Yeast Cleaning up off flavours after fermentation?

[Reviled]

Ive been thinking about this a bit lately, as this winter for some reason ive been having over attenuation issues, which is wierd cos last year I was having the opposite with all my beers finishing at 1030 or 1025ish, this year they are all going down below 1010!!

 

I brewed a stout on monday, and checked the gravity last night and it was down to 1012, there was still a small krausen tho so there was still some fermentation happening, so because I didnt want this to dry out too much, I put the fermenter outside to sort of crash cool it, and checked it this morning its down to 12*c and the 1968 just wouldnt be capable of doing anything at that temp..

 

So my question is, does yeast really clean up off flavours or do anything after its finished munching on sugars? Or is that a myth? Because ive kegged beers after only 3 days of fermentation before and after 2 weeks in the keg at cold temps the beer is still clean and tasting great, sometimes cleaner than other beers I have left a full 2 or 3 weeks in the fermenter???

[JR]

Perhaps it depends on how many off flavors etc were produced in the first place. A nice healthy fermentation where everything was right might not produce too much crap that needs cleaned up where a less healthy one might need a while to get cleaned up.

I really dont know though - I'm just making stuff up.

Cooling early and stopping fermentation will only work for beers you are kegging (as it seems you are) its just a recipie for overcarbonation if you are putting it in bottles.

[Reviled]

Cheers JR, thats pretty much along the lines of what I was thinking, it was suggested on AHB that the yeast can absorb by products such as diacetyl and acetyladehyde (spelling?), so I think it may be fair to say that if there is none of these present from a clean fermentation there shouldnt be any problems crashing it early??

And yes youre right JR, I would definately not even think about this if I was bottling, the joys of kegs eh ;o) lol

[Andrew]

The only thing yeasts will really mop is Diacetyl. If you've got acetaldehyde (and/or others) they're there for good. At the end of fermentation your beers should sit for an extra day, known as the "diacetyl rest" this lets the yeast mop up the diacetyl. This will still happen at 12oc to some extent, unless you cool down to around 4-5 oc some yeast will still be playing around but very slowly unless its a lager yeast which as you know will happyily ferment at that temp.

[Reviled]

Cheers Andrew, the more info uncovered about this the more it seems that it is a myth that the yeast will clean up off flavours after fermentation..

Only strain that ever gave me diacetyl was 1026 British cask, and that other strain that Martin was using for a few beers, other than that ive never found the need for a diacetyl rest as I barely get it in my beers? *touch wood*

[Stu McKinlay]

The only thing yeasts will really mop is Diacetyl.

I disagree with this...
Acetaldehyde is produced early in fermentation, like VDK (diacetyl), and is also reabsorbed later. Letting the yeast achieve full attenuation is important, unless you are trying to brew a beer that you specifically want to showcase either of those characteristics... e.g. some english ales, or a Budweiser clone ;-)

Acetaldehyde can also be caused post-fermentation by oxidation of alcohol. I'm not sure I've ever read it but I have a feeling that diacetyl can be caused by this too... Darryl will know more there.
Interestingly (or not), I have noticed a character similar to acetaldehyde that I get off hops (NZ hops especially) early after fermentation. It might stick around for a week or two and then disappears completely. I'm pretty sure that I'm more sensitive to acetaldehyde than most... it's pretty rare, almost unheard of, for me to pick it in a NZ commercial beer these days but I do have to do a double take now and then (the hop character that is similar, seems a richer ("fatter"?) aroma than acetaldehyde... I've discussed it with people but never really worked out what it is).


Back to the initial question...

You will have some yeast left in the beer so, in theory, it should tidy iself up... But 1968, which is a mad flocc'er, may give you some trouble if you're forcing the end of fermentation early. If you're having trouble with attenuation, I'd suggest that you probably look at your yeast health, cleaning/sanitation, and mashing temperature (callibrate you thermometer) before trying any "tricks" to force the final gravity

[Andrew]

Your right Stu but I was refering to the end of fermentation. If you have acetaldehyde at the end of fermentation, its going to be there for good after attenuation.
I'd agree with the rest of your comments. I should learn to explain myself better but then thats why I,m a brewer and not a beer writer.

[Reviled]

Cheers Stu, appreciate the feedback and some interesting comments there...

Im thinking the reason for my over attenuation is recently its been pretty cold outside, and I think my mash temp is dropping a bit in the first 15 mins, so I might try mashing a degree or so higher, but its too late for the stout, hoping to keg it tomorrow, well see what happens...

[Fairlane]

This may not answer your question, but hey... I've got time on my hands...
Now this may come across a bit textbook like but I'm basically paraphrasing a research assignment I did on yeast autolysis a couple of years ago...


Autolysis is where the yeast cell's protoplasm begins to break down under attack from it's own enzymes when the conditions are such that it can no longer grow or multiply. The yeast will then start to metabolize it's own storage reserves such as glycogens or fats, and when these are used up the cell begins to die.

Chemically, the following tends to occur
- the internal organelles of the yeast get disorganised allowing the liberation of hdrolytic enzymes
- inactivation of chemicals which inhibit these enzymes
- enzymatic degradation of intracellular molecules
- an increase in the yeast cell porosity allowing release of the degraded molecules
- further degradation of these molecules outside of the cell.

The products released include
- carbohydrates
- nitrogenous substances (amino acids, peptides)
- nucleic acid components
- fatty acids (octanoic, decanoic)
- vitamins (thiamine, niacin, biotin)

The most important of these components (as far as we are concerned) is the fatty acids. They can be described as giving aromas that are yeasty, fatty, and goaty (if you know what your goat smells like...). They also react with ethanol to produce volatile esters which will affect the aroma of the beer negatively.

Amino acids can also act as precursers for the production of lactones such as Sotolon (better known as 4,5 dimethyltetrahydrofuran -2,3 dione) which gives a nutty burnt odour and / or ethoxy-5-butryolactone which gives a green walnut odour.

Some amino acids have sulphur as a component and when they break down they can form compunds such as hydrogen sulphide (cabbage) or mercaptans (burnt rubber).

Temperature is the most impoprtant condition for control of the autolytic process... in food industries where the processing temperature is over 50deg C, and with a non ethanol environment, the process may take as little as 24hrs. In the wine industry (champagne etc...) the temperature of wine storage is approximately 10degC with a 12 to 14% ethanol environment and the autolysis may not be noted for up to 6 months.

So to try to make a relevant point.... I think that if you have your beer in keg and at low temperatures, you're not going to see many adverse effects from yeast inclusion. If anything the temp will aid settling and give you a cleaner beer. And if you've racked it off the ferment yeast, then the quantity involved will be too low to do any harm anyway...

Whole fresh yeast cells may be addded to reduce diacetyl, but then you need to deal with more autolysis... It would be preferable to condition the beer for longer periods.

Acetaldehyde is actually another product of autolysis and is seen mostly in sherry. Sherry is aged with a "flor" yeast which sits on top of the wine in the barrel, and when it dies it sinks to the bottom of the barrel and releases nutirents for continued flor growth. Along with terpenes and lactone, acetaldehyde is one key component to that nutty, slightly oxidised sherry character.

dh

[Reviled]

Wowza, that was quite a read Fairlane, thanks for that mate! Its actually answered a couple of my questions and I found that really helpful!

[Fairlane]

it took a while to type I can tell you... I'm tired now, must be coffee time :)