What is a stuck fermentation? It’s where the fermentation process finishes unexpectedly early (often at 1.020 for beer kits), leaving a large amount of the sugar un-fermented and with a lower finishing alcohol level. A stuck fermentation can be problematic for a number of reasons, firstly the end product is often too sweet and secondly, if you decide to bottle the wine, beer or cider which has stuck, you risk the bottles exploding should fermentation restart for any reason.
What causes a stuck fermentation?
There are many factors which can result in a stuck fermentation, but here are some of the most common reasons the yeast stops working:
- Out-of-date yeast
- Poor temperature control (generally too cold)
- Too much sugar added at the start of fermentation
- Lack of nutrients for the yeast
- Insufficient aeration before yeast added
If you are making beer, wine or cider from a kit, then out-of-date yeast is generally not an issue, unless the kit has been sitting around for a long time. It’s always advisable to check the sell-by date. If the sell-by date is getting close or has even passed, then you should replace the yeast. If you are using packets or tubs of yeast, keep them in the fridge to maximise their lifespan, and keep an eye on the expiration dates. Tubs of yeast should be used within three months of being opened.
Keep it warm
Most wine, beer and cider yeast has an optimum working temperature, which is usually 20°C, although the yeast sachet or kit instructions should specify the ideal fermentation temperature. The exception is lager yeast, which has a lower optimum, of between 12 to 15°C. If the fermentation temperature falls too low, then the yeast will become dormant and fermentation will stop. Moving the fermentation vessel to a warmer area should encourage the yeast to wake up again, but sometimes a little extra help is needed.
Some wine recipes, very high gravity beers such as barley wines, and spirit kits which contain large volumes of sugar, can prove a challenging environment for yeast. For wine and spirits, the best course of action is to only add half or three-quarters of the sugar at the start, then once fermentation is proceeding vigorously, add in the rest. For beer, make a yeast starter a day or two before hand, to ensure the yeast is in optimum condition before pitching.
Vitamins and minerals
While grapes, most wild fruit and crushed grain will contain plenty of nutrients for the yeast, due to manufacturing processes, wine, beer and cider kits can often be deficient in certain vitamins and minerals, which the yeast need, as can wines made form flowers or vegetables. Add in a yeast nutrient at the start, such as Vitamin B1 or Diammonium phosphate for wines and ciders (or use Super Yeast Compound), or Yeast-Vit for beer kits.
Stir it up
In the first stages of fermentation, yeast needs lots of oxygen, which needs to be dissolved in the un-fermented wine / cider or beer. If brewing in a demijohn, shake the contents vigorously, several times, before adding the yeast, to aerate the mixture. If using a larger fermentation vessel, use a perforated beer paddle to stir the must or wort as energetically as you can. Pouring in the water from a height can also help. Dedicated brewers will even use a stainless steel air stone, with an in-line sterile filter to bubble oxygen through the wort!
Restarting a stuck fermentation
One of the first things you need to check is the temperature of the fermenting liquid. You can use a stick-on LCD thermometer on the outside of your demijohn or fermentation bucket, or you can use a sterilised glass thermometer and dip it into the liquid for a reading. If needed, more your fermenter somewhere warmer, or apply heat with the use of a heated brew belt, or similar. Fermentation should re-start within a day or two.
Feed the yeast
If the temperature is not the issue, then try adding some yeast nutrient or Yeast-vit. Add to the liquid and then give the wort or must a good stir with a perforated beer paddle, to rouse the yeast. It doesn’t matter if you stir up the sediment on the bottom, as it will settle back out again, and there may be good yeast hiding there! Make sure your stirring paddle is sterilised, before stirring, though.
Pitch more yeast
Add more yeast. Sometimes a stuck fermentation can be revived by the addition of new yeast. If making wine or cider, use a re-start or high alcohol yeast and follow the directions on the packet. If brewing beer, try a high-attenuating beer yeast, such as Safale US-05. Make sure you follow the directions on the packet, if the yeast needs rehydrating, before being added to your brew.
An alternative is to make a yeast starter, by rehydrating new yeast in warm water (30°C), allowing it to sit for 15-20 mins, then add a small amount of sugar or malt extract (for beer). Transfer to a sterilised bottle with an airlock and once the airlock starts becoming very active, add a little more sugar/malt. Leave for a day and once you have a really active fermentation, add to your stuck brew.
If you are brewing beer, there is another technique you can try, which is adding dry beer enzyme. This is usually an enzyme called amylase, which will break down unfermentable dextrins in the wort, into fermentable sugars. While this will often get fermentation going again, it does have a couple of drawbacks. Firstly, it will reduce the sweetness of the beer, making it taste ‘dry’, secondly, by removing the unfermentable dextrins, it will reduce the beer’s body. To counteract this, you could add in 200g or so of lactose, prior to bottling (it will need to be dissolved in hot water and then stirred into the brew). Lacotse is also unfermentable and while it adds very little sweetness, it will add more body.