Rotundone is responsible for the attractive “pepper” character in grape berries and wine and is especially important for cool-climate Shiraz. Part 1 of this two-part series covered the sensory impact of rotundone and the main viticultural factors that influence rotundone concentrations in the berries before harvest. Part 2 summarizes some of the research findings regarding the influence of oenological practices on rotundone and the stability of the compound during ageing.

 

Rotundone extraction

Rotundone is found almost entirely in the grape skins1 and the extraction of rotundone during fermentation is critical for increasing rotundone levels in the wine. Studies showed that rotundone is mainly extracted during the initial stages of fermentation whereafter no further increases occur. The presence of ethanol and/or yeast-related effects are likely involved in facilitating the extraction of the hydrophobic rotundone2, especially considering that no significant increases were observed during the two days between crushing and active fermentation. For the purpose of extracting maximum concentration of rotundone, maceration prior to fermentation and extended maceration post-fermentation should not be relied upon.

It is important to note that only about 10% of the rotundone in grape skins is extracted during fermentation under normal winemaking conditions, regardless of the starting concentration1. Fortification of the fermenting must during the early stages of fermentation has been proven to significantly increase the extractability resulting in a 19% extraction as opposed to the non-fortified conventional fermentation2.

 

Rotundone in non-grape materials

Non-grape tissues such as grapevine leaves and stems contain high concentrations of rotundone compared to the grape berries at both vériason and harvest. This is of particular interest to the winemaker who can use winemaking techniques such as whole-bunch fermentation and the addition of vine leaves and stems to the fermentation tank in order to elevate the rotundone concentration (up to sixfold).3,4 Winemaker, Izele van Blerk of KWV prefers doing whole bunch fermentation on Shiraz grapes and reported a noticeable increase in spiciness in cool-climate Shiraz5.

This inclusion of stems and leaves should still be done with caution as it can also result in higher concentrations which confer “cut-grass” and “herbal” aromas. It can also increase tannin concentration, which if overdone, can lead to excess astringency.

 

Yeast and rotundone

To date, there is no evidence that yeast alters rotundone concentration (other than the indirect effect of ethanol production increasing the extractability)2. The impact of yeast and various yeast strains on rotundone is a topic that needs further investigation.

 

Separation mechanisms

Separating the wine from the skins resulted in a 10-30% decrease in rotundone concentration of a Vespolina wine1. This decrease is likely due to the rotundone being bound to the insoluble particles that were subsequently removed.

Filtration also significantly decreased the rotundone concentration of the Vespolina wine and a decrease of between 50 and 60% of the amount extracted during fermentation was observed1. Alternative methods for wine filtration and fining should be considered to ensure maximum retention of rotundone before the wine is bottled.

 

Stability

It seems that rotundone concentrations remain relatively stable during extended ageing6. Researchers spiked a Shiraz wine with 100 ng/L of rotundone and bottled the wine using various closures. The concentration of rotundone remained stable during the first 12 months of storage, while a 6% reduction in concentration was observed after 39 months. A natural wine (no rotundone added), with an initial concentration of 161 ng/L still contained 152 ng/L of rotundone ten years after bottling. It thus seems that the pepper characteristics of wine at bottling are not likely to change dramatically over time provided proper storage conditions are used.

 

Conclusion

These results represent great practical value for the wine industry, providing options for the production of a “spicier” wine using winemaking techniques such as fortification. That said, it is still important to remember that only about 10% of the total rotundone found in the berry is extracted into the wine. Therefore any process that increases the amount of rotundone in grapes will result in elevated levels in the finished wine2. Optimising the viticultural processes to enhance rotundone formation in the grape berry should thus be considered before opting to rely solely on adjusting winemaking techniques.

Scientists have been working to understand the factors that determine the concentration of rotundone in Shiraz wines and there are still many questions that remain unanswered. It appears that both viticultural and oenological techniques may be used to manipulate the peppery character of the wine and it would be of value for the South African industry to investigate ways in which vineyard and winery practices can be adjusted to optimise levels.

 

References

(1)      Caputi, L.; Carlin, S.; Ghiglieno, I.; Stefanini, M.; Valenti, L.; Vrhovsek, U.; Mattivi, F. Relationship of Changes in Rotundone Content during Grape Ripening and Winemaking to Manipulation of the “peppery” Character of Wine. J. Agric. Food Chem. 2011, 59 (10), 5565–5571. https://doi.org/10.1021/jf200786u.

(2)      Zhang, P.; Luo, F.; Howell, K. Fortification and Elevated Alcohol Concentration Affect the Concentration of Rotundone and Volatiles in Vitis Vinifera Cv. Shiraz Wine. Fermentation 2017, 3 (3). https://doi.org/10.3390/fermentation3030029.

(3)      Capone, D. L.; Jeffery, D. W.; Sefton, M. A. Vineyard and Fermentation Studies to Elucidate the Origin of 1,8-Cineole in Australian Red Wine. J. Agric. Food Chem. 2012, 60 (9), 2281–2287. https://doi.org/10.1021/jf204499h.

(4)      Zhang, P.; Fuentes, S.; Wang, Y.; Deng, R.; Krstic, M.; Herderich, M.; Barlow, E. W. R.; Howell, K. Distribution of Rotundone and Possible Translocation of Related Compounds amongst Grapevine Tissues in Vitis Vinifera L. Cv. Shiraz. Front. Plant Sci. 2016, 7 (June), 1–12. https://doi.org/10.3389/fpls.2016.00859.

(5)      Mocke, B. The quest to become whole again https://www.wineland.co.za/quest-become-whole/.

(6)      Jeffery, D. W.; Siebert, T. E.; Capone, D. L.; Pardon, K. H.; Van Leeuwen, K. A.; Solomon, M. Grape and Wine Pepper Aroma – Analytically Challenging but We Sniff It out in the End. AWRI Tech. Rev. 2009, 180, 11–16.

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