What are the causes of fermentation stoppage when brewing beer?

Fermentation That Never Properly Starts

A non starting fermentation is still a stuck fermentation but the reasons why it never starts properly in the first place may be different to a fermentation that stalls toward the end. As you might guess most of the reasons point to the yeast and the factors that affect yeast health so let’s start with the basics and work onward from there.

One thing to point out here is that just because the airlock isn’t bubbling it doesn’t mean the beer isn’t fermenting. The only real way to check the state of a fermentation is with a hydrometer reading.

The Viability and Quality of Yeast

This is the first thing to look at if the fermentation starts sluggishly and then gets stuck or fails to even start. Every package of yeast you buy will have either a production date or a use by date. This information is vital in telling you whether you are pitching healthy, viable yeast cells or simply poor quality and dead yeast cells. If you have had a package of yeast for a while it may be that the viability of cells has dropped too low so there aren’t enough healthy yeast when pitched into your wort to properly start fermentation.

If you have a packet of yeast that’s old or a dud, repitching a new package should start the fermentation as normal.

DEGONG fermentation tanks

Temperature Of The Wort

This is the next thing on the checklist if you are having a painfully slow, non starting fermentation or stuck fermentation. If the temperature inside the fermenter is too low it can inhibit the yeast activity. Different strains of yeast work best at differing temperature ranges. If you leave the fermenter in a cold garage it may be too cold for the yeast to become active.

Ideal temperature ranges for most ale yeasts are around 18°C – 21°C, lagers are lower at around 7°C – 14°C. If your beer is sitting in too cold of a spot then it is likely the yeast will be struggling to get going.

Similarly, higher temperatures will have potential to cause problems. If you pitch the yeast whilst the temperature of the wort is still too hot there is the possibility that these higher temperatures will actually kill the yeast, in this case, fermentation will have no chance of even beginning.

If the fermenter is in a spot that is likely to fluctuate in temperature or fall out of the ideal range of the yeast move it to somewhere more suitable. If you think you may have pitched the yeast into too hot a wort you will need to repitch new, healthy yeast.

Lack of Oxygen in the Wort

Yeast cells need oxygen to reproduce. This is why you are advised to aerate the wort as it is going into the fermenter.

Most brewers rely on splashing or movement to aerate the wort as it is poured into the fermenter, this is the simplest way to introduce oxygen needed by the yeast. Other brewers use aeration devices on a drill for example, to whip air into the wort and some even use pure oxygen through an aeration stone.Most brewers can make do with aerating by pouring the wort from a height to get oxygen into solution but as the gravity of a beer rises the additional stresses put on the yeast can mean a lack of oxygen can cause a stuck fermentation as there is not sufficient resources for them to reproduce.

If you skip this simple step of introducing oxygen just prior to pitching yeast it can stall the yeast, increase the lag time before the beer starts fermenting and cause a stuck fermentation. The best practice, in this case, is to aerate the wort and pitch fresh, healthy yeast.