This seems to make most sense as an all-grain post.
Consider two mash schedules and yeast combinations:
A single-malt grain bill mashed at 158 and fermented with a highly attenuative yeast (say ~80%?)
A single-malt grain bill mashed at 145 and fermented with a minimally attenuative yeast (say ~68%)
First impulse, what might you expect from these two circumstances?
What about flavor profile? Final gravity?
Just looking to explore a bit of theory here…
Thanks,
John W
I would expect #2 to attenuate more than #1. Mash temp (and grist composition) makes far more difference than the attenuation rating of the yeast. That’s just a way of comparing one yeast to another given a standard wort.
+1 I’ve brewed beers that had 80% attenuation with a yeast that was rated for 74% tops. Low mash temp, plenty of yeast and a moderate and controlled ferm temp all work to get maximal attenuation.
Kai
+1
2 has the lower final gravity and is sweeter.
Switch the yeast around and 2 has the lower final gravity and 1 is sweeter. So basically the yeasts rated attenuation tells you about how sweet a beer it makes and the mash temperature tells you about what the FG might be and sweetness and FG are only loosely correlated.
This is partly where my question came from, i.e., the nature of the sugars that remain after fermentation.
I have the book “Brewing Science and Practice,” by Briggs, Boulton, Brooks, and Stevens. In chapter 4, “The Science of Mashing,” on page 135 there is a figure (fig. 4.20) that shows the various sugars created at various mashing temperatures. (glucose, maltose, maltotriose, sucrose, maltotetraose).
I know that a part of yeast attenuation comes from what sugars they are able to ferment…and I saw in the same book where sugars have varying degrees of relative sweetness, as well as differing taste detection thresholds.
And so I was totally thrown off by the appearance that lower mash temps made more of the sugars that taste sweeter, and high mash temps just the opposite. And then throw in the variable sugar fermentation character of yeasts, and you’ve got one confused homebrewer!
So I thought I would throw it out here, and see if anyone had some sound experience with mixing up mash temps and yeast attenuations.
And maybe some practical advice as to how one might exploit combinations of mash temps and extreme yeast attenuations.
Thank you all for responding.
Anything else you can add?
John W
There is little difference in the types of sugars that brewing yeast can ferment. To my knowledge the only pronounced difference lies in lager yeast’s ability to ferment raffinose and melibiose. But those are not present in wort, at least not in significant amounts. All yeasts ferment the primary 3 sugars that are present in wort: glucose, maltose and maltotriose.
There are however differences in their ability to utilize maltotriose which makes this the sugar that is commonly left behind by lower attenuating yeasts.
The sugars that taste sweeter, glucose and maltose, are very digestible for the yeast and will be taken completely by even the low attenuating yeasts. In fact, maltose won’t be touched until pretty much all of the glucose is gone.
That’s what you get from reading these books ;). Many things don’t seem to line up with what we find in home brewing literature and how things have been taught to home brewers. But that’s why those books are so interesting.
I have experience with 2 same recipe beers where one had a low attenuation limit (i.e. low wort fermentability) while the other had a higher attenuation limit (i.e. higher wort fermentability). I made both beers finish at the same attenuation and the beer that had the higher attenuation limit tasted a bit sweeter since it had more residual fermentable sugars.
Kai
Thank you for the post Kai, it’s very helpful.
I think the varying ability of yeast to ferment maltotriose and that some lager yeast can ferment raffinose and melibiose is where I got the idea that yeasts vary in the sugars they can ferment…it’s something I had only casually looked at in the past, at least from the standpoint of attenuation, so it’s a weak area in my knowledge. Thanks for the help.
Plus, I’ve generally put attenuation as a lower concern for yeast selection…my first criteria has always been flavor/aroma production, as I’ve always heard that yeast is a primary contributor to flavor/aroma.
[quote]That’s what you get from reading these books . Many things don’t seem to line up with what we find in home brewing literature and how things have been taught to home brewers. But that’s why those books are so interesting.
[/quote]
Agreed. It’s a very interesting book, and I’ve enjoyed having it…but it’s not exactly a practical guide to homebrewing!
[quote]I have experience with 2 same recipe beers where one had a low attenuation limit (i.e. low wort fermentability) while the other had a higher attenuation limit (i.e. higher wort fermentability). I made both beers finish at the same attenuation and the beer that had the higher attenuation limit tasted a bit sweeter since it had more residual fermentable sugars.
[/quote]
And I assume the OGs where nearly identical? And when you say the sweeter one “had more fermentable sugars” would that imply it still had some unfermented glucose?
