Stuck Fermentation: Diagnosis, Intervention, and Prevention

Introduction

Stuck fermentation—when yeast activity ceases prematurely, leaving significant residual sugar—represents one of the most challenging winemaking problems. A stuck fermentation increases spoilage risk, delays processing, complicates blending, and can result in wine loss. For enologists, understanding the causes of stuck fermentation, implementing effective restart protocols, and preventing occurrence through proactive management are essential skills. This guide provides a comprehensive approach to stuck fermentation diagnosis, intervention, and prevention.

Defining Stuck and Sluggish Fermentation

Definitions

Stuck Fermentation: Fermentation that has ceased with measurable residual sugar (>4 g/L) and shows no activity for 24-48+ hours.

Sluggish Fermentation: Fermentation proceeding very slowly (density drop <0.5 °Baumé/day); at risk of stopping.

Distinguishing from Complete Fermentation

Complete (Dry):

• Residual sugar: <2-4 g/L
• Specific gravity: ≤0.995 (typically)
• No fermentation restart upon warming

Stuck:

• Residual sugar: >4-10+ g/L
• Higher specific gravity than expected
• May or may not resume with intervention

Causes of Stuck Fermentation

Nutrient Deficiency

Yeast Assimilable Nitrogen (YAN):

• Minimum: 150-200 mg N/L (depending on sugar level)
• Deficiency: Most common cause of stuck fermentation
• Higher sugar = higher YAN requirement

YAN Requirements by Sugar Level:

Initial Sugar (g/L)	Minimum YAN (mg N/L)
<200	150-200
200-250	200-250
250-300	250-300
>300	300+

Other Nutrients:

• Vitamins (thiamine, biotin, pantothenic acid)
• Minerals (magnesium, zinc)
• Sterols and fatty acids (survival factors)
• Oxygen (early fermentation)

Alcohol Toxicity

Mechanism: Ethanol damages yeast membranes; inhibits enzyme function

Thresholds:

• 14-15%: Significant stress for most strains
• 16-17%: Lethal for many commercial strains
• 18%+: Only specialized strains survive

Interaction: Alcohol toxicity worsened by temperature, nutrient deficiency

Temperature Extremes

Too Hot (>30°C reds; >20°C whites):

• Rapid nutrient consumption
• Volatile compound loss
• Enzyme denaturation
• Yeast death

Too Cold (<12°C):

• Metabolic slowdown
• Sluggish fermentation
• Increased duration

Temperature Spikes: Sudden changes stress yeast

Osmotic Stress

High Sugar Musts (>250 g/L):

• Water drawn from yeast cells
• Metabolic stress
• Reduced viability
• Common in: Late harvest, botrytis-affected, dried grape wines (see High-Alcohol Fermentation Challenges)

Inhibitory Compounds

Fungicides:

• Residual vineyard treatments
• Especially captan, newer fungicides
• Check pre-harvest intervals

Sulfur Dioxide:

• Excessive SO₂ at crush
• Free SO₂ >50 mg/L inhibitory

Natural Inhibitors:

• Medium-chain fatty acids (from yeast)
• Some grape-derived compounds

Brettanomyces Competition:

• Competes with Saccharomyces
• Produces inhibitory compounds

Yeast Issues

Poor Inoculation:

• Insufficient cell count
• Incorrect rehydration
• Old or damaged yeast

Strain Selection:

• Low alcohol tolerance (for high-sugar musts)
• Low temperature tolerance
• Killer factor sensitivity

Killer Yeast:

• Indigenous yeast produce killer toxin
• Kills inoculated yeast
• Results in stuck fermentation

Diagnosis Protocol

Step 1: Confirm Stuck Status

Measurements:

1. Specific gravity / density (2 readings, 24h apart)
2. Residual sugar (enzymatic method preferred)
3. Alcohol level
4. Temperature

Stuck Criteria:

• No density change over 24-48 hours
• RS >4 g/L
• Temperature favorable (not cold-arrested)

Step 2: Assess Cause

Check List:

Factor	Test	Target
YAN	Analysis	>200 mg/L remaining at stuck point
Alcohol	Analysis	Note if >14-15%
Temperature	Reading	15-28°C (reds); 12-18°C (whites)
pH	Analysis	Note extremes
SO₂	Free SO₂	<20 mg/L during fermentation
Yeast viability	Microscopy	Check cell count; methylene blue

Step 3: Microscopic Examination

Methylene Blue Staining:

• Live cells: Clear/unstained
• Dead cells: Blue-stained
• Viability: >70% needed for restart potential

Cell Count: Should be >10⁶ cells/mL for active fermentation

Step 4: Identify Problem

Most Common Causes (in order):

1. Nitrogen deficiency
2. High temperature during fermentation
3. Alcohol stress (high-sugar musts)
4. Combination of factors

Intervention Protocols

Protocol A: Mild Stuck (Yeast Still Viable)

Applicable When:

• Yeast viability >50%
• Residual sugar <50 g/L
• Alcohol <15%

Steps:

1. Add nutrients: DAP (25-50 g/hL) + organic nitrogen (25-50 g/hL)
2. Adjust temperature: Warm to 18-22°C
3. Gentle aeration: Splash racking or pump-over
4. Wait 24-48h: Monitor for activity restart

