Common Spoilage Issues in Wine Fermentation

Wine fermentation is a delicate biological process where yeast converts grape sugars into alcohol and carbon dioxide. But this process doesn’t always go smoothly. From premature yeast shutdown to bacterial invasions that turn your wine into vinegar, spoilage during fermentation is a real and common challenge. Understanding these spoilage mechanisms – what causes them, how to detect them, and how to prevent them – is essential for anyone involved in winemaking, whether at an industrial or home scale.

What is wine fermentation spoilage? 🔗

Wine spoilage refers to any unwanted change in the wine’s flavour, aroma, colour, or stability caused by undesirable microorganisms, chemical reactions, or environmental conditions. These faults can occur at any stage – during primary fermentation, malolactic fermentation, aging, or even after bottling. The result is reduced wine quality, off-putting sensory characteristics, and in severe cases, a product that’s completely undrinkable.

As noted by researchers at Penn State Extension, wines host complex microbial communities including yeasts, lactic acid bacteria (LAB), and acetic acid bacteria (AAB), all of which can shift from beneficial to harmful depending on conditions. Even Saccharomyces cerevisiae, the standard wine fermentation yeast, can produce foul-smelling sulfur compounds when stressed.

Stuck fermentation: when yeast stops working 🔗

A stuck fermentation occurs when yeast becomes dormant or dies before it has finished converting all the sugar in the must into alcohol. This is different from an “arrested fermentation,” where a winemaker intentionally halts the process – for instance, when producing fortified wines like Port. A stuck fermentation is unplanned, unwanted, and often difficult to fix.

According to Wikipedia’s entry on stuck fermentation, once fermentation stalls, dying yeast cells release a chemical compound that inhibits the growth of new yeast, making restarts extremely challenging.

Common causes of stuck fermentation 🔗

There are several factors that can cause a fermentation to stall. Here are the most significant ones:

Temperature extremes: Yeast is highly sensitive to temperature. If the must gets too hot – approaching 40°C (104°F) – yeast cells begin to die. Conversely, if the temperature drops too low (below about 13°C or 55°F), yeast activity slows dramatically and may stop altogether. As explained by WineMaker Magazine, temperature swings during fermentation are among the most frequent causes of stalled fermentations, and the effect worsens as alcohol levels rise.

Nitrogen deficiency: Yeast needs nitrogen to grow and reproduce. When the must lacks sufficient yeast-assimilable nitrogen (YAN), fermentation slows and can stop entirely. Grape varieties like Chardonnay and Riesling are naturally prone to low nitrogen-to-sugar ratios, making them higher-risk candidates for stuck fermentations. Winemakers commonly add diammonium phosphate (DAP) to supplement nitrogen levels.

Excessive sugar (high Brix): Overripe grapes carry very high sugar levels, which translate into elevated alcohol during fermentation. Most yeast strains cannot survive or reproduce in environments above 16-18% alcohol by volume (ABV). If the initial sugar level is too high, the yeast may die off before the wine reaches dryness.

Microbial competition: Wild yeasts and bacteria naturally present on grapes can compete with the inoculated yeast strain for nutrients and resources. This competition can weaken the primary fermentation yeast and slow down or halt the process.

Lack of oxygen and nutrients: In the absence of oxygen, yeast relies on lipids found in grape solids. Heavily clarified white musts – stripped of grape solids – can starve the yeast. Red wines, fermented on their skins, face a lower risk because the skins provide both solids and some oxygen exposure.

Restarting a stuck fermentation 🔗

Prevention is always better than cure, because restarting a stuck fermentation is notoriously difficult. However, a few approaches can help. Moving the fermenter to a warmer location (around 20-21°C or 68-70°F) often helps if cold temperatures were the culprit. Adding a yeast energiser that provides extra nitrogen and micronutrients can revive sluggish yeast. In more stubborn cases, winemakers pitch a fresh, highly alcohol-tolerant yeast strain such as Lalvin EC-1118, often by first building a vigorous yeast starter and gradually acclimating it to the stuck wine before combining the two.

As specialists at Westgarth Wines explain, for red wines, aerating the must may help reactivate dormant yeast. Another effective technique is to progressively blend the stuck wine into a vessel where a healthy, active fermentation is already underway.

