Project Number
P04000097
Project Title
Effect of yeast and lactic acid bacteria on volatile compounds associated with smoke taint.
Project Leader
Du Plessis, H
Team Members
Du Plessis, H
Ngo, M
Start date
2016
Objectives and Rationale
Wines from grapes exposed to bushfire smoke can exhibit undesirable smoky, burnt aromas and an ashy character on the palate. This project formed part of a larger collaboration to develop chemical/analytical and sensory methods for evaluating smoke taint levels in grapes and wine for the South African wine industry through sensory and GC-MS methods. It also attempted to provide the local industry with winemaking strategies to reduce smoke taint. The specific focus of this project was to investigate the effects of different yeast and lactic acid bacteria combinations on the volatile compounds associated with smoke taint and to use alternate wine production practices to reduce smoke taint in wines.
Methods
Chenin blanc, Merlot and Shiraz grapes were smoked and laboratory and small-scale fermentation trials were performed. The standard chemical parameters were analysed using Fourier-transform infrared spectroscopy, non-volatile phenolic compounds were measured with high-performance liquid chromatography (HPLC) and the concentrations of volatile phenolic compounds were determined using gas chromatography–mass spectrometry (GC-MS). The wines were also subjected to sensory evaluation.
Key Results
Yeast strains can produce high, intermediate or low levels of volatile phenolic compounds. Spontaneous fermentations can cause high concentrations of volatile phenolic compounds in wines. Lactic acid bacteria strain and malolactic fermentation (MLF) affect the concentration of volatile phenolic compounds found in wines and whether these compounds are perceived as negative or positive. Wines that did not undergo MLF or underwent sequential MLF contained lower levels of volatile phenolic compounds than those that underwent MLF as a co-inoculation. Volatile phenolic compound levels can be reduced by the production of blanc de noir or rosé from red grapes.
Key Conclusion of Discussion
The yeast strain and MLF can affect the levels of volatile phenolic compounds in wine and can positively or negatively affect the wine quality.
Recommendation to Industry / Key take-home message
Winemakers should have the smoke-exposed grapes analysed before harvest to determine the levels of phenolic precursors and perform laboratory-scale pre-harvest fermentations to assess the wines’ potential volatile phenolic compound levels. Winemakers should consult yeast suppliers to assist them with selecting low volatile phenolic compounds producing yeast strains or should consider the following yeasts: NT 112, RX60 or WE 372 for red wine production and UCLM S325, Merit or VIN 13 for white and rosé wine. Yeast strains with β-glucosidase activity should be avoided. Production of blanc de noir or rosé wines should be considered, but if red wines are produced, please note that induction of MLF will increase the volatile phenolic compound levels. All wines (white, rosé or red) should be prepared for early consumption (consumed within 6-12 months). Wines acceptable for consumers at bottling could become unacceptable after 6 to 12 months. Smoke-exposed grapes should not be used to produce premier-class wines, and smoke-tainted wines should not be blended with non-smoke wines.