Hypothetically if a selected yeast with attenuation of say 70 % was put in to a solution containing only fermentable sugars only (glucose say). Would the yeast attenuate 100% and if not what would be limiting factors? For arguments sake say the fermentable sugars were extraced from malted barley and therefore contained suitable nutrient to allow healthy fermentation (or does this not follow). Go on. Go tech too if you can... I love it.
yeast attenuation
- Thread starter evilhorse
- Start date
Help Support Homebrew Talk - Beer, Wine, Mead, & Cider Brewing Discussion Forum:
s3n8
Well-Known Member
There are some other factors to consider, for instance alcohol tolerance. Some yeast are more tolerant than others.
Also, the published attenuation stats are "apparent attenuation" not true attenuation. If you had a 100% fermentable solution, it would finish with a SG of well below 1. Many wines and ciders will finish at .990 or so, which would mean apparent attenuation would be more than 100%.
Hopefully some others with better knowledge of chemistry will chime in soon
.
Also, the published attenuation stats are "apparent attenuation" not true attenuation. If you had a 100% fermentable solution, it would finish with a SG of well below 1. Many wines and ciders will finish at .990 or so, which would mean apparent attenuation would be more than 100%.
Hopefully some others with better knowledge of chemistry will chime in soon
.
There are several factors that can affect attenuation.
The health of the yeast when they enter the wort.
The amount of nutrient and oxygen present for the first round of reproduction
Pitching rate. If you under pitch the yeast can become stressed and not finish. Over pitching can have bad effects as well. Use the Mr Malty pitching rate calculator.
The temperature for fermentation can affect attenuation
Not letting the primary run long enough
Do some reading from John Palmers online How To Brew and listen to the attenuation podcasts from The Brewing Network for more technical details.
The health of the yeast when they enter the wort.
The amount of nutrient and oxygen present for the first round of reproduction
Pitching rate. If you under pitch the yeast can become stressed and not finish. Over pitching can have bad effects as well. Use the Mr Malty pitching rate calculator.
The temperature for fermentation can affect attenuation
Not letting the primary run long enough
Do some reading from John Palmers online How To Brew and listen to the attenuation podcasts from The Brewing Network for more technical details.
so from your reply s3n8 Im reading into it that you think a yeast stated to attenuate at 70% 'could' attenuate 100% if the sugars were all fermentable and conditions were favourable (eg: nutrient, temp, innoculum size, low alc etc). This makes sense to me, I cant see why a yeast would stop fermenting without cause?
So what is the % attenuated refering too?
Im my basic understanding the brewer can control the proportion of fermentable and unfermentable sugars with the selection of malts a and mash conditions. In my logic would one not select yeast wih '100%' attenuation to consume all fermantable sugars (yeilding alcohols, esters to a desired amount) and leave unfermentables for body/balance/malt flavour. If a beer were attenuated of only 70% of fermentables it would surely be unstable (susceptible to spoilage/refermentation with wild strains etc).
Oh is maltose fermantable? How much?
Sorry lots of questions.
So what is the % attenuated refering too?
Im my basic understanding the brewer can control the proportion of fermentable and unfermentable sugars with the selection of malts a and mash conditions. In my logic would one not select yeast wih '100%' attenuation to consume all fermantable sugars (yeilding alcohols, esters to a desired amount) and leave unfermentables for body/balance/malt flavour. If a beer were attenuated of only 70% of fermentables it would surely be unstable (susceptible to spoilage/refermentation with wild strains etc).
Oh is maltose fermantable? How much?
Sorry lots of questions.
s3n8
Well-Known Member
That is the way I understand it, if you made a 100% fermentable solution say with dextrose and water, the yeast would consume all of the sugar, thus would attenuate well below 1.000 FG. Since the sugars produced during mashing are a complex concoction of maltose and many other sugars (again, i sucked at chemistry, so a little help from the brew science crowd would not hurt here) different yeast consume different sugars. The % attenuation numbers are for typical worts, and can be affected quite a bit by other conditions such as mash temp, fermentation temp, pitching rate, oxygenation, etc. Its not that they get bored and stop, its that they hit a wall where they cant do any more work given the circumstances.
This idea of underpitching and over pitching makes no scientific sense to me. Yeast are millions of independant microorganisms. To say they would become "stressed" from "underpitching" doesn't make sense. You can probably infer that a yeast cell has a lifespan and that may, possibly, by pitching too little yeast they wouldn't be able to multiply at a fast enough rate to reach full attentuation before they die or somethign... but I can't wrap my head around what would make "underpitching" an issue other than a potential slow start.
