Yes, sluggish and stuck fermentations are one of the most common, persistent, and frustrating issues that winemakers encounter. They happen for so many reasons, and possibly for a combination of so many reasons, that it’s often difficult for a winemaker to diagnose and troubleshoot the cause. It’s even harder, unfortunately, to come up with a solution for dealing with them (more on that later). In the meantime, let’s talk about possible causes and their prevention, so you can enter the fermentation season with a little more confidence.
Sluggish and Stuck Fermentation Causes and Solutions:
1) Brix too high and yeast die off due to escalating alcohol.
Solution: Try to pick your grapes a little earlier or add some water to your fermentation. If it’s red must, please mix carefully as water pockets can cause bacteria to grow because the low pH of the juice won’t deter them. Water has a pH of around 7 which bacteria love, so be sure to mix in any water additions well.
2) Pitched yeast, or your indigenous house yeast culture, weren’t alcohol-resistant enough to ferment to completion.
Solution: Some yeast strains just won’t ferment in an environment where the alcohol is much higher than 14%. Be sure to choose a yeast strain that can. Most winery supply companies that sell yeast maintain a yeast selection spreadsheet or graphic where they point out the pros and cons of each strain. Strains like EC-1118 (aka Prise de Mousse) and Uvaferm 43 are naturally selected for their strong fermentation characteristics and ability to ferment at higher alcohol levels, sometimes up to 16–17% in ideal conditions.
3) Yeast didn’t have the right nutrition.
Solution: More often than not, you need to feed your yeast, more than just what the grapes will provide. Yeast need the right mix of vitamins, minerals, and biotin, just like we do, to perform at their best. Though you don’t want to over-feed, it’s cheap insurance to boost your must with a little bit of a complex yeast nutrient. Many supply companies sell them. Look for one that doesn’t just have DAP (diammonium phosphate, a source of nitrogen) but includes things like yeast hulls, vitamins, minerals, and amino acids. Though it’s impossible to know exactly how much to add unless you send a juice sample out for a YAN (yeast assimilable nitrogen) test, it’s always a safe bet to add a conservative dose like 0.2 g/L, though of course that does depend on the specific product’s composition you are using. When in doubt, ask your supplier for dosing suggestions.
4) Yeast died due to wrong fermentation temperature.
Solution: Yeast are pretty temperature sensitive. Too cold and they won’t get started. Too hot and their cellular membranes become increasingly sensitive to the alcohol in the fermenting wine. Make sure your fermentation is always between 55 and 85 °F (13 and 30 °C). Ideally you’ll have your fermentation temperature act like a nice bell curve, starting out between 55–65 °F (13–18 °C), peaking at 85 °F (30 °C) or so, and then cooling gradually back down to finish somewhere around 70–75 °F (21–24 °C).
5) Yeast had competition from other spoilage microbes.
Solution: Try adding 15-20 ppm SO2 to your must prior to inoculating with your selected yeast in order to retard the growth of spoilage bacteria.