Acetaldehyde – how to limit its formation during fermentation

Steve Byrne and Greg Howell
Vintessential Laboratories

Introduction
Although most winemakers are aware of acetaldehyde, it still causes some negative impacts on wine quality. It is the main carbonyl compound formed in wine during fermentation, although it can also be produced in wine via other mechanisms. High levels of this aldehyde can have poor sensory effects on wine. There are several tools that a winemaker can use to limit the production and impact of acetaldehyde in wine.

Impact of acetaldehyde on wine
Acetaldehyde is the main aldehyde present in wine. At low concentrations (below 70 mg/L) typical of freshly fermented wine, acetaldehyde can impart a fruity aroma. At higher concentrations (from 100 to 120 mg/L and above) it can become pungent and objectionable, the aroma being described as a bruised green apple, sherry-like, oxidative or nutty character (1).

Acetaldehyde is the main binder of added sulfite molecules. This binding produces a molecule that has low volatility and low odour, thus limiting the sensory effect of acetaldehyde. However this binding also reduces the efficacy of the added sulfite.

Properties
Acetaldehyde is a colourless liquid with the chemical formula C₂H₄O and the systematic name ethanal (not to be confused with ethanol). It is frequently used in manufacturing as an intermediate in the synthesis of other chemicals, including perfumes and dyes. In large doses it is not pleasant – and in some literature it even gets the blame for the onset of hangovers (although technically speaking by the time a hangover kicks in, acetaldehyde has left the body).

Carbonyl compounds (aldehydes and ketones) in wine such as acetaldehyde bind with bisulfite to form hydroxysulfonates, which render the acetaldehyde compound odourless. A high ratio of bound SO₂ to free SO₂ can indicate that a large proportion of added SO₂ has had to remedy an oxidation problem by ‘mopping up’ acetaldehyde and other carbonyl compounds.

Acetaldehyde is not the only compound in wine that binds to SO₂ – different sugars, acids and polyphenols also form bound complexes. In most cases though, acetaldehyde is the major compound associated with bound SO₂ (see Table 1).

Table 1: Proportion of sulfur dioxide typically bound to wine components (2).

Formation of acetaldehyde

During fermentation, sugars are primarily converted to ethanol and carbon dioxide. Fermentation is not, however, a simple process and many intermediate steps occur. Acetaldehyde is involved in one of these intermediate steps.

Acetaldehyde is not only formed in the fermentative production of ethanol, it is also present in the pathway of production of acetic acid from ethanol by bacteria. Acetic acid bacteria (AAB) usually produce significant levels of acetic acid under conditions of high oxygen and low alcohol, however in wine environments with higher alcohol and low oxygen, there is a tendency for AAB to favour production of acetaldehyde.

Whilst yeast typically reduces acetaldehyde to ethanol, under certain conditions of high oxygen, yeast can also convert ethanol back to acetaldehyde via an oxidative pathway. This is typical of yeasts appearing on the surface of ullaged tanks or barrels. In some instances the activity of these yeasts is encouraged and the oxidative processes utilised in the production of certain styles of wine such as flor sherry.

The natural process of wine ageing also produces acetaldehyde. This is a chemical rather than a microbiological reaction, whereby an oxidizing agent such as hydrogen peroxide is formed as a by-product of phenolic oxidation and then reacts with ethanol to produce acetaldehyde.

How to minimise the formation of acetaldehyde
There are a couple of steps involved in fermentation that present the opportunity for the winemaker to influence the production of acetaldehyde. Sugar is first converted to pyruvate by yeast via the glycolytic pathway. The pyruvate is then decarboxylated to acetaldehyde with the associated release of carbon dioxide. This acetaldehyde intermediate is then reduced to ethanol.

During this process, excess acetaldehyde can be produced if SO₂ is added during fermentation or if there are increases in pH or fermentation temperature. Yeast strain can also influence the concentration of acetaldehyde – commercial strains can be chosen based on their metabolic characteristics, including whether they produce high or low levels of acetaldehyde.

Analysis of Acetylaldehyde
In our labs, acetaldehyde is analysed by Gas Chromatography, utilising a Flame Ionisation Detector (GC-FID). The instrumentation is sophisticated, but the turnaround time of analysis is fast, so it allows winemakers to make quick decisions about remedial treatment of affected wines.

Last year, 68% of wines analysed in our labs had acetaldehyde levels of below 50 mg/L. 18% had levels between 50 mg/L and 120 mg/L and 13% had concentrations greater than 120 mg/L, indicating possible oxidation problems. The highest value recorded last year in a red table wine was 500 mg/L.

Analysis of bound SO2
If acetaldehyde concentration is not routinely monitored during wine storage, then an analysis of both free and bound SO₂ is an important consideration for indirectly assessing the extent of oxidation.

Interestingly, there is still a tendency for some winemakers to not measure bound SO₂. Last year over 40% of routine samples received in our labs for SO₂ analysis only requested free SO2, without regard for the bound or total SO₂ content. Measuring the bound SO₂ in addition to the free level not only aids in monitoring oxidation, it also helps to track the effectiveness and accuracy of SO₂ additions, as well as ensuring that your wines are meeting regulatory market requirements for total SO₂ concentration.

One reason for the lower incidence of measuring bound SO₂ within wineries appears to be the reluctance of some labs to upgrade their existing SO₂ aspiration apparatus to be able to measure both free and bound SO₂. All that is required is a gentle heat applied to the acidified wine sample to break the bond between acetaldehyde and SO₂, and then measuring the released SO₂ in the same way as free SO₂ is determined. It’s important that only a gentle heat is applied to the sample as vigorous boiling can result in distillation of volatile acids from the sample and lead to an overestimation of bound SO₂. The other requirement for the bound test is a cooling system for the heated vapours; this typically uses non recycled water.

Conclusion
Acetaldehyde is a well-known but infrequently analysed compound in the Australian wine industry.

Several processes during the life of a wine are involved in its production and certain fermentation conditions and winemaking interventions can be used to influence the levels produced. Monitoring the effects of oxidation and in particular acetaldehyde production should be a consideration for all winemakers and can be achieved by analysing the compound itself or by indirect analysis of free and bound sulfur dioxide.

References
1.    Wine Production and Analysis, Zoecklein et al, 1999, Aspen Publishers, p328

2.    Knowing and Making Wine, Peynaud, E; 1981, Wiley, p270

Steve Byrne is General Manager of Vintessential Labs. Greg Howell founded Vintessential Laboratories in 1995, he can be contacted by email on [email protected]. More articles on related topics are available on the Vintessential website: www.vintessential.com.au/resources/articles/