All you're doing when you pitch yeast is introducing a nominal amount of "good" yeast cells to an environment they flourish in. Therefore, they reproduce until the environment is no longer suitable to sustain reproduction at that level. At that point they go dormant and floculate and wait for the environment to change to their liking again. Theoretically speaking you should be able to introduce 1 good yeast cell to "the party" and have a fully attenuated yeast cycle. You will jsut have a very long fermentation. The same goes for "overpitching". You're simply just adding more viable cells at the beginning thus decreasing the time it takes for your yeast to multiply and attenuate to their genetically "engineered" level. I can't see how either scenario can change how the yeast attenuate.
Maybe I'm over-simplifying it.... is there a resident biochemist who can give me the details I'm missing?
Yes, 100%, it's 95% of the fermentables in wort. You can make a nearly 100% fermentable wort by adding Beano or some other form of alpha galactosidase. You will end up with a low-charbohydrate, zero-body, zero malt beer. That's how light beers are made.
thanks s3n8.
Cheaton. Yes I agree 1 is enough. But I am told that the lag phase associated with low pitching rates is a good window for establishment of wild strains (that may include spoilage organisms or poor strains), and that as many yeast eliminate/outcompete other strains a low innoculum may allow non selected strains to dominate. Ive had lacto probs before which I fixed with a big innoculum to give rapid onset/vigorous fermentation. Id be keen to hear arguments against overpitching? I cant think of any?
Can anyone provide a breakdown of the various sugars and their relative proportions in a given wort and the finished beer?
david 42? surely more than 5% of fermentable sugars are from dextrose/glucose, isnt the point of mashing at specified temp to create a a desired balance between the highly fermentable dextrose and the less fermentable maltose? I have read that maltose is 100% fermentable but usually will stop short unless forced by manipulation of fermentation conditions (heat, nutrient, 02...). maybee maltose is more fermentable than I have thought?
Cheaton. Yes I agree 1 is enough. But I am told that the lag phase associated with low pitching rates is a good window for establishment of wild strains (that may include spoilage organisms or poor strains), and that as many yeast eliminate/outcompete other strains a low innoculum may allow non selected strains to dominate. Ive had lacto probs before which I fixed with a big innoculum to give rapid onset/vigorous fermentation. Id be keen to hear arguments against overpitching? I cant think of any?
Can anyone provide a breakdown of the various sugars and their relative proportions in a given wort and the finished beer?
david 42? surely more than 5% of fermentable sugars are from dextrose/glucose, isnt the point of mashing at specified temp to create a a desired balance between the highly fermentable dextrose and the less fermentable maltose? I have read that maltose is 100% fermentable but usually will stop short unless forced by manipulation of fermentation conditions (heat, nutrient, 02...). maybee maltose is more fermentable than I have thought?
Absolutely agree! And the one cell starting the process is purely from a theoretical, all things being perfect scenario. SWMBO is a Biochemist. She mentioned there are many factors the make "underpitching" a bad idea. But its purely from a fermentation starting point of view. She couldn't come up with a reason why, if fermentation had begun, that "underpitching" could cause the attenuation to be off. But absolutely, the competition factor is spot on. But the idea that "underpitching" can cause yeast "stress" and therefore low attenuation doesn't seem scientifically sound unless your slow start allowed other organisms to out-compete the yeast for resources. In which case you've got more issues than low attenuation.
I would guess (ans SWMBO agrees) that low attenuation would more likely be attributed to either environmental problems with the substrate (wort) or genetic problems with the yeast. Environmental problems can be far more than just fermentables/non-fermentables. Could be PH levels, mineral levels, presence of chemicals (chlorine, bromine, flouride) in the water. But I doubt its how much yeast you pitch.
I would guess (ans SWMBO agrees) that low attenuation would more likely be attributed to either environmental problems with the substrate (wort) or genetic problems with the yeast. Environmental problems can be far more than just fermentables/non-fermentables. Could be PH levels, mineral levels, presence of chemicals (chlorine, bromine, flouride) in the water. But I doubt its how much yeast you pitch.
Well I certainly have no intention of engaging in a discussion of the biology of yeast with some bio-chemists. Suffice it to say I would enter such a discussion heavily under armed.
It is possible that it is something of an urban myth, however I have encounterred many times in readings and listening to brewing podcasts the notion that under pitching can lead to under attenuation. It usually surrounds the fact that initial multiplication in the wort is limited by the initial oxygen supply, which may be somewhat of a limiting factor for increasing the size of the colony quickly enough. The theory goes that the final size of the colony that begins it's assault on the fermentables effectively becomes gorged and lazy and may flocculate early. Again this is my understanding, and I fully realize I have never personally conducted any experiments to prove or disprove the validity of that notion. At any rate, that is where the earlier assertions I made came from.