Acetic acid bacteria: the vinegar makers 🔗

Acetic acid bacteria (AAB), primarily from the genera Acetobacter and Gluconobacter, are among the most common spoilage organisms in wine. These bacteria oxidise ethanol to produce acetic acid – the same compound that gives vinegar its sharp taste and smell. Even small amounts of AAB activity can push a wine’s volatile acidity (VA) above acceptable thresholds, ruining its sensory profile.

A research review published by PubMed notes that AAB are well adapted to sugar- and ethanol-rich environments and are naturally found on grapes as part of the microbial flora. Their presence becomes problematic when oxygen is available – these are strictly aerobic organisms, meaning they require oxygen to grow and metabolise.

When do acetic acid bacteria strike? 🔗

AAB can cause problems at several stages. On damaged or rotten grapes, their populations explode, reaching up to 10⁶ colony-forming units (CFU) per berry. During healthy, fast-starting fermentations, AAB populations decline rapidly because the anaerobic environment (rich in carbon dioxide) suppresses their growth. However, during stuck or sluggish fermentations, where CO₂ production is insufficient to exclude oxygen, AAB can proliferate and produce acetic acid.

Post-fermentation, AAB remain a threat during aging and storage. Wines stored in small oak barrels absorb small amounts of oxygen through the wood, which can reactivate dormant AAB metabolism. According to ScienceDirect, red wines matured in oak cooperage tend to have higher volatile acidity than white wines for this very reason.

Volatile acidity and ethyl acetate 🔗

The primary spoilage product from AAB is acetic acid, measured as volatile acidity (VA). But AAB contamination often brings another compound: ethyl acetate, formed when acetic acid reacts with ethanol. While acetic acid itself may not be easily detected at low levels (commercial vinegar has 30-90 g/L of acetic acid, compared to the legal limit in wine of about 1.2-1.4 g/L), ethyl acetate can be detected at much lower concentrations, producing a distinctive nail-polish-remover smell.

Additionally, AAB can produce acetaldehyde – a compound that at levels above 100-125 mg/L gives wine an oxidised, sherry-like character with green-apple and metallic notes.

Oxidation: the silent spoiler 🔗

Oxidation is one of the most pervasive chemical spoilage pathways in wine. It occurs when wine is excessively exposed to oxygen, triggering chemical reactions that degrade flavour, aroma, and colour. Oxidised white wines turn brown and develop stale, nutty, or bruised-apple aromas. Red wines lose their vibrant fruit character and take on flat, prune-like notes.

Oxidation often goes hand-in-hand with AAB spoilage because both are driven by oxygen exposure. However, oxidation can also happen purely through chemical reactions without bacterial involvement – particularly during transfers, racking, filtration, or improper storage where containers aren’t fully sealed.

Proper use of sulfur dioxide (SO₂) is the primary defense against oxidation. SO₂ acts as both an antioxidant and an antimicrobial agent, protecting the wine from oxygen damage and suppressing harmful microorganisms simultaneously.

Other microbial spoilage organisms 🔗

Lactic acid bacteria (LAB) 🔗

While certain LAB, particularly Oenococcus oeni, play a beneficial role in malolactic fermentation (converting sharp malic acid into softer lactic acid), other LAB species like Lactobacillus and Pediococcus can cause serious faults. These include excess production of diacetyl (an overpoweringly buttery flavour), mousey taint (an unpleasant aftertaste resembling rancid nuts), geranium taint (from the metabolism of sorbic acid), and ropiness (a slimy texture in the wine).

LAB are facultative anaerobes – they grow with or without oxygen, which makes them harder to control than AAB. Their spoilage is most common during stuck fermentations and in finished wines with low SO₂ and residual sugar or malic acid.

Brettanomyces 🔗

The yeast Brettanomyces bruxellensis is one of the most feared spoilage organisms in winemaking. It produces volatile phenols that give wine undesirable aromas often described as barnyard, horse sweat, band-aid, or medicinal notes. According to Iowa State University Extension, while some consumers tolerate low levels of Brett-derived aromas in certain wine styles (particularly aged reds from the Rhône or southern Italy), most consider its presence a significant fault.

Brettanomyces is particularly challenging because it is tolerant to sulfur dioxide, can survive in bottled wine, and may grow on cellobiose – a sugar released during the toasting of oak barrels. It can infect red wine months after barreling and can even be transmitted by fruit flies.

Hydrogen sulfide and reductive faults 🔗

During fermentation, yeast that is starved of nitrogen may produce hydrogen sulfide (H₂S), which has a highly recognisable rotten-egg smell. The sensory threshold for H₂S is very low – just 8-10 µg/L. If not addressed, H₂S can further react with other wine compounds to form mercaptans (thiols), producing even more offensive odours like onion, rubber, and skunk.

