by Francois van Jaarsveld and Francois October
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).
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 …