Why do fermentations get stuck?

cnoyes · 02-10-2008, 09:03 PM

I'm wondering if somebody can explain chemically or biologically how fermentations get stuck. The common theory is that it was racked to secondary too soon, but I don't understand how this works. The only yeast you're leaving is the yeast that is already flocculated out and in the very bottom. For yeast to metabolize anything, doesn't the yeast have to be in contact it? So wouldn't the yeast that is still suspended be the only yeast that really matter?

I think this question is also related to the practice of leaving the beer in the primary to condition instead of using a secondary.

DAAB · 02-10-2008, 09:25 PM

I would have said the most common reason for stuck fermentation is under-pitching.

As for racking to a secondary before the stable gravity point, the active yeast isn't in suspension at all times in a homebrew enviroment. This short description by Graham Wheeler of what's going on in a fermenter should explain things...

Quote:

A major one is that our fermenters are much shallower than the ideal, commercial ones, which by convention is six feet. It means that the majority of the yeast is either sitting on the top, or the bottom and not in suspension. The yeast is carried up in the wort when it produces a bubble of carbon dioxide, and falls back down again when it releases it. This critical distance is six feet. By the time it has risen five or six feet it releases the bubble and sinks, and by the time it has sunk five or six feet it has generated another bubble and starts to rise again. This keeps a lot of yeast in suspension, whereas in our case, with only about 18 inches of depth, it doesn't get this opportunity, and consequently a lot of our yeast is sitting on the bottom, waiting to produce another bubble. So it is a possibility that an inadequate amount of yeast is carried across.

cnoyes · 02-10-2008, 09:32 PM

Ahhhhhhh. So the yeast do "swim". They don't just stay sitting on the bottom.

That's what I was looking for, thanks. What was the source on that?

|-|edghog · 02-10-2008, 09:39 PM

Another thing that I have heard is that a yeast colony (if you can call it that) actually does a survey of it's environment before it starts feeding in order to set itself up for a future hibernation stage.

So, if their feeding cycle is disturbed there is a chance that it may cause the colony to collapse and go into hibernation early.

Not sure if this is exactly true but I remember reading it somewhere once.

Hedghog

Jack · 02-10-2008, 10:34 PM

I heard the most common reason was that the yeast ran out of oxygen before they finished fermenting. I thought that is why commercial brewers and hard-core homebrewers pay so much attention to the oxygen concentration of their wort.

cnoyes · 02-11-2008, 12:56 AM

Quote:

Originally Posted by Jack

I heard the most common reason was that the yeast ran out of oxygen before they finished fermenting. I thought that is why commercial brewers and hard-core homebrewers pay so much attention to the oxygen concentration of their wort.

According to howtobrew.com, the oxygen is for the stage before primary fermentation, when the yeast are reproducing.

**Schlenkerla** · 02-11-2008, 01:38 AM

The list can be long, here is the most common.

Not enough nutrients (wine & cider)
Too cold
Too hot
Too much alcohol
Too much maltose
Not enough yeast
Not enough oxygen
Too acidic
Yeast is too Old

**Bobby_M** · 02-11-2008, 02:01 AM

Out of spite mostly.

In my understanding, each cell has a finite energy store and can only consume so much sugar. Therefore, you either start with enough cells to pull off the whole ferment or you create an environment that allows them to reproduce to a large enough colony to pull off the ferment. That's acheived by flooding the wort with O2.

ohiobrewtus · 02-11-2008, 02:02 AM

IMO, the most common reasons for a stuck fermentation would be underpitching yeast or pitching a yeast that is not appropriate for the style (expecting a yeast to ferment out something with an OG of 1.100 for example).

DAAB · 02-11-2008, 01:10 PM

Quote:

Originally Posted by cnoyes

Ahhhhhhh. So the yeast do "swim". They don't just stay sitting on the bottom.

That's what I was looking for, thanks. What was the source on that?

oopps, I intended to include the authors name in that post (i've no rectified it).

It was Graham Wheeler, the author of Homebrewing the CAMRA guide, discussing the dropping system HERE.