Supplementing the must with adequate nitrogen (through DAP or complete yeast nutrients) and racking the wine off its lees promptly are effective measures to prevent reductive faults.

Prevention strategies for wine fermentation spoilage 🔗

Preventing spoilage is far easier than correcting it. A multi-layered approach works best:

Sanitation: Thorough cleaning and sanitising of all equipment that contacts the must or wine is fundamental. Soap alone is not enough – winemakers should use no-rinse sanitisers like potassium metabisulfite solutions to eliminate bacteria, moulds, and wild yeasts from equipment surfaces.

Grape quality: Sorting grapes carefully at harvest to remove damaged, mouldy, or overripe fruit reduces the microbial load entering the fermentation vessel. Rotten grapes carry significantly higher populations of spoilage organisms.

Sulfur dioxide management: Applying a measured dose of SO₂ (typically around 50 ppm) to the must at crush helps suppress unwanted wild yeasts and bacteria before inoculation with a selected yeast strain. As recommended by the University of Minnesota Grape Breeding and Enology program, proper and timely SO₂ management is one of the most effective tools against volatile acidity.

Temperature control: Maintaining a stable fermentation temperature – ideally between 13°C and 30°C depending on the fermentation stage – prevents yeast stress and discourages spoilage organisms. Stainless steel tanks with cooling jackets are standard in modern wineries for this purpose.

Nutrient management: Measuring yeast-assimilable nitrogen (YAN) before fermentation and supplementing if needed ensures yeast have the resources to complete fermentation. Complete nutrients that include amino acids, vitamins, and minerals are preferred over DAP alone.

Oxygen control: Minimising oxygen exposure throughout fermentation, aging, and bottling is critical. This includes keeping vessels topped up, using inert gas blankets (CO₂ or nitrogen), and ensuring tight seals on all storage containers.

Detecting spoilage early 🔗

Early detection gives winemakers the best chance to intervene before spoilage becomes irreversible. Regular monitoring of key parameters – such as residual sugar (using a hydrometer), volatile acidity levels, free and total SO₂, and temperature – is essential throughout the winemaking process.

Sensory evaluation also plays an important role. The smell of nail polish remover (ethyl acetate), vinegar (acetic acid), rotten eggs (hydrogen sulfide), or barnyard (Brettanomyces) are all warning signs that should prompt immediate testing and corrective action.

Emerging technologies are also making real-time detection possible. A recent study published in Microbial Biotechnology and covered by Phys.org described a living biosensor made from engineered bacteria that glows in response to acetic acid. The biosensor works in both liquid wine and the headspace air above it, providing early warning before spoilage becomes advanced.

The bigger picture: spoilage as a microbial balancing act 🔗

It’s worth noting that many spoilage organisms are not inherently “bad.” They are part of the natural microbial ecosystem of grapes and wine. The distinction between beneficial and harmful often depends on context, concentration, and control. Oenococcus oeni is essential for malolactic fermentation but can produce excessive diacetyl if not managed. Some non-Saccharomyces yeasts, once dismissed as spoilage organisms, are now commercially available and valued for the aromatic complexity they contribute to wine when used in controlled fermentation settings.

Effective winemaking, therefore, is less about eliminating all microorganisms and more about managing the microbial environment – creating conditions that favour the desired organisms while suppressing those that cause harm.

What do you think? Have you ever tasted a wine that clearly exhibited spoilage characteristics like vinegar notes or barnyard aromas, and did it change the way you think about the importance of fermentation management? What spoilage prevention strategy do you consider most critical for producing consistently good wine?

References

1. https://extension.psu.edu/whats-in-the-wine-microbiome
2. https://en.wikipedia.org/wiki/Stuck_fermentation
3. https://winemakermag.com/wine-wizard/curing-a-stuck-fermentation
4. https://westgarthwines.com/blogs/news/alcoholic-fermentation-stuck-fermentation
5. https://pubmed.ncbi.nlm.nih.gov/18237809/
6. https://www.sciencedirect.com/topics/food-science/acetic-acid-bacteria
7. https://www.extension.iastate.edu/wine/defining-spoilage
8. https://enology.umn.edu/news/volatile-acidity-wine-making
9. https://phys.org/news/2026-01-early-wine-spoilage-dark.html