Volatile Acidity: Prevention, Detection, and Control

Problem Definition

Volatile acidity (VA) refers to steam-distillable acids in wine, primarily acetic acid with smaller contributions from formic, butyric, and propionic acids. At elevated concentrations, VA produces vinegar-like and nail polish remover (ethyl acetate) aromas that are universally considered faults. The legal limit varies by appellation but typically ranges from 1.1-1.4 g/L (as acetic acid) for dry wines.

The problem is particularly acute in:

Extended maceration reds with high pH (Grenache, Sangiovese)
Oak-aged wines with Brettanomyces contamination
Wines with fermentation delays or stuck fermentations
Hot-climate wines with elevated sugar and reduced acidity

Technical Context

Sources of Volatile Acidity

1. Yeast metabolism (unavoidable)

Saccharomyces cerevisiae produces 0.1-0.3 g/L acetic acid as normal fermentation byproduct
Stressed fermentations (high sugar, low nitrogen, temperature extremes) increase production 2-3×
Certain strains produce more VA than others

2. Acetic acid bacteria (AAB)

Acetobacter and Gluconobacter species oxidize ethanol to acetic acid
Require oxygen; proliferate at wine surfaces or in partially-filled containers
Optimal growth at 25-30°C; pH 5.0-6.0 (but active at wine pH)
Can produce >1 g/L acetic acid within days under favorable conditions

3. Lactic acid bacteria (LAB)

Lactobacillus and Pediococcus can produce acetic acid via heterolactic fermentation
Oenococcus oeni (MLF) produces minimal VA (0.1-0.2 g/L) under normal conditions
Elevated VA from LAB indicates spoilage organisms, not clean MLF

4. Brettanomyces

Produces both acetic acid and volatile phenols
Activity accelerated by residual sugar and low SO₂
Common in barrel-aged reds, particularly with poor cellar hygiene

Sensory Thresholds

Compound	Detection Threshold	Recognition Threshold	Rejection Threshold
Acetic acid	0.2-0.3 g/L	0.6-0.8 g/L	1.0-1.2 g/L
Ethyl acetate	50-90 mg/L	120-150 mg/L	180+ mg/L

Ethyl acetate forms via esterification of acetic acid with ethanol. Its presence amplifies negative perception of VA.

Chemical Context

The relationship between acetic acid and ethyl acetate:

CH₃COOH + C₂H₅OH ⇌ CH₃COOC₂H₅ + H₂O

The equilibrium shifts right (more ethyl acetate) at:

Higher temperature
Lower pH
Higher alcohol concentration
Longer time

This means wines with elevated VA will develop increasing ethyl acetate during aging.

Options and Interventions

Prevention

During harvest and crush:

Eliminate damaged, rotten, or Botrytis-affected fruit (sort rigorously)
Minimize time between harvest and processing
SO₂ addition at crush: 30-50 mg/L for clean fruit; 75-100 mg/L for compromised fruit
Maintain cold temperatures during transport and processing

During fermentation:

Inoculate promptly with active yeast culture
Avoid stuck fermentation (maintain YAN, temperature control)
Full vessels; minimize headspace
Complete fermentation to dryness unless intentionally stopped

During aging:

Maintain molecular SO₂ at 0.5-0.8 mg/L
Keep containers full; top weekly
Control cellar temperature (14-16°C for barrel aging)
Regular sanitation of barrels, pumps, and hoses

Detection and Monitoring

Analytical methods:

Steam distillation (Cash still): Reference method; measures total VA
Enzymatic assay: Specific for acetic acid; rapid
HPLC: Quantifies individual volatile acids
Sensory: Regular tasting; most sensitive detection for low levels

Monitoring schedule:

Post-fermentation: Baseline measurement
Monthly during barrel aging
Before any blending operation
Before bottling (final QC)

Remediation Options

Reverse osmosis (RO) with ion exchange:

Removes acetic acid and ethyl acetate
Legal in most New World jurisdictions
Not permitted in most European PDOs for quality wines
Can strip desirable aromatics; requires careful management

Blending:

Dilute high-VA wine with low-VA wine
Calculate proportions to achieve target below legal limit and sensory threshold
Does not fix underlying fault—only masks it

Biological remediation (experimental):

Certain yeast strains can metabolize acetic acid
Not commercially established; research stage

Filtration and SO₂:

Sterile filtration removes AAB and Brett
SO₂ addition inhibits further production
Does NOT reduce existing VA

Trade-offs and Risks

Reverse osmosis:

Reduces VA effectively but alters wine matrix
May require re-addition of volatiles
Regulatory restrictions in EU appellations
Cost: €0.50-2.00 per liter depending on treatment level

Over-sulfiting:

Excessive SO₂ to prevent VA can cause H₂S formation under reductive conditions
High bound SO₂ levels affect sensory quality
Legal limits apply (typically 150-200 mg/L total SO₂)

Barrel management:

New barrels have lower microbial load but higher oxygen ingress
Old barrels may harbor Brett/AAB in stave depth
Barrel hygiene protocols (hot water, steam, SO₂ wicks) essential

Appellation constraints:

Châteauneuf-du-Pape AOC: High-alcohol, high-pH wines are inherently VA-susceptible; extended aging traditional but risky
Barolo DOCG: 38-month minimum aging requires vigilant monitoring
Chianti Classico DOCG: 30-month aging for Gran Selezione increases exposure time

Practical Implications

Variety-specific considerations:

Grenache: Low acidity (pH 3.5-3.9), high alcohol (14-16%), and oxidation-susceptible. VA risk elevated. Requires higher SO₂ management and cooler cellar temperatures.
Sangiovese: Moderate acidity provides some protection, but extended maceration and aging (Brunello: 5+ years) create extended risk window.
Pinot Noir: Thin skins and susceptibility to Botrytis increase risk from compromised fruit. Whole-cluster fermentation with moldy stems can elevate VA.

Appellation-specific implications:

Châteauneuf-du-Pape AOC: Traditional extended aging in large foudres requires rigorous topping protocols. Many producers now use concrete or stainless steel to reduce risk.
Barolo DOCG: 18-month minimum oak aging mandate requires barrel hygiene program. Modern barrique programs increase oxygen exposure vs. traditional botte.

References

du Toit, W.J., & Pretorius, I.S. (2000). “Microbial Spoilage and Preservation of Wine: Using Weapons from Nature’s Own Arsenal.” South African Journal of Enology and Viticulture, 21, 52-96. DOI: 10.21548/21-special_issue-2118
Drysdale, G.S., & Fleet, G.H. (1988). “Acetic Acid Bacteria in Winemaking: A Review.” American Journal of Enology and Viticulture, 39(2), 143-154. AJEV Link
Ribéreau-Gayon, P., Glories, Y., Maujean, A., & Dubourdieu, D. (2006). Handbook of Enology, Volume 2: The Chemistry of Wine and Stabilization and Treatments (2nd ed.). Wiley. Publisher Link
Bartowsky, E.J., & Henschke, P.A. (2008). “Acetic Acid Bacteria Spoilage of Bottled Red Wine—A Review.” International Journal of Food Microbiology, 125(1), 60-70. DOI: 10.1016/j.ijfoodmicro.2007.02.027