WINETECH Technical Yearbook 2019

spectrometer, were performed one day prior to smoke exposure, as well as one and seven days after smoke exposure. RESULTS In this study the application of kaolin and/ or smoke had no impact on ripening and berry development. Smoke exposure did not yield grapes with significant levels of free volatile phenols at one or seven days after smoke exposure. None of the volatile phenols measured, were detected in control grapes and only very low levels in the exposed grapes. This finding was quite surprising. In contrast, significant differences were found in the glycoconjugate concentrations between control and exposed grapes, as well as between grape varieties. Glycoconjugate compositional differences (different sugar combinations) were also observed between varieties and time points that included measurements at maturity. The researchers suggested from these results that glycosylation occurs very soon after smoke exposure, hence the near absence of free volatiles one day after exposure. Overall the glycoconjugate concentrations were much higher in Merlot grapes in compar i son to Sauv i gnon b l anc and Chardonnay that had received the same amount of smoke. The researchers postulate

that the observed varietal differences can be ascribed to differences in berry physiology: skin thickness and the presence (or not) of cuticular waxes. The compositional differences in glycoconjugates observed between varieties could also potentially be ascribed to a variation in enzymes responsible for glycosylation. The study presented an interesting anomaly: the low volatile and glycoconjugated phenol concentrations one day after smoke exposure did not explain the increase in concentrations observed during further r i pen i ng . Th i s f i nd i ng suggest s that intermediate forms of the “smoke taint” phenols exist for a short period of time that are not detectable by the current GC-MS and LC-MS/MS methods. The kaolin treatment had no effect on the uptake of volatile phenols and glycoconjugate formation in the case of Sauvignon blanc, and only a moderate effect on the uptake of some phenols in the case of Chardonnay. Kaolin applied as an aqueous suspension (mixed with water) to fruit and foliage before smoke treatment did have a significant effect on the uptake of volatile phenols in the case of Merlot. Reductions of 58-92% of most volatile phenol glycoconjugates measured at maturity were observed.

Spectral reflectance measurements were only successful differentiating between control and smoke-exposed grapes in the case of Chardonnay at one and seven days and Sauvignon blanc at one day after

grape varieties, as well as within the same grape variety, depending on the time of measurement. Kaolin was able to significantly reduce glycoconjugate concentrations in only one grape variety and the differentiating between control and smoke-tainted grapes using a handheld spectrometer was also effective for one grape variety only. REFERENCES Ristic, R., Van der Hulst, L., Capone, D.L. & Wilkinson, K.L., 2017. Impact of bottle aging on smoke-tainted wines from different grape cultivars. Journal of Agricultural and Food Chemistry 65, 4146-4152. Mayr, C.M., Parker, M., Baldock, G.A., Black, C.A., Pardon, K.H., Williamson, P.O., Herderich, M.J. & Francis, I.L., 2014. Determination of the importance of in-mouth release of volatile phenol glycoconjugates to the flavor of smoke-tainted wines. Journal of Agricultural and Food Chemistry 62(11), 2327-2336. Van der Hulst, L., Munguia, P., Culbert, J.A., Ford, C.M., Burton, R.A. & Wilkinson, K.L., 2019. Accumulation of volatile phenol g l ycocon j ugates i n grapes fo l l owi ng grapevine exposure to smoke and potential mitigation of smoke taint by foliar application of kaolin. Planta , https://doi.org/10.1007/ s00425-018-03079-x .

exposure, but not at seven days. SIGNIFICANCE OF THE STUDY

Merlot have had the highest glycoconjugated pheno l concent ra t i ons a f te r smoke treatment compared to Sauvignon blanc and Chardonnay. Kaolin application to foliage and grapes can potentially mitigate volatile phenol uptake, as demonstrated in the case of Merlot. Spectral reflectance measurements have potential, but further research is needed. SUMMARY In a recently published study (January 2019) Australian researchers investigated the accumu l at i on of vo l at i l e pheno l glycoconjugates in different grape varieties following exposure to smoke ten days after veraison (Van der Hulst, et al ., 2019). In addition they investigated possible mitigation of smoke taint by foliar application of kaolin, as well as whether they could detect smoke-tainted grapes in the vineyard using a handheld spectrometer. The study found significant differences in glycoconjugate concentrations between

– For more information, contact Karien O’Kennedy at karien@winetech.co.za.