To add another twist to using mash temp/yeast traits, how can one exploit mash temperatures and yeast’s alcohol tolerance? Suppose you mash for high fermentability in a high gravity beer, and then use a low alcohol tolerance yeast?
I feel fairly confident about this topic since it touches on questions that I had as well when I started breweing. As a result I paid close attention whenever I came across information that helped me to better explain why some yeasts ferment further than others. Key to the understanding was that all yeasts are able to eat the same sugars that are present in wort. In fact when put in a fast ferment test environment (high pitch rate, warm temps, agitation) a low attenuator like WLP002 will ferment as far as a good attenuator (WLP830, for example). Because of this it is not the types of sugars that yeast can ferment that makes a difference but the way yeast behaves in fermentation. In particular flocculation, alcohol tolerance and how well it can metabolize maltotriose.
I don’t like how attenuation numbers are reported for home brewers and I like even less how they are used. There is no uniform standard for measuring these numbers. White Labs started to report them more detailed, at least for some strains. What I’d like to see is a standard for testing and I’d like to see not the abselute attenuation but the difference between attenuation and attenuation limit (i.e. fermentability) of the used wort. This measured the residual fermentable sugars.
[quote]I have experience with 2 same recipe beers where one had a low attenuation limit (i.e. low wort fermentability) while the other had a higher attenuation limit (i.e. higher wort fermentability). I made both beers finish at the same attenuation and the beer that had the higher attenuation limit tasted a bit sweeter since it had more residual fermentable sugars.
[/quote]
Yes, the OGs were identical.
Sugars are consumed serially, I think that’s in the Briggs book as well, which means glucose is consumed before maltose and maltose is consumed before maltotriose. This means that there won’t be any glucose left in the beer once it has attenuated somewhat. Even maltose won’t be left once the attenuation gets close to the attenuation limit.
I think you could get the yeast to stall before it consumed all maltose. The result would be a rather sweet beer. But I doubt that there are practical benefits. One thing to keep in mind that if you keep the yeast from attenuating as much as it can by curtailing its health or ability to resist alcohol you may also cause it to produce a different flavor profile. It may also have adverse effects on mouthfeel and head retention.
Kai
To complicate matters more…Belgian strains lend a unique flavor profile to fermented wort. I’ve always thought I had a pretty good understanding of yeast behavior until I started brewing Belgian beer and then it all went out the window. 
I agree that most yeast ferment the available sugars (glucose, sucrose and fructose) and then selectively work on the remaining sugars based on fermentability and attenuability but I like to think that the attenuation and fermentability work together to produce varying results depending on the yeast strain and grist. German hefe yeast or Chimay yeast have a distinct flavor profile that can be enhanced by mash temp and ferm temp. I also believe there’s a synergy that exists between fermentability and attenuability of a yeast/mash combination.
There may be, and if so I suspect it would be one of those “intangibles” that can’t be easily quantified or explained…it just “happens.” But it sure would be nice to try to “explain” what happens and how to cause it to happen. One can dream.
[quote]Because of this it is not the types of sugars that yeast can ferment that makes a difference but the way yeast behaves in fermentation. In particular flocculation, alcohol tolerance and how well it can metabolize maltotriose.
[/quote]
{my emphasis added. jw}
Ahhh…see, this may be more of what I was looking for…
So one can ask, what guidelines might direct formulation of recipes, given a brewer’s choice of yeast and mashing regimes/temperatures?
As an example on yeast behavior and how it can be used to influence a beer style, take Yorkshire squares and Burton unions: both fermentation methods evolved to suit the yeast in use…IIRC, Yorkshire square yeast is highly flocculent and the square system keeps it “roused” until the brewers drop the beer out and leave the yeast behind; and the Burton yeast has poor flocculation ability, but gets removed by the union system.
[quote]I don’t like how attenuation numbers are reported for home brewers and I like even less how they are used. There is no uniform standard for measuring these numbers. White Labs started to report them more detailed, at least for some strains. What I’d like to see is a standard for testing and I’d like to see not the abselute attenuation but the difference between attenuation and attenuation limit (i.e. fermentability) of the used wort. This measured the residual fermentable sugars.
[/quote]
I agree…I’ve never really been able to use the reported percentages to predict anything about my beers. The numbers to me serve as little more than “low, medium, or high” attenuation. We somehow need to drop a hint to the yeast producers…we can’t expect change unless we say something. ???
Thanks a lot for your posts guys!!
