Stuck Fermentation

Stuck fermentation defined | How to tell if wine is "stuck" | Causes of stuck fermentation | Prevention and treatment of stuck fermentations | References

Have you ever started a wine - and fermentation seems to start normally enough - but all of a sudden, the signs of fermentation (bubbles in your airlock, or falling Specific Gravity, for example) seem to slow down or stop too soon? If so, you've experienced what's known as a "stuck fermentation."

Definition
By definition, a stuck fermentation is a fermentation that has stopped before all the available sugar in the wine has been converted to alcohol and CO2. Were you to give up on the wine at this point, it would taste semi-sweet and pretty bad. That would be a shame, and what's more, a waste of good juice!

How did this situation occur? More importantly, what can you do to restart fermentation and salvage your wine?

Is It Really Stuck?
Before we dive into these questions, we should first make sure that our wine is stuck. Ask yourself these questions before you start dumping yeasts, additives, and chemicals willy-nilly into your carboy:

1. What is the SG (specific gravity) of your wine? Do you have proof that the SG is no longer falling, or is tremendously sluggish? If you don't know (or aren't sure how to do this), we recommend you see our article on how to use the hydrometer. It explains the ins and outs of measuring your wine's SG.
2. Do you have a good airtight seal at your airlock? Is your airlock firmly seated in the bung, and is the bung securely seated in the mouth of the carboy? If not, this might explain why you don't see bubbles in your airlock.
3. Are you fermenting in hot weather or in a hot area? Yeast works faster under higher (yet tolerable) temperatures, so your wine may actually be finished fermenting before you realize it.

Luckily, stuck fermentations don't occur very often - but when they do, it's important to make corrections right away and get the fermentation going again.

Causes of Stuck Fermentations
More than likely, the cause of a stuck fermentation centers around the wine yeast. Either something in the wine environment is preventing the yeast from working properly, or there is a problem with the yeast itself.

Even if the proper yeast is used, most experienced vintners know that wine yeast is pretty particular when it comes to fermenting wine to dryness - the proper environmental conditions (such as cleanliness and temperature) must be met, and nutrients (such as a balanced source of DAP [diammonium phosphate], amino acids, minerals, and vitamins) need to be available for the yeast to continue their hard work.

Wine yeast is most happy when:

• It's not too hot, and not too cold
• There's lots of food to eat
• No killer agents are present
• They live in sanitary conditions
• Oxygen is available (to kick off fermentation)

Sounds a lot like humans, huh? Using a little common sense, then (which vintners seem to have a lot of!), we can easily extrapolate the major causes of a stuck fermentation:

1. Extreme fermentation temperatures - too high or too low.
2. Using unsanitized equipment - dirty or unsanitary equipment increases the possibility that microbiological factors such as wild killer yeasts and bacteria will spoil your wine.
3. Using old yeast - weakened or expired/out-of-date.
4. Incorrect yeast used - match the proper yeast for your wine (in the case of buying wine concentrate kits from grapestompers, this is automatically done for you).
5. Yeast not rehydrated before pitching - always rehydrate yeast according to manufacturer's suggestions.
6. Yeast rehydrated at too low or too high a temperature - this can kill a large percentage of yeast cell population.
7. Temperature shock when rehydrated yeast is introduced to must - try to allow no more than a 5-7° C differential between yeast mixture and must.
8. Sulfite levels too high - adding too much metabisulfite; failing to wait 24 hours after Campden applied to must before pitching yeast; or high must pH, which can lead to high fermentation rate.
   
9. Pesticide residue on the exterior surface of grapes or fruits - wash all grapes or fruits well before processing.
10. Lack of nutrients, including a lack of nitrogen or certain amino acids.
11. Extremely high starting SG - too much sugar in must at the outset.
12. Sugar has all been utilized - you don't want your starting SG to be too low either!
13. Too much CO2 in your wine - don't forget to degas.
14. Naturally occurring sorbate in must - as in the case of blueberries.

Prevention of Stuck Fermentations
Here are some things the home winemaker can do to prevent stuck fermentations:

• Monitor and ensure proper fermentation temperatures.
• Ensure proper sanitation - learn how to sanitize equipment
• Use fresh yeast.
• Use the proper yeast for the wine you're making - don't guess or use a packet of yeast just because it's handy.
• Properly rehydrate yeast before pitching.
• Pitch the yeast within 20 minutes of rehydrating it.
• Maintain proper free SO2 levels - the amount of metabisulfite to add to your wine depends on pH of wine.
• Add yeast nutrient before pitching yeast - Item # 2733.
• Keep your starting SG to reasonable levels (1.090 - 1.100 or lower). If you don't currently have a hydrometer, buy one (they're inexpensive - about $5) and learn how to use it.
• Aerate the must properly by vigorous stirring, just before pitching the yeast. This will introduce the oxygen needed to "kick off" fermentation.

