By Kevin Kourofsky
Even though wine yeast don’t really know this, fermentation has three stages: initiating, maintaining and finishing. Keeping the yeast happy by maintaining a sound, clean and ongoing fermentation is the most important aspect of winemaking. Failure to maintain an ongoing fermentation is called a ‘stuck’ fermentation, defined as “… a condition, undesirable in the production of dry wines, occurring when yeast activity has ceased.” In humbler terms, it is when the wine appears to be finished but either tastes, or tests, sweet. More undesirably, it’s when a wine thought to be finished fermenting is bottled and when opened foams very obviously in the glass and possibly on your spouse’s new rug. This can happen to any of us, as it did to me very recently, though no rugs were injured in my case.
A stuck fermentation can happen in any of the three stages mentioned above. But with a little care it can be avoided and with a little detective work can be fixed. In my case, a little more care in monitoring the fermentation would have revealed the cause of my stuck fermentation. This March, I decided to purchase some Chardonnay juice from Chile to ease some winemaking of the coming fall harvest season. Such juice is usually concentrated for shipping purposes and reconstituted. Despite this, one can make very good wine from concentrated juice and in the case of Chardonnay, the process brings out more pear and baked apple flavors, which are favored in my household. I chose not to induce malolactic acid fermentation (ML) and started a strong yeast fermentation. Strangely, the fermentation seemed to be taking longer than usual, but continued to bubble away in the airlock. Had I monitored the process more closely, I would have realized that the wine remained sweet to the taste. My testing confirmed at least 2% residual sugar (RS) in the wine and that the sugar levels remained constant. The bubbling was solely an unwanted, spontaneous ML fermentation. The ML bacteria had stepped on the yeast and stressed it. I had a full-on stuck fermentation.
Preventing a Stuck Fermentation at Initiation
Inducing a fermentation should be a gradual process. First you want to make a welcoming home for the yeast. The pantry should be stocked with plenty of oxygen and nutrients that the yeast will need to reproduce. Although finished wine and oxygen are enemies, in fermenting wine they are great friends. I start all my wines, even white wines, in an open container to allow maximum oxygenation.
Among the multiple nutrients required by yeast, nitrogen is the most important. But, yeast will tend to over eat. So, it’s recommended that you wait until the sugar is 1/3rd depleted before adding nutrient. Pure nitrogen, such as DAP, should be avoided in favor of a balanced meal of Fermaid K (includes some DAP) or Fermaid O (no included DAP), or similar products. Since I can’t test to see if there is enough nitrogen in the must (called YAN) to start the fermenting process, I add a small amount of the balanced non-DAP product into the must yeast to start. Call it insurance. I then add the recommended amount of nutrient after the yeast is well established and in need of nutrient, after the must drops its first third of sugar. The only time I use DAP is if there is a hydrogen sulfide (H2S) problem when the must doesn’t have enough nutrients (rotten egg smell) or at the end of fermentation when the yeast is struggling in the higher alcohol environment of end-stage fermentation.
You should also take care to choose an appropriate yeast for the job at hand. If your must has very high sugar, then a powerful-fermenter yeast is called for. This is also true if your end-stage wine will be high in alcohol. Alcohol can smother yeasts. And remember to use the appropriate amount of yeast to get the job done.
Freeze dried yeast needs to be carefully enlivened. These yeasts are very vulnerable to sugar or water that may have chlorine in it or to cold temperatures. When inoculating the must or juice, keep in mind that the temperature of the must should be in the comfort range for the particular yeast you are using. The temperature should be very close to the temperature of the reconstituted yeast solution. I also use a yeast starter product such as Go-Ferm to start the yeast as a nursery for the baby yeast.
Failure to Maintain a Fermentation
There are several reasons why a fermentation can simply cease. Monitoring the fermentation will help you notice problems before they occur. This is especially true if you smell any off odors. Yeast that is struggling tends to produce H2S with a rotten egg smell or (very bad) a raw garlic reek. A brushy smell may be early stages of this problem.
If it’s early days in the fermentation and the alcohol levels are relatively low, then a splash rack may help. This procedure is used to oxygenate the juice. Simply roughly rack the juice into a container and stir. You may also want to check your records to be sure you added nutrients at the proper time and in the proper amount. If not add them. If you have added nutrient, investigate whether your yeast has a high nitrogen requirement. Either way, a bit of DAP here might help restart the process. It may require a couple of splash racks to get the patient well.
Be sure the ambient temperature of the must is appropriate for the yeast. Some yeasts are more sensitive to cold temperatures. Too much cold can put the yeast into hibernation.
If this fails, then you may have to perform a partial or full restart as described below.