In the book Yeast, White says that flocculation is the most important determinant of attenuation. I’d never really thought about that but it makes perfect sense, the longer the yeast is suspended and working the better it finishes the job. He goes on to say that yeast strains have been selected for their different flocculation characteristics based on how and when they are harvested (bottom vs top, early vs late). This simple concept has really helped me to better understand how to best utilize the various strains available. Things like, you don’t rush a highly attenuating yeast since it takes longer to drop out. You might want to lager and/or fine it to get your best beer. Or, rouse your fast-floccing yeast a bit early on if you think you want to be on the dry iside.
I think for an average ABV brew, sweetness is probably more a function of the sweet sugars that are crosslinked to proteins or other carbs, making them unfermentable while leaving the sweet moeity available for the taste bud. Your crystals being stewed and dried probably provides a fair amount of crosslinking, as would a long boil or the reduction of a portion of the wort for a Wee Heavy.
Once you get into high ABV, limitation of the yeast could mean residual fermentables that would be sweet.
Yes, flocculation or any premature sedimentation, is a key characteristic that determines how much fermentable sugars are left behind after fermentation.
Key to understanding flocculation is that for most flocculant yeast flocculation is inhibited by the presence of sugars. In particular maltose is a strong flocculation inhibitor. Maltotriose, on the other hand, doesn’t inhibit flocculation. Because of that it can easily happen than the yeast starts to flocculate and settle once all maltose is consumed. But at that point there is still maltotriose present.
I see that a lot in my lagers using WLP830, for example, from one day to the next the cell count would drop from 10 M/ml to just over 1 M/ml while there is still about 1 Plato worth of fermentable extract left. And at that point a slow fermentation process starts.
For my current Doppelbock I want to try Kraeusening with non flocculating lager yeast (WY2042 – Danish Lager) after a primary fermentation with WLP833. I want to see if that can speed up the maturation phase.
An important tool to understanding and taking action on all this is the fast ferment test. I have and am spending a lot of time on researching mashing, mash pH and soon yeast but I don’t think that any of this will be as valuable as the information a brewer can gain from the fast ferment test. To me not doing this test feels as if I haven’t taken an original gravity sample. In fact I rather have the result of this test, which is the lowest possible FG, then the original gravity if I had to choose between the two.
Kai
While it might be true that sugars (or their absence) act as a signal for flocculation to occur, but it doesn’t explain the pronounced differences between strains. I think that is something that is bred into the strain via selection.
Seems like temperature would also be a factor, although I suppose the temp simply follows the sugar levels inasmuch as rapid fermentation (and the heat it generates) slows at the time the most abundant and most fermentable sugars become depleted.
I don’t believe there is a pronounced difference among the strains, especially if you don’t exceed their alcohol tolerance.
English ale yeast will produce a higher AA in a tripel wort than trappist yeast will in a mild wort.
Flocculation is just one aspect and WLP002, for example, is a very stubborn flocculator where even the presence of maltose doesn’t easily revert flocculation.
I learned this the hard way. Normally when I have flocculant yeasts and want to count them I need to unflocculate them. This is best done in brewing wort and I would put the yeast with a liter or two of fresh wort on the stir plate. After a while all clumps are broken up and the yeast is evenly distributed which allows a sample for counting to be drawn.
Not so with WLP002. Even after an hour on the stir plate it won’t de flocculate. Maybe the wort was too cold since the yeast did deflocculate while being grown. But I ended up pitching it w/o being able to count the cells. No real loss here since I was still able to weigh the sediment but I was really surprised how strong of a flocculator WLP002 is.
Some yeast also flocculate in the presence of alcohol or the absence of glucose. The motivation for the yeast is simple: low nutrients and/or toxic environment → let’s clump together to protect at least some of us.
The ideal brewer’s yeast, at least for many of the big guys, ferments all fermentable sugars and then flocculates to provide for easy separation between yeast and beer. If they flocculate too early or too late it becomes a problem for the brewery. I’d imagine that flocculating too early is more of a problem than flocculating too late. And based on what I have read yeasts are more likely to lose their flocculation ability over time than being able to gain this ability. I think they say that the yeast is becoming dusty and it is a sign of genetic drift.
Kai
This might be a good time to mention that Chris White and Jamil Zainisheff are this month’s guests in “Ask the Experts”. You guys might want to try directing some of these questions to them for their take on it.
I am not sure I understand. You are saying there is not a big difference in flocculation characteristics between yesat strains? Or that there isn’t a difference between them in terms of their reaction to running out of sugar? Your example seems to indicate that there either isn’t a strict relationship between flocculation and attenuation, or that there isn’t a difference in flocculation and attenuation is in fact closely coupled to floc.
I do think a yeast cake is still metabolically active for some time, so maybe flocculation doesn’t completely determine attenuation.
I already submitted two Denny, I don’t want to pester them to death.