Protocol B: Moderate Stuck (Reinoculation Needed)

Applicable When:

• Yeast viability <50%
• Residual sugar 50-150 g/L
• Alcohol 14-16%

Steps:

1. Prepare restart yeast:

   • Select high-alcohol tolerant strain (EC1118, Uvaferm 43, Lalvin DV10)
   • Rehydrate according to manufacturer (with Go-Ferm or equivalent)
   • Acclimate in small must/wine mix

2. Acclimation Protocol:

   • Mix: 50% water + 50% stuck wine
   • Add rehydrated yeast
   • Wait 2-4 hours (minimal fermentation start)
   • Add more stuck wine (75% wine)
   • Wait 2-4 hours
   • Inoculate full volume

3. Support:

   • Add nutrients (complex; organic preferred)
   • Maintain 18-22°C
   • Monitor daily

Protocol C: Severely Stuck (High Alcohol)

Applicable When:

• Alcohol >16%
• Residual sugar high
• Near or at yeast tolerance

Steps:

1. Dilution (if legally permitted):

   • Add low-alcohol wine from same variety
   • Reduce alcohol to <15%
   • Proceed with Protocol B

2. Without dilution:

   • Use specialized restart yeast (Uvaferm 43)
   • Extended acclimation (48-72h)
   • Highest possible inoculation rate
   • Accept partial fermentation may be outcome

Protocol D: Cold-Arrested

Applicable When:

• Temperature was too cold (<12°C)
• Yeast dormant but viable

Steps:

1. Warm gradually (2°C/day)
2. Target 20-22°C
3. Add nutrients
4. Often resumes without reinoculation

Specialized Restart Products

Yeast Strains for Restart

Strain	Alcohol Tolerance	Notes
EC1118	18%	Gold standard restart strain
Uvaferm 43	18%+	Specifically designed for restart
DV10	17-18%	High performance
K1-V1116	18%	Low nutrient requirements

Support Products

Rehydration Nutrients:

• Go-Ferm/Fermaid O: Sterols, fatty acids
• Essential for restart yeast health

Fermentation Nutrients:

• Organic nitrogen (yeast-derived): Preferred for restart
• DAP: Inorganic; less preferred alone
• Complex nutrients: Fermaid K, Superfood

Detoxification Products:

• Yeast hulls: Absorb toxic medium-chain fatty acids
• Activated charcoal: Last resort (removes color, flavor)

Prevention Strategies

Pre-Fermentation

Must Assessment:

• Measure YAN (adjust to 200-300 mg/L)
• Note sugar level (adjust YAN accordingly)
• Check for fungicide residues
• Assess potential alcohol

Yeast Selection:

• Match strain to must characteristics
• High sugar → high alcohol tolerance
• Cool fermentation → cold-tolerant strain

Must Preparation:

• Appropriate SO₂ (<50 mg/L free)
• Temperature control ready
• Nutrient program planned

During Fermentation

Monitoring:

• Daily density checks
• Temperature logging
• Sluggishness = warning sign

Nutrient Protocol (Staged Additions):

1. At inoculation: 1/3 total YAN (with rehydration nutrients)
2. 1/3 sugar depletion: 1/3 total YAN
3. 2/3 sugar depletion: Final 1/3 (organic nitrogen preferred)

Temperature Management:

• Cooling capacity adequate
• Avoid spikes
• Gradual adjustments

High-Risk Fermentations

High Sugar Musts (>260 g/L): Common in Sauternes AOC with botrytized grapes.

• Use osmotolerant yeast
• Higher inoculation rate
• More YAN
• Cooler fermentation
• Extended acclimation

High-Brix Grapes:

• Consider partial saignée
• Chapitalization alternative (where legal)
• Accept potential RS

Outcome Assessment

Successful Restart

Indicators:

• Fermentation activity resumes
• Density decreases steadily
• Achieves target RS (<4 g/L)

Timeline: May take 2-4 weeks longer than normal

Partial Success

Outcome: Fermentation advances but does not complete

Options:

• Accept RS (style adjustment)
• Second restart attempt
• Blend with dry wine

Failed Restart

Options:

• Sweet wine style
• Fortification (if legal/desired)
• Distillation
• Disposal (worst case)

Conclusion

Stuck fermentation remains one of winemaking’s most challenging problems, but systematic diagnosis and intervention can salvage most stuck wines. For enologists, the key is prevention through proper must assessment, nutrient management, temperature control, and appropriate yeast selection. When stuck fermentation does occur, prompt diagnosis, careful restart protocol selection, and patient management provide the best chance of successful completion.

References

• Bisson, L.F. (1999). “Stuck and Sluggish Fermentations.” American Journal of Enology and Viticulture, 50(1), 107-119. AJEV Link

• Malherbe, S. et al. (2007). “Factors Affecting Stuck and Sluggish Fermentation.” South African Journal of Enology and Viticulture, 28(2), 72-79. DOI: 10.21548/28-2-1464

• Lallemand (2020). “Wine Fermentation Troubleshooting Guide.” https://www.lallemandwine.com

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Last Updated: January 10, 2026
Research Grade: Technical reference
Application: Fermentation troubleshooting, prevention protocols, restart procedures