Stuck Fermentation — Causes & Restart Techniques

A stuck fermentation is the term used in winemaking when yeast becomes dormant before fermentation has completed. This differs critically from an 'arrested fermentation,' where the winemaker intentionally halts conversion, as in the production of fortified wines. Operationally, a stuck fermentation is identified when SG or Brix does not change for more than 24 hours despite fermentable sugar remaining above 4 g/L, the widely accepted threshold below which a wine is considered dry. A sluggish fermentation, by contrast, continues at a reduced rate. Both conditions create risk, as an unfinished fermentation leaves residual sugar available to spoilage bacteria and wild yeasts, potentially driving up volatile acidity and generating off-aromas.

• Monitor SG or Brix daily; a halt of 24 hours or more with sugar above 4 g/L confirms a stuck state
• Distinguish from sluggish fermentation through trend analysis; any ongoing Brix decline, however slow, indicates yeast are still active
• Residual sugar above 4 g/L in a finished wine creates microbiological instability and a risk of re-fermentation in bottle
• Stuck fermentations at higher residual sugar (above 10 g/L) present greater commercial loss and more urgent spoilage risk than stalls near dryness

Research has identified more than 15 potential causes of slow or stuck fermentations. The two most commonly cited are excessively high temperatures killing yeast outright and nitrogen-deficient must unable to sustain yeast metabolism. Nitrogen is the most important macronutrient for yeast after sugar, and musts below 140 mg N/L YAN are considered high-risk for fermentation arrest. Grape varieties such as Chardonnay and Riesling are naturally prone to low nitrogen-to-sugar ratios, compounding this risk. Overripe grapes produce musts with high sugar loads that approach yeast alcohol tolerance thresholds of 16-18% ABV, at which point yeast reproduction ceases. A further, frequently overlooked cause is the accumulation of medium-chain fatty acids (hexanoic, octanoic, and decanoic acids) produced as yeast metabolic byproducts, which disrupt yeast membrane permeability and inhibit sugar uptake. Microbial competition, pesticide residues, overly clarified juice lacking grape solids, and premature fining agent additions are additional documented triggers.

• Temperature extremes, both too high and too cold, are among the most common causes; each yeast strain has a defined optimal temperature range that must be maintained
• Test YAN before fermentation; the minimum threshold for completing fermentation is 140 mg N/L, with higher-Brix musts requiring up to 250 mg N/L
• Clarified white juice, lacking grape solids and oxygen, deprives yeast of the lipids they need when anaerobic conditions prevent sterol synthesis
• Killer factor yeasts and lactic acid bacteria can both produce compounds that inhibit Saccharomyces cerevisiae and trigger an apparent stuck state

Successful fermentation restart depends on two critical factors: proper pre-conditioning of the rescue yeast and understanding the root cause of the arrest, so that the selected strain's tolerances match the specific stresses of the stuck wine. The gold-standard restart approach involves careful stepwise acclimation rather than direct inoculation into a hostile, high-alcohol environment. Recommended restart yeast strains with proven alcohol tolerance and vigor include Uvaferm 43 Restart, EC-1118 (Prise de Mousse), K1-V1116, and Fermivin Champion. Before inoculation, the stuck wine should be treated with yeast hulls at approximately 0.2-0.3 g/L for 24-48 hours to adsorb inhibitory fatty acids, then racked. A 'mother restart tank' is prepared by mixing a portion of stuck wine with water to dilute alcohol and provide a lower-stress environment; the rehydrated rescue yeast is added and allowed to begin active fermentation before the stuck wine is blended in gradually. The temperature of the two volumes must not differ by more than 10 degrees Celsius to avoid cold-shocking the restart culture.

• Never pitch rehydrated yeast directly into a stuck wine; acclimation through a stepwise mother restart tank dramatically improves success rates
• Add yeast hulls to the stuck wine 24-48 hours before the restart attempt to adsorb toxic medium-chain fatty acids that inhibit yeast
• Select a restart strain with demonstrated alcohol tolerance; Uvaferm 43 Restart, for example, tolerates up to 17% ABV and operates from 55-95 degrees Fahrenheit
• Maintain restart temperature at 20-25 degrees Celsius (68-77 degrees Fahrenheit) and add complex nutrients such as Fermaid O or Fermaid K to the stuck wine to replenish nitrogen reserves

