Fermentation stall is when the yeast fails to reduce the gravity of the beer to its potential final gravity. If you take a gravity reading over the course of a few days and do not see the gravity drop, then your fermentation has stalled.
Any failure to catalyze the yeast to work or pressure applied to it beyond its limits can trigger a fermented beer to stall. The most common causes are
Yeast health is critical. Within reason, this is more important than how much you pitch.
The best way to ensure yeast health is to brew with the freshest yeast possible. If you are using some old yeast, use a fermentor. In fact, just use a starter anyway.
Don't put a packet or vial of 12 month old yeast into a starting gravity of 1.066 and expect great results. It may stall or just brew crappy beer.
If you make wort with a lot of adjuncts, you may experience slow or stagnant fermentation. To avoid this, add yeast nutrients to the wort during the last five minutes of the boil.
To solve this problem, use a yeast energizer. Energizers are different from yeast nutrients. The energizer can be added directly to the fermenter to get the yeast running again.
The temperature of the fermented beer has a great influence on the characteristics of the fermentation. Here is a simple example. If you place a beer strain at 50 degrees Fahrenheit, you may only need to heat it up to a more normal beer temperature.
Sometimes just raising the temperature a degree or two is enough to get things going.
Yeasts need oxygen to function. In fact, they can replace oxygen with other things, such as lipids. Nutrient-rich wort will require a smaller amount of oxygen.
Most of us have been told to leave the residue outside the fermenter. So, at least you should shake your carboy before you pitch your yeast.
If your tank environment is properly controlled, you can aerate the wort at this point and allow the yeast to breathe and thus multiply.
This is probably the first piece of advice for young apprentices whose fermentation has stalled. Gently stir the yeast in your beer fermenter. The idea is to get them back in suspension and in an environment where they can "do their thing" with sugar and nutrients. It's easy and worth a try.
Timing is critical. If the beer won't start, simply raise the temperature to a point where it can ferment. For example, it's okay to raise the beer temperature from 66 F to 68 F, but don't reach 75 F and expect to taste a great beer.
If your beer stalls after 3 or more days of "normal" fermentation, you can raise the temperature more aggressively.
Temperature is very strain dependent. Work within the parameters recommended for your strain.
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