I will now back out of the discussion and watch from the sidelines, as it appears headed to a technical level beyond my participation.
In the end it's all just bugs and beer.
Cheers
It is possible that it is something of an urban myth, however I have encounterred many times in readings and listening to brewing podcasts the notion that under pitching can lead to under attenuation. It usually surrounds the fact that initial multiplication in the wort is limited by the initial oxygen supply, which may be somewhat of a limiting factor for increasing the size of the colony quickly enough. The theory goes that the final size of the colony that begins it's assault on the fermentables effectively becomes gorged and lazy and may flocculate early. Again this is my understanding, and I fully realize I have never personally conducted any experiments to prove or disprove the validity of that notion. At any rate, that is where the earlier assertions I made came from.
I will now back out of the discussion and watch from the sidelines, as it appears headed to a technical level beyond my participation.
In the end it's all just bugs and beer.
Cheers
Haha. Sounds like you're pretty well armed! I'm no biochemist, I just live with one hehe. Like I said, I'm probably just over simplifying it, and yeast ain't the SWMBO's forte (ask her about enzyme kenetics, protein binding, and signal transducer pathways and you're in for a 2 hour discussion). Your explanation does make sense. It sounds completely plausable. I hadn't considered that angle. SWMBO is currently teaching a class on fermentation in food production. She'll be an expert by the time its done, but in the meantime I'll have to steal her book and do some reading on this.....
From our discusssions. I am here: yeats any generally goes at the smallest sugars first (monoscacarides eg: glucose acheived by alpha enzyme activity in the mash) when these are gone it will move into disaccharides (eg maltose form beta enzyme activity in the mash) and so forth with tri saccarrides... the length of the chain determines the fermentability. You can control the mash temp to gain the desired balance of sugars and thus the resultant beer style as most yeast will stop fermenting at some time after they have consumed all the most easily fermentable sugars and are unable to sustain fermentation of the more complex sugars at a rate that allows them to survive in the wort at which point they flocculate. Attenuation expressed as a percentage therefore refers to the ability to ferment (cheifly) maltose and to a lesser extent other longer chain sugars, or in other terms the strains ability to cleave poly saccahrides (malose and up) down to down to monosaccarides. As a side topic the adding of dextrose priming sugar restarts fermentation breifly (as these are most easily consumed) untill equilibrium is reached again, this to me suggests that the environemt is still suited to fermentation (therefore cessation is not due to a change in mash environment bar the loss of easily fermentable sugars). Please tell me if ive got it wrong? Id be keen to see the composition of a wort (% of various sugars) pre ferment and post ferment and yeast strain if anyone knows where to get one?
I did only a quick skim of the responses, so forgive me if I'm repeating something and/or if this is too off-topic...
Just wanted to add that "apparent attenuation" is so-called because alcohol is less dense than water, so as the yeast convert the sugars into alcohol, the beer becomes less dense in two ways - 1) decrease in sugars, and 2) increase in alcohol. Therefore, true attenuation is actually usually less than "apparent attenuation". This is also why you can get wines and ciders that end with a sub-1.000 gravity - obviously you can't have more than 100% attenuation, it just appears that way because the wine/cider has som much alcohol.
Just wanted to add that "apparent attenuation" is so-called because alcohol is less dense than water, so as the yeast convert the sugars into alcohol, the beer becomes less dense in two ways - 1) decrease in sugars, and 2) increase in alcohol. Therefore, true attenuation is actually usually less than "apparent attenuation". This is also why you can get wines and ciders that end with a sub-1.000 gravity - obviously you can't have more than 100% attenuation, it just appears that way because the wine/cider has som much alcohol.
dantodd
Well-Known Member
apparent attenuation figures are based on a standard wort and fermentation practice. There are a number of ways to exceed (or fail to meet) the published AA% for a particular yeast. Under pitching or poor oxygenation will result, generally, in lower attenuation than published. While you can increase attenuation by over pitching, rousing and temperature alteration. I am no biochemist so I can't explain the full panel of reasons and biological mechanisms causing these changes but there is a very good podcast from "The Brewing Network" on yeast. I don't recall the guest but you may be able to find it by looking through the archives. The guest spoke specifically about getting normal attenuating yeast to go very dry by manipulating the fermentation and outlined some of this discussions with Chris White about why the yeast were able to attenuate much higher than they advertised.
