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Offline donstavely

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Curious about yeast metabolism
« on: October 28, 2021, 04:11:02 PM »
I am trying to learn more about yeast metabolism in pizza doughs and preferments.  I understand that there is a complex interaction between yeast respiration, fermentation, and enzyme and bacterial action, in the creation of desirable flavor compounds.  I understand (sort of) how temperature and time affects this interaction, which is discussed a lot on this forum. 

On the other hand, I don't read much about the the role of sugars versus other carbohydrates in yeast metabolism.  Straight flour contains almost no sugars, yet it can make a perfectly leavined dough.  So clearly yeast can thrive on the carbs in flour (mainly starches?)  Many recipes add several percent sugar as a browning and flavor enhancer.  It is sometimes used to activate or "jump-start" yeast before it is added to the dough, but is absent from poolishes and bigas.  I am not even sure if the percent added sugar dramatically affect fermentation rate.

So I guess my main question is, shouldn't the presence or absence of sugar make a big difference in both the proofing process and in the flavor components of the final dough?

I would appreciate either a specific reply here or a pointer to a more general resource.  Thanks!       
« Last Edit: October 28, 2021, 04:14:22 PM by donstavely »

Offline donstavely

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Re: Curious about yeast metabolism
« Reply #1 on: October 28, 2021, 04:24:23 PM »
In partial answer to my own question, I found this article which goes pretty deep into the chemistry:

https://blogs.scientificamerican.com/guest-blog/enzymes-the-little-molecules-that-bake-bread/

It doesn't touch on the impact of these reactions on flavor compounds, though.

Offline HansB

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Re: Curious about yeast metabolism
« Reply #2 on: October 28, 2021, 07:50:51 PM »
Amylase enzymes, found in flour, convert starch in the flour into sugars that the yeast consumes. That's why you do not need added sugar for fermentation.

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Offline drainaps

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Re: Curious about yeast metabolism
« Reply #3 on: October 31, 2021, 03:13:43 AM »
In a not-so-short answer to your question, and cutting a few corners, thereís a cascade of DIFFERENT enzymes breaking down flourís "long-chain sugars" aka polysaccharides, aka starch, aka amylose and amylopectin, into single-molecule sugars. Yeast is only able to feed on single sugars. The breakdown cannot be done in one single reaction but goes in a kind of cascade, and different enzymes, not only amylase, intervene along the way, depending on how long the sugar is. Some of the enzymes needed in the process are present in the flour, some are synthesized by the yeast cells in order to have single sugars to feast on.

Before we go any further, letís mention that there are 3 single-molecule (monosaccharides) sugars relevant to all things baking and pastry: glucose, fructose and galactose, and 3 double-molecule sugars (disaccharides) that youíll be familiar with already, coming from the combination of  the 3 single sugars: 2 glucoses (maltose, many of us familiar with it), one glucose and one fructose (sucrose = granulated table sugar) and one glucose and one galactose (lactose aka milkís sugar). Iíll leave milk sugar out of this post.

The relevant enzymes that break down the corresponding longer or shorter sugars  are specific to one of the steps of the process only (like a lock and key mechanism). Remembering which enzyme breaks what sugar is easy, as thereís only the replacement of a vowel between the sugar and its enzyme: starch = AmylOse is broken down by AmylAse, MaltOse by MaltAse and SucrOse by SucrAse aka Invertase. The process goes roughly as follows (cutting many corners here as the breakdown of amylose is multi-tiered):

Amylose + Amylase -> Maltose + Maltase -> 2 molecules of glucose.  MaltAse is "generated" by the yeasts.
Sucrose + Sucrase -> 1x glucose + 1x fructose. This, for sugars already existing in flour (around 1-2% of flour by weight) or sugar added in the recipe. Sucrase is also "generated" by the yeasts.

Then (cutting corners again) yeasts generate another enzyme, Zymase, that is the one that (finally) breaks down glucoses into alcohol and carbon dioxyde. This final process releases energy that enables yeasts to keep living and reproduce themselves.

On your question about the presence or absence of sugar in a recipe, itís all down to a couple of things:

(1) The breakdown of starch into maltose via amylase takes time. When you add water to your flour, yeasts start feasting on the naturally existing short-chain sugars in flour, but thatís roughly 1-2% of your flour in weight, so yeast would deplete those sugars pretty fast. This is why fermentation takes time, you need a lot more disaccharides than there are naturally in your flour.

