(Wineland) - Acetaldehyde (ethanal; C2H4O) is a low molecular weight, flavour compound found in a wide variety of aromatic foods and beverages that have, prior to their final stage of production, undergone a degree of fermentation (McCloskey & Mahaney, 1981; Jackowetz et al., 2010). Acetaldehyde has been known to be a product of alcoholic fermentation by yeasts for almost a hundred years, but its presence in wine was not confirmed until 1984 by Dittrich and Barth. It is one of the most important aldehydes (carbonyl compounds) and constitutes more than 90% of the total aldehyde content in wine. Aldehydes, together with a large number of other volatile compounds, are responsible for wine aroma (Liu & Pilone, 2000).

Production

Acetaldehyde is primarily a product of yeast metabolism of sugars during the first stages of alcoholic fermentation. It is the last precursor in yeast fermentation before ethanol is formed, and is produced when pyruvate, the end product of glycolysis, is converted by the enzyme, alcohol dehydrogenase (ADH), to acetaldehyde. Conversely, a secondary source of acetaldehyde production in red wine, which usually occurs after ageing, is oxidation (exposure to air/oxygen) of ethanol, once again facilitated by the enzyme, alcohol dehydrogenase (Jackowetz et al., 2010).

Temperature and acetaldehyde production levels

Controversy still persists regarding the influence of fermentation temperature on acetaldehyde production levels. It was previously reported that acetaldehyde concentration levels, relative to 12, 18 and 24°C, increased significantly at a fermentation temperature of 30°C, which was in direct contrast to reports by Amerine and Ough in 1964 that fermentation temperature does not affect the final aldehyde content. However, it was recently found that cooler fermentation temperatures, in a strictly oxygen-regulated environment, actually led to higher acetaldehyde levels, which could be as a result of a reduced reutilisation of acetaldehyde by the yeasts during the last stages of fermentation (Jackowetz et al., 2010).

Production levels and stage of fermentation

Production levels of acetaldehyde during the early stages of fermentation, differ widely from the final acetaldehyde concentration in wine (Cheraiti et al., 2010) due to reutilisation by the yeast cells (Jackowetz et al., 2010; Li & Mira de Orduña, 2010), as well as degradation by bacteria (Jussier et al., 2006) during the last stages of fermentation.

Sensorial detection

The oenological concentration levels of acetaldehyde vary between different types of wine, e.g. white, red and sherry/port wines. Due to its low sensory threshold (Longo et al., 1992) acetaldehyde has been detected at concentration levels of ca 80 mg/ℓ for white wines, ca 30 mg/ℓ for red wines and ca 300 mg/ℓ for sherries (McCloskey & Mahaney, 1981). The very high acetaldehyde production levels in sherries are due to the fact that this wine style is produced under oxidative conditions. In table wines, high levels of acetaldehyde are undesirable, but at low levels in wine acetaldehyde gives a pleasant, fruity aroma. At higher levels, it nevertheless imparts an irritating odour that has been described as a green, grassy, nutty or apple-like aroma. In sherry/port wines the high acetaldehyde concentrations are considered to be a unique feature of that style (Liu & Pilone, 2000).

The high acetaldehyde levels in sherry/port wines also contribute to the increased colour observed in these wines, compared to normal red wines. Rapid polymerisation of anthocyanins and phenolics (e.g. catechins, tannins) occur in the presence of acetaldehyde, which assists in the formation of condensation products that have higher colour intensities and stabilities (Osborne et al., 2006). Furthermore, acetaldehyde indirectly enhances and stabilises wine colour in that it strongly binds sulfur dioxide, which is known to have a decolourising/bleaching effect in wine (Liu & Pilone, 2000).

Health related problems associated with high acetaldehyde levels in wine

It is crucial for winemakers to monitor and control acetaldehyde levels in wine since, in excess, it can pose several health-related problems. Besides its positive sensorial attributions in wines, numerous studies have shown that the administration of large concentrations of acetaldehyde can lead to a range of behavioural effects, notably those linked with symptoms of hangover such as vomiting, restlessness, nausea, confusion, sweating and headaches. Further, acetaldehyde has been shown to have several fundamental etiologic roles in the pathogenesis of liver fibrosis (Mello et al., 2008) and fetal injury during pregnancy (Quertemont et al., 2005). In addition, chronic alcohol consumption is often observed in patients who suffer oesophageal and gastric cancers as a result of the carcinogenic effect of high acetaldehyde levels in wines. Although no legal limits for concentration of acetaldehyde in wines are currently imposed, the importance of screening acetaldehyde levels in alcoholic beverages has now been given special attention as a result of health concerns (Salaspuro, 2011).

Role of sulfur dioxide (SO2)

The total sulfur dioxide (SO2) content in wine consists of varying levels of free and bound SO2. Other than SO2 being directly added to grape must/wine as a preservative during vinification, its presence in wine can be attributed to yeasts, which produce it to varying extents. Acetaldehyde, being chemically very active, has a strong affinity for SO2. It therefore binds with free SO2 (specifically the bisulfite ion, HSO3-1) to form a complex compound known as ‘hydroxy-sulfonate’, which accounts for the largest percentage of the total SO2 content. This bisulfite-acetaldehyde complex reduces the potent sensory effects of acetaldehyde, and the antimicrobial, antienzymatic and antioxidant properties of SO2 (Jackowetz et al., 2010). A lack of SO2 could lead to spoilage of the wine. Therefore, due to this phenomenon, more SO2 is usually added to a wine containing high concentrations of acetaldehyde, not only to bind it, but also to limit further formation of acetaldehyde. Addition of SO2 could lead to more available free SO2 that will protect the wine’s taste and aroma (Liu & Pilone, 2000). However, as a result of escalating consumer awareness of the adverse health risks related to SO2, efforts have been prioritised to reduce the SO2 contents of wines (Osborne et al., 2006).