Project Number
WW 08-34
Project title
The effect of enological parameters on acetaldehyde kinetics during alcoholic fermentation and its impact on ethanol producing organisms in South African wine and must
Project leader
Van Jaarsveld, F P
Team members
Van Jaarsveld, F P
Jolly, N P
Du Plessis, H W
Hart, R S
October, F M
Ohlson, S
Van Breda, V
Malek, K
Paulsen, C
EXECUTIVE SUMMARY
Acetaldehyde plays a role in the rate of fermentation and the aroma quality of the wine. High concentration levels of acetaldehyde in fermenting must/juice may retard or inhibit yeast ethanol formations, resulting in sluggish or stuck fermentations. It has thus been established that, depending on its concentration levels, acetaldehyde has an effect on yeast metabolism and can therefore influence fermentation kinetics by the inhibition or stimulation of the lag phase and growth rate, which in turn has an impact on the cell yield (biomass). The overall aim of this project is to study the impact of acetaldehyde, as influenced by different yeast strains and oenological parameters, on fermentation dynamics and wine quality, since specific levels of acetaldehyde could result in stuck fermentations, as well as off-odours in wine.
Ten commercial Saccharomyces cerevisiae strains and ten non-commercial non-Saccharomyces, with the exception of Lachancea thermotolerans, which was isolated from the commercial yeast, Rhythm (Chr. Hansen) were selected from the culture collection of ARC Infruitec-Nietvoorbij. These yeasts were screened in a laboratory trial for their acetaldehyde-producing ability during alcoholic fermentation, and resulted in the selection of a high-, medium- and low-acetaldehyde producing yeast. Three levels of acetaldehyde production (high, medium & low) was selected to have definitive parameters when studying the impact that these different acetaldehyde levels has on the fermentation dynamics. The selected yeasts were the Saccharomyces cerevisiae yeasts NT 50 (high), NT 116 (medium) and VIN 13 (low); and the non-Saccharomyces yeasts Torulaspora delbrueckii (high), Candida valida (medium) and Candida guilliermondii (low).
Initially, it was decided to use Pinotage and Sauvignon blanc as the two local cultivars on which the study would be performed. However, Chenin blanc replaced Sauvignon blanc as cultivar of choice, since it was felt that the typical methoxypyrazine aromas of Sauvignon blanc could adversely impact the sensorial detection of acetaldehyde in wine.
For the cellar trial Pinotage and Chenin blanc grapes were harvested from Nietvoorbij vineyards and, after crushing, the musts were frozen away at subzero temperatures and later vinified. The above-mentioned selection of Saccharomyces yeasts was used individually for vinification of grape must, as well as in varying combinations with the non-Saccharomyces yeasts, and resultant wines analysed chemically and evaluated sensorially. The initial sensory evaluation/discussion, amongst judges from Nietvoorbij and the University of Stellenbosch, showed noticeable differences between treatments, in terms of aroma and residual sugar.
Statistical evaluation of the data from the laboratory and cellar trials clearly showed that yeast strain, time of fermentation and fermentation temperature have an impact on levels of acetaldehyde. It is, therefore, concluded that wines co-inoculated with selective yeasts at low fermentation temperatures will have lower levels of acetaldehyde than wines only inoculated with Saccharomyces cerevisiae yeasts.
Since SO2 has a very high affinity for acetaldehyde, the impact of various concentrations of SO2 on the levels of acetaldehyde in fermenting must was monitored in a cellar trial, and the resulting effects on fermentation dynamics and final wine quality were monitored. Although it is known that SO2 impacts wine quality, it was also found in this experimental trial that the varying levels of SO2 have a direct effect on the acetaldehyde levels during fermentation and in the finished wine.
As expected, there were differences in acetaldehyde levels between yeast strains, although the levels were still within acceptable ranges or limits found in wines. Higher acetaldehyde levels were found in samples inoculated with Saccharomyces cerevisiae (because the rate of fermentation of S. cerevisiae is much faster than the co-inoculated samples), as well as samples exposed to high SO2 levels, and at higher fermentation temperatures. Therefore, to ensure lower levels of acetaldehyde in wine, one must preferably co-inoculate with a non-Saccharomyces yeast strain at lower fermentation temperatures, while maintaining low levels of SO2 during fermentation.
Presentation(s)
Van Jaarsveld, F P, October, F M, Rohwer, J, Du Toit, W. 2013. Effect of enological parameters on acetaldehyde kinetics. Papaer presented at the 35th International Conference of the South African Society for Enology and Viticulture.13-15 November, Somerset West, South Africa.
Article
Van Jaarsveld, F P, October, F M. 2015. Asetaldehied in wyn, Winetech Tegnies, Mnth June (310) (p. 73-76)
Article.pdf