Winetech Technical Yearbook 2022

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4

3

2

Volatile phenols ( µg/L)

1

0

VIN 13

VIN 13 (ST)

QA 23

QA 23 (ST)

X16

X16 (ST)

Treatment

Guaiacol

4-Methylguaiacol

FIGURE 3. Guaiacol and 4-methylguaiacol levels in Chenin blanc wines produced from smoke-exposed juice fermented with three commercial yeasts (VIN 13, QA23 and X16). Smoke treatment is abbreviated as ST.

ABSTRACT Wines produced from smoke-exposed grapes can exhibit unpleasant smoky, burnt or ashtray characteristics. Chenin blanc juice was exposed to smoke and fermented with various commercial wine yeasts. The levels of volatile phenols were determined, and the wines were also subjected to sensory evaluations. The commercial yeast strains produced wines containing variable levels of guaiacol and 4-methylguaiacol. Wines made with QA23 contained the highest guaiacol levels, while those produced with VIN 13 had the lowest levels. Wines produced from smoke-exposed juice contained significantly higher levels of guaiacol and 4-methylguaiacol than unsmoked wines. A sensory panel could distinguish between wines produced from smoked and unsmoked juice and preferred the unsmoked wines. Wines from smoked juice were associated with smoky, sulphur, vegetative and cooked vegetable characteristics. Yeast selection can minimise/mask or maximise the volatile phenol levels in wines. Winemakers should ask their suppliers for assistance with yeast selection and should use strains that produce low or high guaiacol and 4-methylguaiacol levels depending on their chosen volatile phenol management strategies. ACKNOWLEDGEMENTS The authors thank the ARC and Winetech for funding. All the students and interns who assisted with this project are thanked for their technical assistance. Collaborators from Stellenbosch University are thanked for their contributions. REFERENCES https://www.wineland.co.za/yeast-smoke-taint-and-chenin blanc-wines/

results showed that the volatile phenols affected wine flavour even at low levels. The odour detection threshold of guaiacol is 23 µg/L and is associated with burnt, smoky, toasty, phenolic characteristics, 7 while it is 21 µg/L for 4-methylguaiacol, perceived as sweet-spicy, phenolic and leathery. 8 Although the volatile phenols levels in this study were considerably below the odour detection threshold, the sensory panel could distinguish between the smoked and unsmoked treatments and preferred the unsmoked wines. In this trial, none of the three yeast strains effectively masked the negative flavours (smoky, sulphur, vegetative and cooked vegetable characteristics) associated with smoke exposure through the production of varietal or yeast-derived flavour compounds. However, the flavour profiles of the smoked wines produced with the selected yeast were still slightly different from each other. This indicates that even though yeast selection may not eliminate the perception of smoke taint in wines, selecting a low guaiacol-producing strain may minimise volatile phenol levels. Therefore, yeast choice is very important when producing wines from smoke-exposed grapes. Yeast suppliers can help with this choice. CONCLUSIONS Yeast selection can affect the volatile phenol levels and flavour profiles of wines. Yeast selection may therefore be a tool to minimise the levels of volatile phenols in wines, but it may not completely mask the negative flavours associated with volatile phenols. Volatile phenols can have a negative effect on wine flavour, even at levels far below their odour detection threshold. Another strategy is to maximise volatile phenols in wines and then remove them with fining agents as suggested by some suppliers of oenological products. In such a case, a yeast with high guaiacol-producing capabilities is desired. Winemakers should contact their yeast suppliers when working with grapes exposed to smoke and ask them to assist with selecting low or high guaiacol-producing yeast strains for fermentation, depending on their chosen strategy.

For more information, contact Heinrich du Plessis at dplessishe@arc.agric.za.

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WINETECH TECHNICAL YEARBOOK 2022