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).

Acetaldehyde degradation during malolactic fermentation

The concentration of acetaldehyde in wine can be reduced by appropriate yeast strain selection, as well as the prevention of oxidation during vinification. In most cases, the reduction of acetaldehyde after alcoholic fermentation can be accomplished by wine lactic acid bacteria (LAB). Homo- and heterofermentative wine LAB of the genera Lactobacillus and Oenococcus are capable of degrading free and SO2-bound acetaldehyde (Osborne et al., 2000). Metabolism of the acetaldehyde moiety of SO2-bound acetaldehyde by LAB result in the release of free SO2 which in turn inhibit LAB growth.

Saccharomyces cerevisiae

Saccharomyces cerevisiae is the most important wine yeast and is responsible for the metabolism of grape sugar to alcohol (ethanol) and carbon dioxide (CO2). It can grow in high sugar concentrations, as well as at low pH, and can survive in relatively high ethanol concentrations too. Due to these unique characteristics it is able to effectively ferment grape musts (with high sugar concentrations) to ethanol, giving it a competitive advantage over other yeasts.

The large variety of commercially available Saccharomyces cerevisiae strains is partially responsible for the differences in acetaldehyde concentrations in wines, which is attributed to the varied rates at which these yeasts produce acetaldehyde during alcoholic fermentation (Longo et al., 1992).

Although wine yeasts are the primary producers of acetaldehyde during alcoholic fermentation, this metabolite, at high production levels, may have an inhibitory effect on the kinetics of Saccharomyces cerevisiae by either lengthening its lag phase and/or slowing down its growth rate. Conversely, it has been reported that, for ethanol-stressed Saccharomyces cerevisiae, the lag phase was shortened and the growth rate stimulated at low acetaldehyde concentrations, implying that acetaldehyde may play a role in preventing ethanol-induced stress and growth inhibition of yeast cells (Stanley et al., 1993). High concentrations of acetaldehyde, intracellularly and extracellularly, may also retard or inhibit ethanol formation by yeast, resulting in sluggish or stuck fermentations (Liu & Pilone, 2000).

Acetaldehyde triggers the transcription and expression of several HSP genes that are responsible for the synthesis of heat shock proteins (Hsp), of which one protective protein, Hsp104p, has been shown to resist in vitro stress factors (e.g. cold, glucose starvation, oxidative, osmotic, ethanol and/or acetaldehyde stress) in certain yeast cells (Aranda et al., 2002). This, and the fact that the stimulatory/inhibitory effect of acetaldehyde could be a redox-based mechanism (Vriesekoop et al., 2007), needs to be investigated more closely to address growth conditions related to acetaldehyde and ethanol stress.

Non-Saccharomyces yeasts

This group of yeasts includes a wide variety of genera, i.e. Candida, Kloeckera, Hanseniaspora, Pichia, Torulaspora, Saccharomycodes and Zygosaccharomyces. Non-Saccharomyces yeasts are naturally present in all wine fermentations and are metabolically active, therefore their metabolites can affect wine quality. The realisation that non-Saccharomyces yeasts can contribute significantly to the flavour and quality of wine has led to more detailed investigations into its properties, as well as the study of mixed fermentations, which involve the co-inoculation of Saccharomyces cerevisiae with one or more different non-Saccharomyces strains.

Furthermore, most of these yeasts are susceptible to the adverse conditions of wine (e.g. pH, SO2 and ethanol concentrations) and die off eventually, but there are certain species (e.g. Brettanomyces spp. and Zygosaccharomyces spp.) that are tolerant of ethanol and may even be found in bottled wine. Other species, like Saccharomycodes ludwigii can produce large amounts of acetaldehyde that negatively affects wine aroma, while Candida stellata has shown great variability in acetaldehyde production (Romano et al., 1997).

Acknowledgements

The authors thank Winetech, ARC and THRIP for financial support of a project concerning the effect of oenological parameters on acetaldehyde kinetics during alcoholic fermentation, and its impact on ethanol-producing organisms in South African must/wine. The authors also thank Boredi Silas Chidi for assistance with the literature overview and the compilation of this article.

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