sourdough are equally important for gas production in sourdough bread.

I highly recommend you listen to this edition of the Food Programme, broadcast on Sunday just gone, on Sourdough.   At the end of the programme there is a discussion involving Dr. Bill Simpson, who is a micro-biologist with Cara Technologies Laboratory in Leatherhead, Surrey.   This is really your territory, as it is home to one of the world's largest collection of yeasts....and very much bound up in the world of brewing as well as baking.

I summarise below the essence of the discussion concerning "the complicated ecosystem of yeasts and bacteria in a sourdough starter"; the words are not verbatim, but pretty close:

Yeasts and bacteria play nicely together.   The bacteria has to take up a sugar called maltose.   The system requires a special type of bacteria, not normally found in the environment.   The bacteria splits the maltose in 2 and uses one half of that maltose as its own source of food.   The other half is then available as food for the wild yeasts which are around.

The system is about "symbiosis", or "sharing" or "co-operation".   The yeasts need the bacteria to cut up their food for them.   It is the presence of bacteria which allows sourdough yeasts to work in a very different way to the yeasts used in conventional commercial breadmaking

The whole programme is very much worth a listen, and you can use this link to access it:

http://www.bbc.co.uk/radio/player/b01mnpzv

So, because the yeasts are so dependent on bacteria in the first place, I remain convinced that establishing when a starter has reached its peak is somewhat more complicated that it may appear.   I don't really believe it is sensible to separate out yeast and bacteria function; it's all essential, and bound up together.

Take good care

Andy

Doc.Dough's picture
Doc.Dough

While there are elements of this treatise that bear elements of reality, the yeast and the LAB are relatively independent.  Hydrated flour (even highly refined white flour) contains large quantities of both maltose and glucose. The LAB consume the maltose until they drive the pH down far enough that they self limit their own growth rate (pH ~3.8); meanwhile the yeast are consuming the glucose and growing more slowly. The glucose produced by the LAB is trapped in the LAB cells and will diffuse out only when the glucose concentration in the media drops below the concentration in the LAB cytoplasm.  So until the yeast are starving from glucose depletion in the dough, there is not much glucose available from the LAB, and when it does begin to become available it is in small quantities and at exponentially declining rates.

 

ananda's picture
ananda

I don't doubt that you know your subject Doc.Dough, but actually the gentleman who appears in the radio programme is a leading micro biologist, who specialises in collecting and analysing yeasts used in fermentation such as beer and bread.   He did  a pretty good job of explaining sourdough to a lay person like me, and I am very reluctant to accept that he has got it wrong.

I am aware that your discussion of use of glucose by wild yeasts more than makes sense.   However, the new learning for me in this programme was concerning the inter-play between wild yeast and bacteria with regard to the sharing of the maltose.   It has left me much wiser concerning the vital need for bacteria in the mix; your analysis helps me to realise why there is need for low pH.   And even then, I acknowledge that I am stabbing in the dark.

I'm really a very poor scientist, but I have always been aware that good sourdough cultures depend on symbiosis of wild yeast, lactic and acetic bacteria.   I'm afraid that using technical language in the way you have hasn't really helped me to see beyond what I currently already understand

Best wishes

Andy

Doc.Dough's picture
Doc.Dough

Dr Simpson was being interviewed for a radio program targeted at the general public, not speaking to a technical audience.  He does a lot of dumbing down to make his points in the available time (and who knows what was edited out).  However, his statement that the LAB are necessary to break down maltose to feed the yeast is just not correct.  If you are interested, there are many technical papers that delve into the details. Start with Michael Ganzle's dissertation then read through his bibliography, especially:

Saunders, R. M., H. Ng, and L. Kline. 1972. The sugars of flour and their involvement in the San Francisco sour dough French bread process. Cereal Chem. 49:86–91.

The BBC has posted a technique for creating a sourdough starter on the BBC 4 web site. It is based on using yogurt and feeding it flour and water.  It is not a threat to overturn Debra Wink's proven results.

ananda's picture
ananda

Hi Doc.Dough,

I already have the Ganzle papers, but unfortunately I can only access the others you cite by paying out lots of money.   I tried to download them ages ago.

I agree that the page on creating a sourdough is extremely simplified, and I have also benefitted greatly from Debra Wink's fantastic contributions here on TFL.

However, I think Dr. Simpson did a good job of explaining the importance of bacteria here, and obviously I recognise that he had to simplify his explanation significantly.   Perhaps if I quote from Kulp, K. and Lorenz, K. [eds] "Handbook of Dough Fermentations" [2003: pp.29] to further back up what Dr. Simpson is saying in the programme?

