Stuck Fermentation

Stuck fermentation is the premature, unintentional cessation of yeast metabolism during winemaking, leaving sugars unconverted to alcohol and CO₂. It is formally defined as a fermentation in which Brix or specific gravity does not drop for more than 48 hours while fermentable sugar remains. This distinguishes it from a completed dry fermentation (specific gravity below 0.995) or an intentionally arrested fermentation such as in the production of fortified wines like Port, where the winemaker deliberately stops fermentation by fortifying with spirits. Louis Pasteur's foundational work in the 1850s established that yeast converts sugars into alcohol and CO₂, providing the first scientific framework for understanding why fermentations sometimes fail to complete.

• Distinct from a 'sluggish fermentation,' which progresses slowly but continues; a stuck fermentation shows no measurable Brix drop for 48 hours or more
• Unlike arrested fermentation, where the winemaker intentionally halts the process, stuck fermentation is unintentional and exposes wine to spoilage risk
• Saccharomyces cerevisiae, the dominant wine yeast, is favoured for its predictable vigour, alcohol tolerance, and ability to thrive at normal wine pH between 2.8 and 4.0

Stuck fermentation transforms a wine in progress into a commercial liability. Residual sugars invite colonisation by spoilage organisms including Brettanomyces, Lactobacillus, Pediococcus, and Acetobacter, leading to off-flavours, elevated volatile acidity, and microbial instability. Lactic acid bacteria (LAB) can produce volatile acidity above 0.6 g/L, which is already challenging for yeast in high-alcohol conditions, and above 0.8 g/L becomes actively inhibitory to any fermentation restart. Stuck wines also present complications for malolactic fermentation: residual sugar and microbial competition from spoilage organisms can disrupt LAB activity, while the wine remains vulnerable to post-bottling refermentation if residual yeast survive into the bottle.

• Volatile acidity above 0.8 g/L is inhibitory to yeast and must be controlled before any restart attempt can succeed
• Spoilage organisms including Brettanomyces and Zygosaccharomyces can utilise residual sugars and any excess nitrogen left after a stuck fermentation
• Winemakers must control undesirable LAB activity before attempting to restart a stuck fermentation, as active bacteria compete with and inhibit reinoculated yeast

The primary causes of stuck fermentation are: high temperatures near 104°F (40°C) that slow and kill yeast; alcohol toxicity as ethanol accumulates above the strain's tolerance (typically 16-18% ABV, though multiple stressors can cause stalling earlier); and nutrient depletion, particularly nitrogen deficiency in must with YAN below 140-150 mg/L. Medium-chain fatty acids (octanoic and decanoic acids) produced by yeast themselves can also accumulate and act synergistically with ethanol to inhibit fermentation. Cold shock, over-clarified must below 50 NTU, pesticide residues, microbial competition from spoilage organisms, and excessively low pH (below 3.2) are additional contributing factors. Diagnosis involves monitoring specific gravity or Brix daily; a plateau for 48 or more hours with residual sugar present signals a stall.

• Over-clarified juice below 50 NTU causes yeast to struggle staying in suspension during early fermentation, increasing the risk of a premature stall
• Heat shock changes yeast plasma membrane composition, reducing their subsequent tolerance to ethanol and causing them to stall at lower-than-normal alcohol levels
• Saccharomyces cerevisiae cannot ferment pentose sugars such as arabinose, which remain as residual sugar and can create misleading Brix readings

Prevention is considerably more effective than remediation. Select yeast strains rated for high alcohol tolerance, such as Lalvin EC-1118 (Prise de Mousse) or Uvaferm 43, which can ferment to 18% ABV under good conditions. Rehydrate dry yeast using Go-Ferm at 30 g/hL (at a ratio of 1.25 parts Go-Ferm per 1 part yeast) in clean 43°C water, cooling to 40°C before adding the yeast, and allowing 20 minutes to stand before pitching. Supplement nitrogen with DAP and complex organic nutrients to achieve YAN above 150 mg/L, with additions timed at inoculation and at the one-third sugar depletion point. Maintain fermentation temperature between 68-72°F (20-22°C), especially as fermentation approaches completion. Once stuck, warm the must to around 20°C, ensure any undesirable LAB are controlled, then prepare a fresh active starter using a high-tolerance strain and progressively blend it into the stuck wine.

• Proper rehydration: dissolve Go-Ferm in 43°C clean water, cool to 40°C, add dry yeast, stand 20 minutes, then slowly acclimate to must temperature before pitching
• Nutrient additions should be timed at inoculation and at one-third sugar depletion; nitrogen added after half the sugar is consumed is largely unavailable to yeast
• Aeration during fermentation supports yeast membrane integrity and reduces the accumulation of medium-chain fatty acids that inhibit fermentation
• Fermentations stuck above 3°Brix with alcohol below 11.5% are relatively straightforward to restart; lower sugar and higher alcohol combinations are increasingly difficult

Certain wine styles face inherently elevated stuck fermentation risk due to their high initial sugar levels. Amarone della Valpolicella, produced from dried Corvina-based grapes in the Veneto, regularly starts fermentation at sugar levels capable of yielding 16-17% ABV or more, placing sustained pressure on yeast throughout the process. Tokaji Aszú production in Hungary involves must with exceptionally high residual sugar from botrytised berries, requiring careful yeast management to achieve the desired degree of fermentation completion. High-Brix red wines such as Zinfandel, Primitivo, or Grenache harvested above 27-28 Brix are particularly vulnerable, as the potential alcohol level may approach or exceed the tolerance of commonly used yeast strains, especially if fermentation temperatures also swing.

• Amarone della Valpolicella is a textbook high-risk style: uncontrolled stuck fermentation can leave undesirable residual sugar in a wine designed to be technically dry or near-dry
• Saccharomyces bayanus, used in EC-1118, tolerates alcohol levels of 17-20% and is often employed for high-Brix grapes, sparkling wine production, and restarting stuck fermentations
• Wild yeast species such as Hanseniaspora and Candida typically die out around 3-5% ABV, leaving Saccharomyces to complete fermentation; their presence early in fermentation does not prevent sticking later

Stuck fermentation sits at the intersection of microbiology, chemistry, and physical winemaking conditions. Ethanol disrupts yeast plasma membrane fluidity and the function of membrane-associated transporters and enzymes, with the effect becoming increasingly severe as alcohol concentration rises. Concurrently, medium-chain fatty acids (particularly octanoic and decanoic acids) produced by yeast themselves accumulate in the fermenting must and act synergistically with ethanol to inhibit yeast growth and metabolism. Yeast hulls (inactive yeast cell walls) are used as a remediation tool precisely because they adsorb these inhibitory fatty acids from the must, helping to detoxify the environment for a reinoculated yeast culture. Analytical tools including daily hydrometer or refractometer readings, lab-measured YAN, and viable yeast cell counts allow winemakers to track fermentation health and intervene before a sluggish fermentation becomes fully stuck.

• Decanoic and octanoic acids, produced by yeast during fermentation, act synergistically with ethanol and are a well-documented cause of fermentation inhibition and arrest
• Yeast hulls (yeast ghosts) are added to stuck musts specifically to adsorb inhibitory fatty acids from the medium before re-inoculation
• Heat shock during fermentation forces changes in yeast plasma membrane composition that permanently reduce the cells' tolerance to ethanol, causing arrest at lower-than-expected alcohol levels