The aromas and flavours associated with particular grape varieties and regional specific wines are more often than not a result of large numbers of compounds, of varying origin, interacting with one another and forming various olfactory and gustatory qualities. There are however a number of compounds (in this case sesquiterpenes) which have an individual aromatic quality associated to them. The distinct aroma of pepper so often associated with cool climate Shiraz/Syrah is the result of one of these sesquiterpenes …
As ever, in this post I hope to share with you the findings of my research on this topic in as approachable a format as possible.
Rotundone is a sesquiterpene, sesquiterpenes are a class of terpenes ( a large and diverse class of organic compounds) that consist of three isoprene units. Like monoterpenes, sesquiterpenes may be acyclic (containing no rings of atoms) or contain rings, including many unique combinations. Sesquiterpenes are found naturally in plants and insects, as semiochemicals (defensive agents or pheromones) Rotundone was originally discovered in the tubers of Cyperus rotundus, a species of sedge, and is also present in black and white peppercorns and a number of herbs. Extensive experimentation has shown that Rotundone is exclusively found in the skin of a grape and does not appear in either the stem or the pulp
As discussed in a previous post (petrol aroma in Riesling) pre-winemaking process, sesquiterpenes (although also found free) are primarily bound to sugars, rendering them aromatically inactive. Post-winemaking process, formation of aromatically expressive (free not bound) sesquiterpenes arises primarily as a result of acid hydrolysis (using wine as an acid medium) of these sesquiterpenes.
Rotundone has a particularly low sensory threshold of around 8 ng/l in water and 16 ng/l in wine, interestingly it is estimated that around 25% of people are unable to identify Rotundone at all. This particularly low sensory threshold bears significant importance when levels of Rotundone in various regions/varieties are observed against this sensory threshold, as in the graph below. In almost all of the regions in which levels of Rotundone in Shiraz were measured the amounts detected were far above the sensory threshold with some as much as ten times higher (161 ng/l)
As Rotundone is only present in the skin of the grapes, the current hypothesis, supported by some initial supporting evidence (shown in the graph below where wines were shown to have differing levels of Rotundone even from the same winery in the same style) is that depending on climatic and growing conditions, the presence of Rotundone can be higher or lower (this can also depend on clonal variety)
As with several other terpenes, Rotundone increases during the late stages of ripening, and can continue to develop after the grapes are fully ripe closest to harvest. Current studies point toward vintages which experience a cooler overall temperature producing wines with higher levels of Rotundone, in particular concentration of Rotundone is negatively correlated with daily solar exposure and grape bunch zone temperature and positively correlated with vineyard water balance. Applying a basic understanding of cellular respiration it appears that Rotundone is prone to accelerated perspiration at higher temperatures and so bunch cover, exposure and overall temperature will have a large impact on concentration levels.
So, how can levels of Rotundone be influenced by viticulturists?
Starting in the vineyard, given that Rotundone concentration has been linked to vineyard water balance it is possible that precision irrigation and site selection could not only influence Rotundone concentration but also mitigate the impact of climate change and allow viticulturists to maintain desirable levels of Rotundone despite dry vintages. It is also evident that there are large variations in concentrations of Rotundone within single vineyard plots with concentrations varying by as much as 500 ng/kg intra-site. This appears to suggest that factors such as topography, soil, clonal vigour and canopy management play a fundamental role in Rotundone concentration. More research is needed to truly understand the more nuanced influence these varying factors have in order for viticulturists to strategise around them.
Moving on to the winery, studies have shown Rotundone is exclusively located in the skins of the grapes, it is rational to hypothesise that extended maceration techniques, where the skin is left in contact with the juice for a longer period of time, could potentially increase the Rotundone concentration of the final wine, and thus increase it’s peppery character.
It is also likely that management of the must pre, intra and post fermentation can drastically alter concentrations of Rotundone. Only a small amount of Rotundone is measured in the must during pre-fermentation (which is likely a result of the crushing process prior to this stage) Rotundone concentrations increased dramatically from cap formation to the end of fermentation, reaching about 12-15 times the initial concentration. Studies show that after separation from the wine skins, 10-30% of the Rotundone was lost, likely as a result of being bound to the skin and lees particles that were subsequently removed. Additional Rotundone losses occurred during wine filtration,which brought the Rotundone content in the final wine to 50-60% of the amount extracted during the fermentation process.
In light of the aforementioned, although more research is required to be certain, it would be rational to suggest that pre-fermentation maceration (cold soak), yeast selection, fermentation temperature and length, cap management (pump-over etc.) and post-fermentation fining and filtering are all able to influence concentrations of Rotundone in a finished wine.
I hope that this article has added value to your understanding of wine, if anyone has any further information regarding Rotundone or would like to ask any questions please feel free to comment below, thank you for reading.