(2) The type (and brand !) of flour impacts how fast the whole thing happens. How wheat is milled (and how many broken starch granules there are in your flour) also has a huge impact. How humid has been the weather as wheat grows also has a huge impact (more of less amylases). As you can imagine it would be too complicated for the regular baker to manage all of this: you canít have fermentation in one hour today and need to wait for three hours for the same results next month because of a different batch of flour.

 What flour mills do is basically concoct a  flour "recipe" depending on the wheat crop and all the variables, so that their flour is predictable and bakers get consistent results in every batch, within a reasonable time frame. Itís a bit like making Coca-Cola or blending mass-market whisky, you expect as a consumer the same taste from every single can/ bottle.

 This predictability is a lot more critical for professional flours than supermarket flours as you can imagine. Nobody cares if grandmaís loaf takes forever to rise. This is why one should never use unbranded supermarket flours. Theyíre usually inconsistent: suppliers might change, and quality parameters are more widely defined / accepted than is the case for professional flours.

Back to the flour recipe, and to your question, mills already adjust for short-chain sugar presence in the raw flour on the bag, so that fermentation has a smooth start. They might add maltose, for instance.  They also adjust for enzyme levels (adding for instance amylases if need be) so that flour is not too slow to start fermenting.

I honestly see little justification for the addition of sugar or maltose to a pizza dough by yourself. If the mill has done its work properly, donít touch it. Yes, you might get more residual mono and disaccharides once your fermentation is done and your dough goes in the oven, and that might help a little with colouring, but honestly, if you need to correct your flour too much, what you actually need is to switch flours. Mills know a lot better than we do.

The case for correcting flour by yourself might be different in countries where imported flours have ridiculous prices or arenít available at all, and local flours arenít up to scratch for bread making because there's no local bread making culture. A few Asian countries come to mind (Malaysia or China for instance). If you're baking there (I've been baking there 😅) your recipes might be using diastatic malt, sugar, gluten.... just to name a few, to make corrections that the mill doesn't even care about, because their customers don't need that, and because of the often prohibitive price of French bread flours in that part of the world.

 Before I go, just say that yeast is a tricky beast, in that it can generate the energy it needs via fermentation or respiration, and respiration is the least-effort path. Respiration though generates no carbon dioxide for yeasts, hence no dough rise, but it generates water. This is why sometimes dough surface (where yeast is in contact with air oxygen) is wet, and this is why you want your dough in a ball, well-protected  and  far away from air currents, so that yeasts ferments and doesn't respire.

Hope this helps. Itís been a bit long but I wanted to try and make it clear.

I am trying to learn more about yeast metabolism in pizza doughs and preferments.  I understand that there is a complex interaction between yeast respiration, fermentation, and enzyme and bacterial action, in the creation of desirable flavor compounds.  I understand (sort of) how temperature and time affects this interaction, which is discussed a lot on this forum.

On the other hand, I don't read much about the the role of sugars versus other carbohydrates in yeast metabolism.  Straight flour contains almost no sugars, yet it can make a perfectly leavined dough.  So clearly yeast can thrive on the carbs in flour (mainly starches?)  Many recipes add several percent sugar as a browning and flavor enhancer.  It is sometimes used to activate or "jump-start" yeast before it is added to the dough, but is absent from poolishes and bigas.  I am not even sure if the percent added sugar dramatically affect fermentation rate.

So I guess my main question is, shouldn't the presence or absence of sugar make a big difference in both the proofing process and in the flavor components of the final dough?

I would appreciate either a specific reply here or a pointer to a more general resource.  Thanks!       
« Last Edit: October 31, 2021, 06:01:58 AM by drainaps »

Offline Pete-zza

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Re: Curious about yeast metabolism
« Reply #4 on: October 31, 2021, 10:14:08 AM »

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Offline donstavely

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Re: Curious about yeast metabolism
« Reply #5 on: November 01, 2021, 03:20:00 PM »
Thank you all for the information.  Pretty complicated stuff!

My real motivation was to try to improve a keto pizza recipe.  My favorite so far is Dennis's v2.0 at Black Tie Kitchen.  He uses 12g of sugar as food for the yeast (even though almond flour contains several percent sugar).  So I was thinking, what if I used a little poolish instead, hoping that the yeast would eat enough of the carbs to keep it keto, but add some bready flavors to the dough.  Hence my questions about yeast metabolizing carbs.

