The total acidity in wine consists of two main components; non-volatile acid (including malic acid and tartaric acid) and volatile acid (VA). Volatile acid comprises of a group of volatile, organic, steam distillable acids. Concentrations mostly vary between 500 to 1000 mg/L, with almost 90% of volatile acidity consisting of acetic acid. The rest is mostly propionic- and hexanoic acid, as well as other fatty acids from yeast and bacterial metabolism, as well as ethyl acetate.
The most common VA concentrations in wine are around 0.4 g/L, with a legal limit of 1.2 g/L (see Table 1 for legal limits). The sensory threshold value in red wine is approximately 0.6 -0.9 g/L. whereas low, almost unnoticeable levels add to aroma complexity. With regards to the sensory attributes of VA; it contributes to the taste intensity of non-volatile acids and tannins, while the perception of VA itself is masked by high concentrations of sugars and alcohol. Acetic acid smells of vinegar, while ethyl acetate smells more like bruised apple and Cutex remover.
Volatile acidity production takes place mainly due to the oxidation of ethanol or the metabolism of acids/sugars. Ethanol is the primary energy source for acetic acid bacteria (AAB). Acetic acid bacteria are microscopic, single-cell organisms with enzymes included in their cell walls. The most common AAB present in wine include Acetobacter aceti, Acetobacter pasteurianus and Gluconobacter oxydans. These organisms are aerobic and need oxygen for survival. Acetic acid bacteria have the ability to oxidise alcohol to acetic acid, which in turn will, via esterification with ethanol, be converted to ethyl acetate. Ethyl acetate possesses a lower sensory threshold value compared to acetic acid and both acetic acid and acetaldehyde (a by-product of ethanol oxidation), are toxic to Saccharomyces cerevisiae and can contribute to sluggish- or stuck fermentations.
Origin and mechanism of oxidation…
During fermentation, the possibility of VA production is increased through the following practices: high risk must, risky winemaking practices and poor management of cellar conditions. Sources of VA after fermentation include cellar practices with specific focus on barrels: the amount of headspace, barrel age, oxidation and sanitary state of the barrels. Most AAB infections will take place in the cellar itself; mainly due to low acids and sulphur dioxide levels, together with oxygen exposure.
There are various sources that can add to the VA concentration in wine; the most conspicuous being:
- wild yeast e.g. Brettanomyces, Kloeckera etc. and as a natural by-product of S. cerevisiae Acetic acid is produced as an intermediary product of the pyruvate dehydrogenase metabolic pathway. This metabolic pathway is necessary and responsible for the conversion of pyruvate to acetyl-CoA. Last mentioned is imperative for anaerobic processes like lipid biosynthesis. This reaction is catalysed by alcohol dehydrogenase, whereby acetic acid is formed via the oxidation of acetaldehyde (produced from pyruvate during fermentation).
- lactic acid bacteria (LAB) during fermentation Heterofermentative LAB possess the ability to metabolise glucose (residual sugar), via the phosphoketolase metabolic pathway, and convert it to CO2, ethanol, acetic acid and lactic acid during malolactic fermentation. The first step in the citric acid metabolism produces acetic acid via citrate lyase activity, during which the conversion of citric acid to oxaloacetate, produces acetic acid.
- acetic acid bacteria Membrane-bound alcohol dehydrogenase oxidises ethanol to acetaldehyde. This intermediary is then oxidised further to acetic acid via membrane-bound aldehyde dehydrogenase.
- non-microbial source The chemical hydrolysis of wood hemicellulose, as well as the oxidation of gape phenolic compounds can result in the production of VA.
Factors that influence VA production…
- Sugar/osmotic pressure. Higher sugar concentrations result in a longer lag phase, which in turn lead to lower viability and growth potential of the yeast cells. Higher sugar concentrations together with low nitrogen levels lead to increased acetic acid concentrations.
- Fermentation temperature. Higher temperatures lead to higher VA concentrations.
- Yeast strain selection. The ability to produce VA id dependant on the specific yeast strain.
- The production of acetate esters e.g. ethyl acetate. This production is dependent on the yeast strain, the presence of indigenous yeast, fermentation temperature and SO2 concentrations.
- High initial acetic acid concentration. Rotten grapes, high sugar concentrations, pH and fermentation temperature at the start of fermentation will lead to increased acetic acid concentrations.
- A large bacterial population. High temperatures during storage of the wine (> 15°C), higher pH levels and lower alcohol and free SO2 concentrations, as well as poor cellar hygiene, will favour the survival of a bacterial population.
- 1. before fermentation:
- monitor sugar and pH in the vineyard
- do not mechanically harvest grapes that could be a potential risk
- maintain sanitary conditions in the cellar e.g. equipment
- use healthy grapes (avoid overripe)
- do not excessively clarify the must, but a degree of clarification will reduce the indigenous microbial population
- 2. during fermentation:
- do acid adjustments if necessary to maintain low pH
- maintain protective SO2 concentrations
- use low VA-producing yeast strain
- use sufficient nutrients during alcoholic fermentation
- ensure fermentation is complete (no residual sugar / temperature fluctuations / re-inoculation)
- reduce exposure to oxygen, but keep in mind that oxygen is necessary for alcoholic fermentation, as well as colour stabilising tannin reactions in red wine, so a degree of oxygen is required
- 3. after fermentation:
- inhibit malolactic fermentation with lysozyme if necessary
- remove wine from yeast lees
- adjust free SO2 levels to 40 ppm
- ensure that wine is being stored in full containers
- ensure sufficient sanitary state of barrels
- correct usage of barrels
- regular top up of barrels
- bottling practices are important e.g. membrane filtration
As mentioned above, there are a variety of preventative measures, but all these techniques are irrelevant if a winemaker sits with a high final VA concentration in his wine. Correctional options include blends, reverse-osmosis and nano-filtration.
1. How to diffuse a volatile situation. Zoecklein et al. 2005.
2. The origins of acetic acid in wine. M. Lambrechts.
3. Volatile acidity in wine. R. Gawel.
4. Current vineyard and cellar events. Sources of volatile acid formation in wine and potential control measurements. C. Theron.