WINETECH Technical Yearbook 2021

centration from the beginning to the end of the ultrasound treatment. This result comes as a surprise considering the efficacy of the ultrasound to extract phenolic compounds. In an attempt to explain the results found in the study the researchers proceeded to exper iment wi th a model wine medium containing known amounts of glutathionylated and cysteinylated thiol precursors. Ultrasound treatment was then applied to the model wine. Results showed a reduction in the non- volatile precursors during the sonication treatment suggesting that the sonication led to the breaking of the precursor compound. This breakage is likely due to the radical and electron-transfer processes generated by the high energy and excited-state species created in the solution following cavitation. The breakdown of the precursor could result in the cleavage of the carbon-sulfur bond, releasing the aromatic volatile thiol into the medium. Indeed, after sonication of the model wine containing the precursors, the mean results for 3MH and 4MMP was significantly higher than those of the control samples. It seems that ultrasound treatment has the potential to release the aromatic thiols from the non-aromatic precursor without the use of yeast activity. The amount of precursors

broken down during sonication, however, does not correspond to the amount of free thiols released (much less thiols are released). The thiol content of the wines made from the sonicated crushed berries were not measured. Further studies will have to investigate the effect of sonication on the free volatile thiols (possibly also inducing the breakdown of the aromatic compound), the reaction of the thiols with formed radicals and/or possible stripping effects due to the volatility of the free thiols during the ultrasound treatment. CONCLUSION Ultrasound is a relatively low cost, non- hazardous and environmentally friendly technology and could be an important technological innovation speeding up some slow reactions required in the winemaking process such as maceration. The application of ultrasound technology can significantly enhance the extraction of various grape components, however, the enhanced extraction of volatile thiol precursors from Sauvignon blanc grape berries due to ultrasound treatment remains to be determined. Tests performed in a model wine solution showed that the ultrasound waves led to the cleavage of the non-volatile precursor, releasing the aromatic thiol. In a real wine

between ultrasound and non-volatile thiol precursors and the effect of ultrasound on the free thiol remains to be established. REFERENCES Roman, T., Tonidandel, L., Nicolini, G., Bel- lantuono, E., Barp, L., Larcher, R. & Celotti, E., 2020. Evidence of the possible interac- tion between ultrasound and thiol precur- sors. Foods 9(1). https://doi.org/10.3390/ foods9010104. OIV, 2019. OIV Resolution OIV-OENO 616- 2019. Geneva, Switzerland. Tao, Y. & Sun, D-W., 2015. Enhancement of food processes by ultrasound: A review. Critical Reviews in Food Science and Nutrition 55(4), 570-594. https://doi.org/1 0.1080/10408398.2012.667849 . Celotti, E. & Ferraretto, P., 2016. Studies for the ultrasound application in winemaking for a low impact enology. In: 39th World Congress of Vine and Wine , Bento Gonçalves, Brazil, pp. 104-106. Ferraretto, P., Cacciola, V., Ferran Batlló, I. & Celotti, E., 2013. Ultrasounds application in winemaking: Grape maceration and yeast lysis. Italian Journal of Food Science 25(2), 160-168.

medium, the presence of other compounds and the formation of radicals during sonication can negate the potential increase in free thiols. Further research is needed to investigate the mechanisms of interaction between ultrasound and thiol precursors and the potential degradation products during winemaking. The prescriptions specified by the OIV regarding ultrasound treatment (OIV, 2019): • The treatment must be carried out on destemmed and crushed grapes to increase the performance of the treatment; • To avoid a temperature increase in the bulk of crushed grapes, this treatment should be done with the bulk in movement; • To boost an efficient cavitation process, the solid/liquid proportion in the bulk of crushed grapes must be adequate. SUMMARY The extraction of non-volatile thiol precursors (which is predominantly located in the skins of the berry) can possibly be enhanced by ultrasound treatment, however, the treatment can also result in the cleavage of the precursor, thereby releasing the aromatic thiol. The exact interaction

For more information, contact Carien Coetzee at carien@basicwine.co.za.

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