Stuck Fermentation

STUCK BEER

In the majority of cases, an apparently stuck beer fermentation is not a problem. We have a lot of people call and say “My beer went crazy for X hours / days, and then stopped completely.” Usually, this scenario simply indicates that the brewer pitched a very active yeast which consumed the vast majority of available sugars rapidly. Then, having nothing else to eat, the yeast simply stopped fermenting and began to settle to the bottom. Check the beer with a hydrometer – if the (Gravity minus 1) is equal to or less than your (original gravity minus 1) times .24, then the active stage of fermentation is most likely complete and you can go ahead and rack your beer to the secondary fermenter.

Here’s an example using a beer with original gravity of 1.054:
1.054 – 1 = .054
.054 x .24 = .013
If gravity is in the range of 1.013, yeast activity is finished and the time has come to transfer the beer to the secondary fermenter.

Remember also that the unfermentable sugars from Crystal (Caramel) or Dextrine (Carapils) malts, Malto-Dextrine or Lactose will add about as many points to the final gravity as they did to the OG – about 6 points for the grain malts and 8-9 points for the powders. Therefore, for each pound of Crystal or Dextrine malt added, add .006 to the expected final gravity, and for each pound of Malto-Dextrine or Lactose add .008 to the expected final gravity.

Here’s an example using a beer with original gravity 1.054 which included 1
lb. of Crystal Malt and 1/2 lb. of Lactose in the recipe:
1.054 – 1 = .054
.054 x .24 = .013
.013 + .006 (1# Crystal malt) + .004 (1/2# Lactose)= .023
Expect a final gravity in the range of 1.023

Different yeast strains have differing attenuations. This simply means that some yeast strains are able to consume sugars that others find unfermentable. Wyeast Labs produces a nice little pamphlet that details the attenuation and other characteristics of each of their yeasts – it’s available free from Highlander with any purchase. Most strains of beer yeast will leave about 24% of the original sugars unfermented, which is where we get our .24 for the above equations.

If you have an apparently stuck fermentation and the numbers don’t fit in the above equation, taking into account unfermentable content as described above, then you probably have a truly stuck fermentation! 🙂

There are several probable causes for stuck fermentations:

  1. Highly unfermentable wort
  2. Rapid temperature change
  3. High gravity wort (and corresponding alcohol levels)
  4. Insufficient aeration of the wort
    A combination of 2 or more of the above is not uncommon.

Highly unfermentable wort is fairly common – as brewers begin to experiment with creating their own recipes, they add too much of a good thing. A brewer will decide that he/she likes the flavor that a pound of Crystal malt adds to a recipe, and triples it on the next batch, giving the finished product a syrupy mouthfeel and sickly sweet taste. Laaglander brand dried malt extract (which can also be labelled as “European” or “Hollander”) has a very high unfermentable content – I’ve made batches of beer using Laaglander DME which finished out at 50% attenuation (only 50% of the original sugars consumed) and tasted like hoppy pancake syrup. (A side note- don’t use Laaglander DME for priming your beer – because of its unfermentable content, the beer’ll be flat and sweet)

Rapid temperature changes can also cause stuck fermentations. Yeast likes a nice, comfortable, constant environment – rapid temperature changes (more than 6-8 deg F / day) can cause yeast to flocculate early and settle to the bottom of the fermenter. If this happens, the only thing to do is move your fermenter to a place with constant temperature and add more yeast.

High gravity wort is another common cause of incomplete fermentation. Since most beer yeast can only tolerate alcohol levels in the 8-10% range, extremely high gravity wort and correspondingly high alcohol levels present a problem for the brewer. If making high octane beer (known in some circles as “head remover”), pitch a large quantity (use a starter) of quality ale or lager yeast at the beginning of fermentation. When the yeast peters out at the high end of its alcohol tolerance, add champagne yeast, yeast nutrient and amylase enzyme. You’ll get the desired flavor profile in the finished product from the original yeast, and desired alcohol levels (up to about 15-17% v/v) from the champagne yeast. To get alcohol levels above about 15-17%, you need to distill your beverage, which is illegal in the US.

The best way to aerate your wort and make the environment friendly to yeast is with a tankfull of pure oxygen. Since most of us don’t have ready access to one of these, a fishtank air pump with a .5 micron filter is the next best bet. These are relatively inexpensive and easy to use, but they take a while to do the job. Those with strong backs can pick up the fermenter and give it a vigorous shake, but this tends to cause backache. Highlander carries a little gizmo called the Syphon Spray Wort Aerator which works wonders. The brewer simply syphons the wort from the kettle to the fermenter, with the Aerator attached to the bottom (receiving) end of the syphon hose. A little pyramid-shaped thingy in the Aerator splashes the wort, introducing oxygen to help the yeast reproduce.