I tried an experiment to see if I could determine how much the yeast would eat.  I mixed exactly 100g of KA bread flour with instant yeast and about 200g of water in a jar.  I wanted it to be thin enough for the CO2 to bubble up, rather than create a foam.  I covered it with plastic wrap with a pinhole to let the CO2 out without evaporating the water or alcohol.  There was no more activity after 24 hours at RT, and the weight of the jar had gone down by 5g.  Since the last step of the fermentation chemistry creates an equal amount of CO2 (molecular weight 41) and ethanol (molecular weight 46), I figure that about 6g of alcohol was created.  So of the original 100g of flour, only 11g were converted by the yeast.  The KABF package says ~13% protein, <1% fiber, and 0% sugar.  So I have to conclude that about 100g - 11g - 13g = 76g of human-digestible carbohydrates remain. 

Does this make sense?  If so, then my hope of the yeast burning through most of the carbs in the poolish is a bust.  :(       


Offline drainaps

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Re: Curious about yeast metabolism
« Reply #6 on: November 01, 2021, 06:34:59 PM »
Makes total sense. A very small % of total starch is metabolized by yeast during bread making. Actually you can rework / retighten an overproofed  bread dough and restart fermentation all over again if it hasnít lost too much water through evaporation because thereís still plenty of food left. Mind you, only broken starch granules count as yeast food (15% of total starch roughly).

Thank you all for the information.  Pretty complicated stuff!

My real motivation was to try to improve a keto pizza recipe.  My favorite so far is Dennis's v2.0 at Black Tie Kitchen.  He uses 12g of sugar as food for the yeast (even though almond flour contains several percent sugar).  So I was thinking, what if I used a little poolish instead, hoping that the yeast would eat enough of the carbs to keep it keto, but add some bready flavors to the dough.  Hence my questions about yeast metabolizing carbs.

I tried an experiment to see if I could determine how much the yeast would eat.  I mixed exactly 100g of KA bread flour with instant yeast and about 200g of water in a jar.  I wanted it to be thin enough for the CO2 to bubble up, rather than create a foam.  I covered it with plastic wrap with a pinhole to let the CO2 out without evaporating the water or alcohol.  There was no more activity after 24 hours at RT, and the weight of the jar had gone down by 5g.  Since the last step of the fermentation chemistry creates an equal amount of CO2 (molecular weight 41) and ethanol (molecular weight 46), I figure that about 6g of alcohol was created.  So of the original 100g of flour, only 11g were converted by the yeast.  The KABF package says ~13% protein, <1% fiber, and 0% sugar.  So I have to conclude that about 100g - 11g - 13g = 76g of human-digestible carbohydrates remain. 

Does this make sense?  If so, then my hope of the yeast burning through most of the carbs in the poolish is a bust.  :(       

Offline Yael

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Re: Curious about yeast metabolism
« Reply #7 on: November 02, 2021, 01:30:01 AM »
Makes total sense. A very small % of total starch is metabolized by yeast during bread making. Actually you can rework / retighten an overproofed  bread dough and restart fermentation all over again if it hasnít lost too much water through evaporation because thereís still plenty of food left. Mind you, only broken starch granules count as yeast food (15% of total starch roughly).

Exactly, damaged starch is limited within the flour; but then would it be possible for him to damage the starch himself? Starch can be damaged while reaching a certain temperature (I remember Tom Lehmann mentioning it), like for a Tangzhong for instance (except that for a Tangzhong starch would gelatinize making it impossible for yeast to feed upon, if my guess is correct). I don't know if this idea would work.
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Offline donstavely

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Re: Curious about yeast metabolism
« Reply #8 on: November 02, 2021, 01:55:14 PM »
Great discussion, guys!  This whole "damaged starch" thing was totally new to me, and yet another variable in this complicated process.

So again how do we humans so efficiently metabolize the leftover starches that the yeast doesn't eat?  It is the a-amylase and b-amylase in our mouth and gut, according to Google.  In normal yeast dough fermentation these enzymes must be present to at least a small degree.  Maybe temperature or pH in dough makes them only effective on the broken starches?

Then I got to thinking about beer making and distilling. The point, at least for lighter beers and all spirits, is to break down as much of the starch into sugars and ferment as much of them into alcohol and CO2 as possible.  The malted barley has natural amylases, but they often need to be augmented with commercial amylase to get good conversion.  Apparently, you need to cook the alpha-amylase (like 155F) to get it to cut up undamaged starches.  Then they use gluco-amylase (in place of beta-amylase) at yeast temperature to get down to simple fermentable sugars.

I read something about gelatinizing the starch as being necessary for the a-amylase to work:
"When you consider that un-gelatinised starch takes days for enzymes to degrade it is clear that hitting the starch gelatinisation temperature for efficient and effective mashing is crucial."
I guess this is part of the need for higher temperature for the a-amylase?

