Every day during harvest, the lab tech goes to each fermenting batch of wine and tests the level of Brix (a measurement of the percentage of sugar). As the wine ferments the Brix steadily drops until it reaches zero, indicating that all of the sugar has been converted by the yeast to alcohol. But occasionally during the fermentation, the chart hanging from the tank where the tech records posts the readings shows the same result day after day. That’s when you know you’re dealing with a stuck fermentation. The yeast has died off, become less active or stopped multiplying. Without treatment, the result would be syrupy sweet juice. It can be scary, but with the right techniques, a skilled winemaker can save the day. Our gold-medal-winning Buchigniani/Garcia Dry Creek Valley Zinfandel gets stuck every year because it comes in at such high Brix.
High Brix means high sugar and sometimes that creates a situation in which the yeast has made so much alcohol that before its finished consuming all the available sugar, the alcohol is high enough to kill the yeast. There are other reasons that primary fermentation can get hung up though. Sometimes the problem is as simple as the tank is too cold for the yeast, and they slow down quit doing their job. Occasionally a harmful bacterium from the vineyard such as Acetobacter or a mold such as Botrytis is producing acetic acids, increasing the volatile acidity and thereby disturbing the yeast. Sometimes there are not enough nutrients in the juice to sustain the yeast. In all cases, as soon as the winemaker has discovered that their wine is stuck, they immediately press the wine off of the skins so that they can better control the process.
Additionally, secondary fermentation (or malolactic fermentation), in which bacteria converts harsh malic acids to softer lactic acids, can outstrip the pace of the yeast. This is often the case with stuck fermentations and the procedure is to let the beneficial bacteria Oeonococcus and Lactobacillus run their course, at which point the primary yeast fermentation can be restored. The wine is then racked off of its lees before restarting the regular fermentation. Rice hulls (which are like the husks of rice grains) are added to neutralize harmful toxins in the juice.
The process of restarting fermentation varies from winemaker to winemaker. At Deerfield we begin by taking some of the wine from the tank and making a mixture of 50% wine, 50% water, fresh yeast, yeast food, and other nutrients which we then allow to start fermenting. This mixture is then added to the tank which is kept at 70⁰F and carefully monitored. When the Brix drops by half of what it was when it became stuck another mixture is prepared. This time the ratio is 75% wine and 25% water. Again when the Brix halves a final mixture is readied. The last mixture includes just wine from the tank and the remaining nutrients the yeast needs. The idea is to slowly acclimate the yeast to the environment of the wine so that it is happy, healthy, and well prepared to do its job. Usually this will resolve the problem and the fermentation will finish.
Maybe we can all learn something from the yeast: When you get stuck, take a bath, give yourself a fresh start, then slowly ease back into it and you’ll solve the problem in no time!