Early detection is essential because the longer a fermentation remains stuck, the greater the risk of spoilage organism colonization and the harder a restart becomes. Daily monitoring of Brix or SG from inoculation through completion is the industry standard. Warmer growing seasons that produce high-Brix, low-YAN fruit are particularly prone to fermentation problems, and YAN assessment before harvest allows winemakers to plan targeted nutrient additions. The general recommendation for a 21-Brix must is 120-220 mg N/L total YAN. Nutrient additions are best split: a portion at inoculation and the remainder at approximately one-third of fermentation, when yeast are most able to assimilate nitrogen. Late additions are rarely effective because increasing ethanol concentrations hinder amino acid uptake. Excessive nutrient additions carry their own risks, including overly vigorous fermentation, altered aroma profiles, and residual nutrients that can feed spoilage organisms after fermentation ends. Legal limits for DAP additions apply in most jurisdictions and must be observed.

• Monitor Brix or SG daily from inoculation; a 24-hour halt with sugar remaining above 4 g/L requires immediate investigation
• Test YAN before fermentation; the recommended range for a 21-Brix must is 120-220 mg N/L, with higher-sugar fruit requiring more
• Add nutrients in split doses: an early addition at inoculation and a second dose at one-third Brix drop, when yeast uptake is most efficient
• Use stainless steel tanks with cooling jackets to maintain consistent fermentation temperature; temperature variation alone is sufficient to cause stalling in sensitive yeast strains

A stuck fermentation, even if successfully restarted, threatens wine quality through multiple pathways. The most acute risk is spoilage: residual sugar provides substrate for lactic acid bacteria, which can generate acetic acid and volatile acidity. Over-oxidation during an unprotected stall promotes the formation of acetaldehyde, associated with bruised apple and sherry-like aromas, with sensory thresholds around 100-125 mg/L. Sotolon, a butenolide lactone and potent aroma compound smelling of fenugreek, curry, or maple syrup at various concentrations, is associated with premature oxidative aging in white wines and can develop in poorly protected stuck fermentations. Stuck fermentations in which spoilage bacteria have taken hold may need to be treated with lysozyme in addition to SO2 before any restart is attempted. Commercially, the consequences of a stuck tank range from significant labor and remediation costs to total loss of the batch, making prevention infrastructure, including YAN testing, temperature monitoring, and strain selection, a sound economic investment.

• Residual sugar above 4 g/L creates microbiological instability; the primary risk is lactic acid bacteria producing acetic acid and elevating volatile acidity
• Acetaldehyde accumulates under oxidative stall conditions and imparts bruised apple and sherry-like aromas above its sensory threshold of 100-125 mg/L
• Sotolon, a powerful odorant associated with premature aging and oxidative conditions, can develop in inadequately protected stuck wines and is difficult to remediate
• If bacteria are suspected, treat with SO2 and consider lysozyme before attempting restart; uncontrolled bacterial growth consumes nutrients that the rescue yeast will need

The most reliable protection against stuck fermentation is pre-fermentation planning. YAN measurement from juice or must samples taken close to harvest gives winemakers the data needed to make informed nitrogen additions using diammonium phosphate (DAP) for inorganic nitrogen and complex yeast nutrients (such as Fermaid O or Fermaid K) for organic nitrogen and micronutrients including biotin, pantothenic acid, and thiamin. DAP alone is insufficient because it provides no micronutrients; a combined approach is best practice when significant YAN supplementation is needed. Yeast strain selection matters: using a cultured strain with a documented high alcohol and temperature tolerance, matched to the anticipated fermentation conditions, is among the most effective preventive measures available. Active temperature management throughout fermentation, particularly avoiding rapid temperature drops during the final stages, prevents the type of yeast stress that most commonly causes mid-to-late stalls. Post-fermentation autopsies, documenting the cause of any stall, refine future vintage planning and reduce repeat incidents.

• Measure YAN before harvest; supplement with a combination of DAP for inorganic nitrogen and a complex nutrient for micronutrients, split across two addition points
• Select a yeast strain with documented alcohol tolerance and temperature resilience appropriate to the planned fermentation style and sugar load
• Maintain active temperature monitoring throughout fermentation; avoid rapid temperature drops, especially in the final third of sugar conversion when yeast are already under ethanol stress
• After any stuck fermentation, conduct a post-mortem review of must chemistry, yeast viability, and fermentation profile to identify the cause and inform future prevention