Here is a video that I found helpful:

   

Offline RHawthorne

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Re: Curious about yeast metabolism
« Reply #9 on: November 08, 2021, 12:17:27 PM »

Then I got to thinking about beer making and distilling. The point, at least for lighter beers and all spirits, is to break down as much of the starch into sugars and ferment as much of them into alcohol and CO2 as possible.  The malted barley has natural amylases, but they often need to be augmented with commercial amylase to get good conversion.  Apparently, you need to cook the alpha-amylase (like 155F) to get it to cut up undamaged starches.  Then they use gluco-amylase (in place of beta-amylase) at yeast temperature to get down to simple fermentable sugars.

I read something about gelatinizing the starch as being necessary for the a-amylase to work:
"When you consider that un-gelatinised starch takes days for enzymes to degrade it is clear that hitting the starch gelatinisation temperature for efficient and effective mashing is crucial."
I guess this is part of the need for higher temperature for the a-amylase?


The whole idea with raising mash temp to the 155-158 degree range is to activate a different group of enzymes which extract more complex sugars that aren't digestible by yeast, and leaving it there, in most cases. This is what gives beer a fuller body and more complex flavors. This is usually done with darker and heavier beers like dopplebock and Scotch ale. And in almost any style of beer, the goal isn't really to get the yeast to consume all the fermentable sugars, but to formulate the mash and fermentation process so that the yeast eats just the right amount of them to hit the target flavor profile and alcohol content. Or at least, that's how it's done in any brewery that makes beer worth drinking.
 For all I know, you could be right, and some of the larger breweries might use some sort of process to break down the polysaccharides obtained from a higher mash temp into simpler sugars, but from what I know of the brewing industry, the preferred option would be to simply add cheap adjuncts like corn or rice to obtain more fermentable sugars, instead of going through a more complex and time consuming process. I was a homebrewer for many years, and I've known several people who worked in the brewing industry, but that was more in the microbrewery sector.
 Anyway, I could go on for some time about brewing beer, but I don't want to take this discussion thread too far off track.
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Offline donstavely

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Re: Curious about yeast metabolism
« Reply #10 on: December 18, 2021, 01:18:11 PM »
In the last month since the last posts on this subject, I have done a total of eight experimental mini pizza doughs. I was hoping that I could find something, besides sugar, that would cause a truly impressive, neapolitan-like rise in a lower-carb dough.

I lately have been using 30% KABF and 1% or 2% added 210 lintner DMP, hoping that the added enzymes would convert more of the undamaged starches to fermentable sugars for the yeast to feed on.  It helped a little, but only a little.  My last attempt borrows from Yael's Tangzhong suggestion, and from beer-making mashing technique.  I mixed the bread flour and malted barley flour with all of the water, then cooked it for an hour in my sous vide rig at 145F.  This gelatinized the starch, and should activate the enzymes. The lower mash temperature is supposed to favor the beta-amylase, which brewers do to produce dryer beers. Again, this only helped a little. 

Here is the bill:
35% almond flour
8% lupin flour
25% vital wheat gluten
30% KA bread flour
2% malted barley flour
2% allulose
2% salt
3% oil
1% instant yeast
2% vinegar  (I read almond flour is slightly basic, regular flour is slightly acidic, and amylases prefer the latter)

None of the doughs had anywhere near the familiar elastic feel and strechability of "real" pizza dough, even with all that vital wheat gluten.  I suspect now that is the lack of a well-formed gluten network, and not just the lack of fermentation, that is keeping things dense. I should also mention that I get zero oven spring, even in a 500F oven.

The last thing I can think of is to add some double-acting baking powder to get some CO2 production in the oven.  I found a starch-free one from Bakewell online.  But again I suspect that the poor gluten network development is still a problem.

Any other insights or suggestions?  I am about to abandon the project and go back to real pizza on infrequent "cheat nights".  Either that of gain back the twenty pounds I lost going low carb  :-\     

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Re: Curious about yeast metabolism
« Reply #11 on: December 18, 2021, 01:40:12 PM »
Don, that is really an unconventional recipe Iím sorry I cannot help you with. I do know that 1-2% of 210 L is way, way too high - at least for conventional flours. Iíve seen 2% of 20L as the high end of a recommended place to begin. I have 60 L and use 0.66%, sometimes less (and often not any at all).The conversion is somewhere around 0.19% for 21) L.


I know and respect what you are trying to accomplish in terms of diet. Once you get more answers here you may find more and better help on the Gluten Free forum. Good luck with your pizza making and low carb plan.
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