Occasionally the brewer will use a yeast that flocculates prior to the end of fermentation, despite the absence of any of the above factors. This most frequently occurs when using a strain cultured from someone’s finished product, such as Sierra Nevada or Samuel Smith’s bottled beers, or a growler from the local brewpub. Stirring the yeast back into solution will often kick the fermentation off again, but it needs to be done GENTLY to avoid introducing oxygen – while oxygen is necessary in the reproductive phase of yeast activity, it is detrimental during fermentation, as it causes business card-like and/or medicinal flavors. If using a highly flocculant yeast such as Wyeast #1728 Scottish Ale or Samuel Smith’s, the yeast will often need to be stirred back into solution several times before it finishes its job.

As a final note, the addition of amylase enzyme as yeast is pitched into the wort, or when trying to un-stick a fermentation, is often of great benefit. Amylase assists the yeast by breaking down sugars to their simplest form, thereby making the sugars readily fermentable. Amylase is also of great benefit in achieving a very low final gravity (much lower than usually expected.)

STUCK WINE & MEAD

The sugars in fruits and honey are much less easily fermentable than those in malt. Despite the wine/meadmaker’s use of yeast nutrient and energizer to avoid stuck fermentation, many of us that make these vinous beverages do occasionally experience an aggravatingly high final gravity.

Mead fermentations “stick” with alarming regularity. The brewer devises a concoction that should be wonderful, adds nutrient and energizer, adjusts the acid level to make it friendly to the yeast , and then waits months for it to ferment. He/she goes to rack it to another fermenter after seemingly interminable waiting, only to discover that the fermentation quit at some ridiculously high gravity. There are two tried and tested ways of restarting a mead fermentation.

Method 1.

  1. Rack your mead to a cleaned and sanitized secondary or tertiary fermenter.
  2. Add .5 tsp yeast nutrient per gallon, and .25 tsp energizer per gallon, to about a pint of boiled water cooled to 100 deg F. Add this solution to your must. Take a pH reading, and adjust the pH to 3.3 (3.4 works).
  3. When the pH is adjusted to the proper level, rehydrate a nice fresh packet or two of Red Star Pasteur Champagne (or Lalvin EC-1118) yeast and repitch it into the batch.

Method 2.

  1. Adjust pH to 3.3-3.4
  2. Add 3/4 tsp. Amylase Enzyme per 5 gallons of mead to boiled water that has cooled to 90-100 deg F, and add this solution to the mead.
  3. Stir mead gently to rouse yeast back into solution. Stuck wine fermentations provide a more complex series of variables than those of mead or beer, the first and foremost of which is the type of yeast used. In fresh-fruit winemaking, the vintner often refrains from pitching yeast, preferring to use the naturally occurring flora on the skins of the fruit. Fermentations using this method seldom (never in my experience) stick, as this naturally occurring flora is ideally suited to the fermentation of the particular fruit on whose skin it appears. Yeast is often added to wine made from concentrate, or to fruit wine which has been sulphited to prevent the naturally occurring flora from causing fermentation.
  4. Rack your mead to a cleaned and sanitized secondary or tertiary fermenter

Either of these methods should kick your fermentation back off again. You
can try rousing the yeast in method #1 rather than repitching, or repitching
rather than rousing in method #2.

Stuck wine fermentations provide a more complex series of variables than those of mead or beer, first of which is the type of yeast used. In fresh-fruit winemaking, the vintner often refrains from pitching yeast, preferring to use the naturally occurring flora on the skins of the fruit. Fermentations using this method seldom (never in my personal experience) stick, as this naturally occurring flora is ideally suited to the fermentation of the particular fruit on whose skin it appears. Yeast is added to wine made from concentrate, or to fruit wine which has been sulphited to prevent the naturally occurring flora from causing fermentation. Because of the high but variable original gravity of wine, (usually in the range of 1.080-1.120) and the differing attenuation and alcohol tolerance of various sweet & dry, red and white wine yeasts, vastly different final gravities are expected depending on the desired finished product. Dry wine, either white or red, can be expected to finish below 1.000 FG, while sweet wines often finish at a gravity of 1.015 or above. Therefore, an apparently stuck wine fermentation is not necessarily undesirable, depending on the character the vintner wants in the finished beverage. Essentially, the winemaker needs to taste his/her product prior to bottling, to see if the sweetness level is what he/she wants. If it’s too dry, it can be easily sweetened to taste. If the fermentation has stopped and the wine is far too sweet, or the gravity is well above the desired final figure, the winemaker should follow these steps.

  1. Perform an Acid Titration Test to determine if the acidity of the wine is appropriate for the style.
  2. Prepare a starter culture of a highly alcohol-tolerant yeast strain .
  3. Add water to the wine to reduce its gravity to 1.080.
  4. Vigorously stir wine to aerate and add Yeast Nutrient (1/2 tsp. / gallon) and Yeast Energizer (1/4 tsp. / gallon). If using Yeast Nutrient tablets, add 1/2-1 tablet per gallon of wine.
  5. When the culture prepared in step 2 above is actively fermenting, add an equivalent amount of “stuck” wine to the culture. When this culture/stuck wine solution is again fermenting, add it to the stuck wine at 75-80 deg F. Repeat step 2 and this step as often as necessary until vigorous fermentation is again observed in the previously stuck wine.