"Yeast clean up after themselves"

I see this repeated numerous times here, but I'm not sure I understand the science behind this statement. Any enlightened responses?

Yeah. I'd be interested too. People say to leave the beer in the primary for a month instead of using a secondary but what good are the yeast that have decided to go dormant and lay on the bottom?

I am not familiar with that statement, but in fermentation, yeast produce several chemical compounds that they later reassimilate and metabolize. One such compound is diacetyl which gives beer a distinct buttery, butterscotch note. This maybe what is being referred to in this statement.

Dr Malt

as DM said, and other chemicls too.

Interesting. Yeast "reassimilate and metabolize" diacetyl? That's fascinating. I'd love to hear more about that.

DaveAllen said:

Interesting. Yeast "reassimilate and metabolize" diacetyl? That's fascinating. I'd love to hear more about that.

Click to expand...

Diacetyl rest brother. A process done to all lagers.

Some metabolic sideproducts are utilized by the yeast after they have exhausted the simplest nutrient sources. Diacetyl and acetaldehyde are typical examples. There might be some metabolism of various esters, but probably to a minimal degree.

Aceteldehyde also. It's part of the process to making ethanol. Racking yeast too early will leave that green apple characteristic.

It's important to note that only healthy yeast will "cleanup" fermentation byproducts (i.e., chemically convert a certain percentage of undesirable chemical compounds with a low flavor threshold to one or more compounds with a higher flavor threshold).

For example, if you conduct a poor fermentation (e.g., significantly under-pitching), the yeast will not be able to reduce the offending compounds to a satisfactory degree.

i don't notice any flavor concerns when i pitch about 250-500mL worth of yeast slurry per 5 gallons. yet when done we get over 2L of slurry. which tells me that is "under" pitched if there is such a thing. i bet one has to just add the smack pack to 5 gallons to have any issue with that?

You always want some cell growth, so you may be fine.

Part of the problem with pitching less yeast is that it takes more time and yeast energy to produce enough cells to get the job done. More growth = more esters, not to mention more oxygen and other nutrients the yeast need to replicate. They deplete the supply and then have to get the job done with fewer numbers, putting stress on the yeast, resulting in the above problems. Diacetyl, aceteldehyde, more esters, and under attenuation.

Many times you will see the rational for leaving the primary yeast cake for a month is that the yeast is still working. Lager yeast work slower because of the cold conditions. 'Monks' seem to get fermentation done pretty quickly letting the temps rise.

So, the "cleanup," so to speak, will only occur if the yeast haven't been stressed. I think I get it. It just seems rather paradoxical: if you pitch a healthy amount of yeast, fewer diacetyls and esters will be produced, and what ones that are, will be "cleaned up" by the remaining healthy yeast if allowed to do so. If you pitch too low an amount of yeast, more diacetyls and esters will be produced, and the remaining yeast will be incapable of "cleaning them up." Probably not so much paradoxical as much as a good reason to make sure there is a sufficient pitched rate of yeast to get the job done right.

Thanks for the info, gentlemen.

From John Palmer (howtobrew.com)

The conditioning process is a function of the yeast. The vigorous, primary stage is over, the majority of the wort sugars have been converted to alcohol, and a lot of the yeast are going dormant; but there is still yeast activity. During the earlier phases, many different compounds were produced by the yeast in addition to ethanol and CO2, e.g., acetaldehyde, esters, amino acids, ketones- diacetyl, pentanedione, dimethyl sulfide, etc. Once the easy food is gone, the yeast start re-processing these by-products. Diacetyl and pentanedione are two ketones that have buttery and honey-like flavors. These compounds are considered flaws when present in large amounts and can cause flavor stability problems during storage. Acetaldehyde is an aldehyde that has a pronounced green apple smell and taste. It is an intermediate compound in the production of ethanol. The yeast reduce these compounds during the later stages of fermentation.

The yeast also produce an array of fusel alcohols during primary fermentation in addition to ethanol. Fusels are higher molecular weight alcohols that often give harsh solvent-like tastes to beer. During secondary fermentation, the yeast convert these alcohols to more pleasant tasting fruity esters. Warmer temperatures encourage ester production.

Towards the end of secondary fermentation, the suspended yeast flocculates (settles out) and the beer clears. High molecular weight proteins also settle out during this stage. Tannin/phenol compounds will bind with the proteins and also settle out, greatly smoothing the taste of the beer. This process can be helped by chilling the beer, very similar to the lagering process. In the case of ales, this process is referred to as Cold Conditioning, and is a popular practice at most brewpubs and microbreweries. Cold conditioning for a week clears the beer with or without the use of finings. Fining agents, such as isinglass (fish bladders), Polyclar (plastic dust), and gelatin, are added to the fermentor to help speed the flocculation process and promote the settling of haze forming proteins and tannins. While much of the emphasis on using finings is to combat aesthetic chill haze, the real benefit of dropping those compounds is to improve the taste and stability of the beer.