Wines sleuths sniff out the pepper molecule

Research being done by the Australian Wine Research Institute (AWRI), with help from Canberra’s Jim Lumbers (Lerida Estate) and Frank van der Loo (Mount Majura Vineyard), could have a profound impact on how vignerons control the level of pepperiness in shiraz – Australia’s signature variety.

Their work is part of the long history we have of digging into the chemistry of wine flavours. And what science unearths sometimes gels with long-used wine descriptions. A few decades back, for example, Australian scientists identified methoxypyrazene as the compound underpinning the aroma and flavour of sauvignon blanc.

Wines made from the variety, especially those from cool areas, had often been described as tasting of capsicum or gooseberry. Subsequent testing found methoxypyrazene at the heart of capsicum and gooseberry flavours, giving a scientific basis for the descriptors used for sauvignon blanc.

I don’t think Australian scientists discovered the terpene family behind riesling’s distinctive floral character. But they’ve certainly wondered why, with bottle age, some rieslings develop a distinctive ‘kero’ aroma. It’s now thought to be caused by the oxidation of terpenes. And since terpenes are partly behind the aroma of kerosene, the descriptor ‘kero’ for old riesling has some scientific basis.

And less than two years ago a group of Australian scientists discovered the molecule (another of the terpene family) behind the widely observed ‘peppery’ character in shiraz.

In a paper published on the BioInfoBank Library website early last year, the scientists write, “An obscure sesquiterpene, rotundone, has been identified as a hitherto unrecognised important aroma impact compound with a strong spicy, peppercorn aroma. Excellent correlations were observed between the concentration of rotundone and the mean ‘black pepper’ aroma intensity rated by sensory panels for both grape and wine samples, indicating that rotundone is a major contributor to peppery characters in shiraz grapes and wine…”

OK, so rotundone makes shiraz grapes and wine taste peppery. But what makes peppercorns peppery? The wine sleuths weren’t buying into the old belief that it resulted from chemical complexity.

Further investigation revealed “Rotundone was found in much higher amounts in other common herbs and spices, especially black and white peppercorns, where it was present at approximately 10,000 times the level found in ‘peppery’ wine. Rotundone is the firsts compound found in black or white peppercorns that has a distinctive peppery aroma”.

The sensory tests revealed two other remarkable facts about rotundone. The first was that 80 per cent of the tasting panel detected it in amazingly tiny concentrations: 16 billionths of a gram per litre in wine or 8 billionths of a gram in water. These tasters could also discern spikes in flavour intensity as the concentration increased.

The second striking observation (marketers please note) was that 20 per cent of the tasters couldn’t detect rotundone at all – even in water at concentrations of 4,000 billionths a gram per litre, 500 times the detectable threshold for the other tasters. Their conclusion that “the sensory experience of two consumers enjoying the same glass of shiraz wine might be very different” could be an understatement -– but it won’t stop the research on rotundone’s affect on wine flavour.

To track it’s development in grapes and wine, Frank and Jim began sending shiraz berry samples to the AWRI from the time of veraison (the stage where grapes begin to soften, develop red colour and ripen). The AWRI hopes to gain a better understanding of when and how rotundone forms and what determines its concentration in the berries.

And because rotundone is believed to be in or near the skin of the grape, its concentration in wine could be affected by the duration of skin contact during winemaking. The time varies considerably – from a minimum of perhaps seven days to three or four weeks.

The duration of contact depends largely on the winemaker’s preference – shorter periods allow for fermentation and extraction of colour and tannin from the skins; longer periods often include pre-ferment or post-ferment maceration, or both, to modify tannin structure.

Whether or not this affects the pepperiness of shiraz should be better understood following the current research.

And to give a long-run perspective on finished wine, Jim Lumbers says “I have donated my vertical of Clonakilla Shiraz Viognier (2007 to 1997) with three gaps kindly being filled by Tim [Kirk, or Clonakilla]”.

Copyright © Chris Shanahan 2009

Be Sociable, Share!