Failure to Finish a Fermentation
Fermentations will naturally slow at the tail end. Added sugar to a must and juice from concentrate especially seem to have a problem finishing. Sweetness can be disguised by acid on the palate and the wine may clear itself, leading one to believe the wine is finished. Testing may reveal that you actually have 2 or 3 brix left in the wine. If your hydrometer shows 1 or 2 brix, then you probably have more sugar than that as the alcohol and yeast in solution may cause the hydrometer to be inaccurate. You really want that hydrometer to touch bottom at 0.990.
Many winemaking guides seem to suggest that a cure for a stuck fermentation is as simple as buying more yeast and chucking it in. In early fermentation that might work, especially if the wrong yeast was chosen or there wasn’t enough yeast added. But it probably won’t work in a late stage fermentation. In later stages of fermentation, the yeast has produced 11 to 12 percent alcohol by volume. The original yeast has grown used to this level of alcohol. Young yeast will not have this protection and the alcohol will probably kill it upon inoculation.
Obviously, with all stuck fermentations, you want to try to revive the yeast you have by racking and additional nutrient. If that doesn’t work, then you have to figure out what went wrong and fix the cause of the stuck fermentation before re-inoculation. For instance, high sulfite levels are a burden to yeast, like high alcohol. In combination, they could be deadly to the yeast. If you have that problem, hydrogen peroxide will reduce sulfite levels. I use the calculator at winebusiness.com for the proper addition. It’s best to know what went wrong and fix the problem rather than going through a failed restart.
After eliminating all known problems, the next step is to re-inoculate with yeast that can withstand your wine’s alcohol level. The procedure for doing this is the same procedure used in making sparkling wine. The traditional method for sparkling wine calls for making a base wine at 10 to 11 percent alcohol. That base wine is clarified and then re-sugared to create a desired level of effervescence by fermenting in a bottle. To get that wine to ferment, a process is used to ready yeast to ferment in such unfavorable conditions.
Here are two ways this can be done. Both require you determine the volume of your stuck wine. Rehydrate the amount of yeast you would normally use for your wine with a strong yeast, such as EC-1118 or DV10, using Go-ferm or similar product. Pour off 0.225% of the volume of the stuck wine and create a mixture of 40% distilled water and 60% of the stuck wine, at 6 RS. (Essentially 6 Brix). Add 2 grams of DAP per gallon. (This method is used by professionals and so the amount in the solution you have will probably be miniscule. Also, for the purposes of a stuck fermentation, the sugar level present in the wine should be accounted for in these steps).
On day two take 1% of the volume and create a mix of 75% wine and 25% water at 5 RS and 2 grams of DAP per gallon. Add it to the prior mixture.
On day three use a mixture of 89% wine and 11% water at 4 RS and the same amount of DAP. Add to prior mixtures.
On day four, use 1% of the volume 100% wine with the same DAP. On day five, if the mixture is showing signs of fermentation with “popping” bubbles at the top, then add the whole mixture to your stuck wine. If not fermenting wait a day. If needed you can repeat 1% at 100% of wine and 2.4% RS and another 2 grams of DAP.
A second method, which I use for my sparkling winemaking, is to calculate the volume of the wine to be re-inoculated. You will need to calculate 2.5% of the whole volume. This will be divided into two parts. Then make a syrup of sugar and water at 10 RS of the same 2.4% volume, thus the same amount as the wine. If making sparkling wine, the wine base would be dry. But if your wine is very sweet, adjust the syrup to be at a final 10% RS accounting for the sugar in the base wine. Rehydrate the new yeast. Then add half of the reserved wine (1.25%) to half of the syrup. The alcohol will be halved, as well as the acid level. The sugar level will now be 5 RS, essentially 5 brix. I also add nutrient here, but in small amounts, similar to the amount in the first method.
On the second day, check for signs of fermentation. If present, then try to bring the solution back to 5 RS before proceeding. I find this difficult and fudge it with a refractometer. (Alcohol makes the readings on a refractometer inaccurate. I extrapolate using a starting reading of the wine) Then add the second part of the mixed wine and water to the mixture.
On the third day the solution should be fermenting, I take an additional step to add a small percentage of 100% wine to slowly acclimate the starter solution to the higher alcohol of the stuck wine before inoculation into the stuck wine.
In either of these cases, if the starter solution won’t ferment, then there is probably a remaining problem in the stuck wine, such as high sulfite levels, inhibiting the re-fermentation.
Save the Wine
A stuck fermentation is a bother and must (pun not intended) be dealt with before other microbes take advantage and make a home for themselves in your fermenting vat. But it’s a relatively easy fix. Keeping an eye on your wine is a lesson worth remembering. Speaking of that, you may be wondering what happened with my Chardonnay. Good news and bad news there. I was successful in bringing the RS to less than 1%. But I was unsuccessful in landing the fermentation. I managed to upset the malic bacteria to the point that they created a diacetyl fault. That fault that makes your wine taste like buttered popcorn. Not good! That may be the subject of a different article. But I’m not done yet: Save the wine!
Kourofsky Wine 