The role of microorganisms in the formation of bitterness in wines


Project Number
IWBT 05-11

Project title
The role of microorganisms in the formation of bitterness in wines

Project leader
Du Toit, M A

Institution
University of Stellenbosch, Department of Viticulture and Enology, Institute for Wine Biotechnology

Team members
Krieling, S J

Project description
Young and Veritas Wine Show wines for 2002 were obtained and tasted for bitterness by an experienced in-house wine tasting panel. There was no evident correlation between the bitter wines (as judged by the panel) and the glycerol/glucose ratios. The wines produced in the laboratory fermentation trials by yeast strains BM 45, WE 372, WE 14 and NT 112 were the closest to glucose/glycerol ratios between 2 and 11. This is a possible indication that wines produced by the above-mentioned yeasts may be at risk for bacterial conversion of glycerol to acrolein, especially if sulphur dioxide levels are not adequate or sterile filtering has not been practiced. The yeasts WE 14 and WE 372 are nearly exclusively used by some producers for red wines, especially Pinotage.

It was found that there are definite differences between the LAB ecology of Pinotage, Merlot and Cabernet Sauvignon. Cabernet Sauvignon had a higher LAB population, while Pinotage, a unique South African variety, had the most diverse population. The two most dominant species isolated were Lb. plantarum and Pediococcus spp., with Pediococcus spp. being isolated mostly from Pinotage. To our knowledge, this is the first time that Lb. plantarum, Lb. paracasei and Lb. pentosus are reported to possess the ability to degrade glycerol. Interestingly, 62% of the GD-possessing strains were isolated from Pinotage.

The results have shown that HPLC analysis coupled with ultraviolet detection may not be sensitive or specific enough to reliably detect and identify bitter compounds, which may form on acrolein exposure of wine. It is therefore suggested that HPLC analysis coupled with mass spectrometry be used to further investigate and quantify these compounds in red wine. From the results it seems that the bitterness found in wines by the present tasting panel is not due to increased levels of any known, presently analysed polyphenols normally found in wine. It would therefore be more advantageous to focus on the observed acrolein-phenol complexes as a possible cause of bitterness in wine. It is also essential that a red wine be produced under conditions that stimulate acrolein formation and that this wine be analysed using all the presently available and suggested methods. In so doing it will be possible to identify positive marker compounds for acrolein induced bitterness and/or potentially bitter compounds.

Presentation(s)
Krieling, S J, Pretorius, I S, Lambrechts, M G and Du Toit, M A. 2002. An investigation into lactic acid bacteria as a possible cause of bitterness in wine. Paper presented at the 12th Congress of the South African Society for Microbiology. Bloemfontein, South Africa.

Krieling, S J, Pretorius, I S and Du Toit, M A. 2002. An investigation into lactic acid bacteria as a possible cause of bitterness in wine. Paper presented at the 26th National Congress of the South African Society for Enology and Viticulture. 13-15 November, Somerset West, South Africa.

Krieling, S J, Pretorius, I S and Du Toit, M A. 2003. Isolation, identification and characterisation of glycerol-degrading lactic acid bacteria from South African red wines. Paper presented at the 1st FEMS Congress of European Microbiologists. Ljubljana, Slovenia.

Du Toit, M A. 2003. Bitterness in red wine. Paper presented at the Anchor Bio-Technologies 7th Technical Symposium. Stellenbosch, South Africa.

Krieling, S J, Pretorius, I S and Du Toit, M A. 2004. Wine lactic acid bacteria and their glycerol degradation ability. Paper presented at the 12th Australian Wine Industry Technical Conference. Melbourne, Australia.

FinalReport.pdf

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