[They write concerning the fermentation parameters of one stage and three stage processes] that the microorganisms remain metabolically active throughout the process.   In such sourdoughs, only a few heterofermentative species [Lb. sanfranciscensis; Lb. brevis ssp. lindneri; Lb. pontis] can be found in numbers...which suggests a relevant contribution to the fermentation process. [They cite Hammes et al 1996 and Vogel et al 1996 here]

[They go on] In contrast to homofermentative lactobacilli, these obligate heterofermentative species exhibit metabolic activities that contribute to the competitiveness of these strains in sourdough and are important for...rapid acidification, balanced production of lactic and acetic acid, and gas production.   The key reactions of the carbohydrate metabolism of Lb. sanfranciscensis and Lb. pontis are an effective maltose metabolism via the enzyme maltose phosphorylase and the capability to use fructose and oxygen as electron acceptors.   [They cite Stolz et al (various) and Hammes 1996 here]

Best wishes

Andy

FlourChild's picture
FlourChild

but I can say that the earlier in the fermentation process that you feed or use the culture, the more you tip the balance in favor of yeast.  A feed after the peak or later in the process tends to tip the balance in favor of acid-producing bacteria.  Or so says James MacGuire, was at a class he taught this weekend.

davidg618's picture
davidg618

I think my reply to andy, above, is applicable here too.

Regards,

David G

Mini Oven's picture
Mini Oven

relation to the culture's ability to hold and retain that gas.  It gives some indication of gas saturation in that culture.  

Visual peaking is seen in starters that can trap the gas being released by yeast.  As we use the trapped gas to indicate a certain level of activity, non-visual peaking or the maximum output of gas might be harder to detect in starters that cannot trap the gas.  Examples would include starters fed with non-gluten flours, very firm starters with surface fissures or conversely very liquid starters with bubbles popping quickly on the surface or starters fed baked bread or heavy chopped grain, sugar water or fruit.   Also rapidly deteriorating doughs will not hold gasses.  

Visual peaking can only happen within certain hydration parameters and with certain flours and is one indication of gas building and gluten or gluten like stretching in the starter.  

So is the visual peak really the peak of yeast activity?  

No, probably not.  If I were to throw a just fed starter into a soda drink bottle and cap it with a balloon, it would inflate the balloon for days until the gasses could find a way out.  But starters are not latex and they break down and deteriorate and any starter that could trap gas would only do it for a specific window of time depending on the flour and temperature.

Doc.Dough's picture
Doc.Dough

Mini,

I think you have it right. And if you change the flour (say from strong to weak) or the hydration level (say from high to lower) the timing of "peak" volume will change. I know from my experiments that a sourdough starter is active enough over a four or five hour period to not make much difference in the final bread, either in terms of fermentation time or final loaf volume (assuming that you allow a little extra time as the starter goes further and further past the point of falling back into the bowl).

FlourChild's picture
FlourChild

I don't think the visual peak is necessarily the peak of yeast activity, but I don't know how one could gauge the action of only one member of a complex culture at home.  Oxygen has a profound effect on yeast reproduction, so how much air is incorporated into the starter as it's mixed would need to be taken into account.  Also, there are a number of learned sources that attribute CO2 production to the bacteria as well as to the yeast.  

To me, the peak represents the point at which the enzymes, bacteria and yeast have all reached a certain balance of activity.  Bacteria and yeasts have produced as much gas as the weakening (from enzymes) structure of the starter is capable of containing.  But it is a better indication of what's going on in a starter than the simple passage of time.  I think there are some new to sourdough who follow a recipe or feeding schedule without paying enough attention to activity in the starter.  

Practical considerations aside, I admit it would be very interesting to hear from a microbiologist on the question of peak yeast activity. :)

dabrownman's picture
dabrownman

What I want to know, as a home baker, when is the best time to use your well fed and  cultivated levain to produce consistently sour bread and how can you reliably tell when that time arrives.   I know, but don't know why besides  personal experience and experiment, that my levains produce the most sour breads when they have reached full volume around 200g total and 100% hydration after the 2nd build  (starting with a 20g seed and 6 hours in teh summer ) and then I add 50 g more flour knocking the hydration down to 66.66%  and  immediately refrigerating it at 38 F for 24 hours.  Once it comes to room temperature 2 hours out of the fridge it is ready to use an consistently makes sour bread.   Why this is I don't or if there is a better way to do it.  Whole rye and whole wheat, or a mix , as far as flours go give the most sour by far too.