The scientific puzzle that is rotundone

Rotundone continues to both baffle and delight the experts.

Since 2007, when an AWRI team revealed that this compound found in grape skins is responsible for the distinctive black pepper flavours in many Shiraz wines, scientists have been working to understand the factors that influence its presence and potency. However, each new discovery brings new questions.

Why, for example, is the impact greatest in cool climate regions and on the cool side of the grapes when standard chemistry tells us that chemical reactions typically run faster when it is hotter? And why does it occur in ripe grapes in Australia, whereas anecdotal evidence from the Rhône Valley suggests it is more common there in cold, wet years when ripening is difficult?

The AWRI collaborates with national and international institutions on a range of rotundone projects and earlier this year published an AWRI Technical Review article to update the state of play. Things move quickly, however.

In the nine months since, a PhD student at the University of Adelaide has demonstrated that air oxidation plays a role in creating the aroma compound, another in Melbourne has confirmed that the chemical activity largely happens in the shade, Japanese researchers have identified a grape enzyme that catalyses the final oxidation step, and the AWRI has tested and discounted the involvement of fungi such as Botrytis and identified the importance of varying light.

‘We didn’t get much response under laboratory light, which is a kind of fluorescent light, but we got a lot of reaction with sunlight, which has a totally different spectrum’, said the AWRI’s Group Manager – Research, Dr Markus Herderich. ‘Then a visiting student demonstrated that effectively you get different oxidation reactions with light and without light.’

Was that expected? ‘Now that we have the result it explains what we see because if you are doing oxygen chemistry you can activate oxygen with light and that’s called singlet oxygen, which gives you a very different reactivity profile compared to having normal oxygen.’

Dr Herderich says researchers now feel they understand the three mechanisms that can lead to the creation of rotundone – oxidation by air, light and enzymes – but none works without a precursor. There is great interest, therefore, in further investigating α-guaiene, a compound very similar in structure to rotundone that can be readily converted to rotundone by oxidation.

‘From my perspective the really big question is why are some vineyards forming rotundone and other vineyards in the same region not? And why are vineyards that are high in rotundone in some years lower in other years despite so many other conditions being unchanged?’

‘If you look within a vineyard, why are some areas within that vineyard always higher in rotundone than others? We have done that experiment over three years and there is always a stable distribution pattern irrespective of how much total rotundone we get.’

‘From a research perspective, the question is “what are the biological or environmental switches that switch on or off the biosynthesis of guaiene and hence control formation of rotundone”?’

Getting closer to the answer could involve some clever science or more basic experiments such as transplanting cuttings from one part of a vineyard or vine to another to see if they retain their characteristics.

‘Is it genetics or something in the soil or location, or a combination of both?’ Dr Herderich said. ‘One of the reasons I like this area of research is that you can actually bring the environment into the terroir discussion.’

For the scientifically minded, rotundone is a type of sesquiterpene, and early stages of its biosynthesis are actually related to the monoterpenes found in floral wine varieties such as Riesling.