Treatment of Stuck Fermentations
And here's what to do if you get stuck... and remember - always start with the simplest things first. Resist the urge to add yeast or additives until you've tried the easy things.

1. Adjust the temperature of your wine. In most cases we've seen, simply warming your wine to 70-75° F for a couple of days will get the ball rolling.
2. Rouse the yeast by swishing or stirring the lees (trub) - sometimes moving the yeast around in the wine will get fermentation going again.
3. WARNING: Although it may be tempting, don't add vitamins (yeast nutrient) during stuck fermentations. Leftover vitamins can stimulate spoilage microbes. Only add a yeast nutrient before or as you pitch your yeast. If you want to add a yeast energizer at this point (which is not the same thing as yeast nutrient), that's OK. Simply go to the local drug store and ask the pharmacist for some Thiamin HCL (thiamin hydrochloride). Add 25 mg. per gallon of wine and mix well.
4. Remove the old yeast by racking the wine, then re-inoculate with fresh yeast, preferably a killer strain like Lalvin EC-1118 or Red Star Premier Cuvee. In a pinch, you could even use a Red Star Champagne yeast. We want to get rid of the old yeast because yeast cells seem able to detect the presence of other dying cells, and are more likely to get "lazy" themselves.
5. If you detect there is a nitrogen deficiency (less than 200 mg/L fermentable nitrogen), addition of DAP (diammonium phosphate dibasic - commonly known in the winemaking industry as Fermaid*) is called for.

If none of the above seem to help restart your fermentation within a couple or three days, it's time to bring in the heavy hitters:

1. Make a yeast starter by pulling off approximately 1/2 gallon of must, and add 1.5 to 2 teaspoons of yeast energizer (thiamin HCL) and 1 packet of "killer" or champagne yeast. Mix well, cover loosely and place in a warm spot. Once you have a vigorous fermentation you can add it back to the original must. OR...
2. Make a different kind of starter: use about a 1/2 cup of warm water, dissolve 1 teaspoon of sugar in the water, add some orange juice to this mix, make sure the temperature is about 90° F, before adding a packet of Red Star Premier Cuvee or Lalvin EC-1118 yeast to this mixture. Wait until it really gets working. Take about a gallon of your must and warm it up to about 68° to 70° F. Now add the yeast starter to the gallon of must, as it starts to work and gets going, SLOWLY add small portions of the stuck fermentation to that which is working. You should not add more than a quart, make sure the temperature of that which you are adding is at least 70° F. As the volume of the working must gets larger, you can add larger portions to the fermentation. Make sure the temperature is at least 70° F before you add it.

Using one of these methods should help get your fermentation restarted.

References
Clicking on any of the reference links below will open a new browser window:

http://wineserver.ucdavis.edu/oldsite/nsurvey.html
http://listproc.ucdavis.edu/archives/ven3w00/log0002/0001.html
http://www.fst.vt.edu/Zoecklein/Fermentationissues.html
http://www.teleport.com/~wineman/feb99.html
http://www.wynboer.co.za/recentarticles/0399ferment.php3
http://www.wynboer.co.za/recentarticles/0299pinotage.php3
http://www.nysaes.cornell.edu/fst/faculty/acree/fs430/lectures/thk35yeastnutrients.html
http://winemaking.jackkeller.net/yeast.asp
http://home.att.net/~lumeisenman/chapt12.html
http://www.lallemand.com/danstar-lalvin/lalvin.html

*Notes:
Fermaid contains diamonium phosphate (DAP), magnesium sulfate, yeast hulls and vitamins.

DAP addition of 1 g/L (8.3 lb/1,000 gal) provides about 258 mg/L fermentable N. This is greater than the supplier's recommended level. In the US the legal limit of DAP is 960 mg/L which is equal to 208 mg N/L. Fermaid K at 2 lb/1000 gal (25 g/hL) = 25 mg N/L while DAP at 2 lb/1000 gal (25 g/hL) = 50 mg N/L. As indicated, fermentable nitrogen concentration in juice or wine can be easily estimated (Zoecklein et al., 1995).

Many musts lack sufficient nitrogen, vitamins and other ingredients needed by yeasts during their growth phase for healthy fermentations. Levels of greater than 200 mg/L fermentable nitrogen are required for healthy fermentations. Supplementation should be made using a balanced source of DAP (diammonium phosphate), amino acids, minerals, and vitamins.