Thiols key to Marlborough Sauvignon character

Much of Marlborough Sauvignon’s distinctive taste can be attributed to high thiol concentrations – so how does one create and preserve these compounds?

Machine-picked Sauvignon Blanc has 10 times more thiols than hand-picked fruit. Picture source: thewine.co.nz

During a presentation called ‘The science of Sauvignon Blanc’ at New Zealand’s first International Sauvignon Blanc Celebration – which was held earlier this month – wine writer and scientist Jamie Goode looked closely at the distinguishing characters of Marlborough Sauvignon, and how to enhance and retain the most desirable traits.

“The simple story,” he began, “is that good Marlborough Sauvignon is distinctive because it is elevated in both methoxypyazines [which give greenness] and thiols [which give passionfruit/tropical aromatics], and both these elements are combined in a balanced way.”

For Goode, thiols are of particular interest in the Marlborough Sauvignon story, and a relatively new aspect to discussing wine flavour: “Five years ago people weren’t talking about thiols,” he said, before attributing much of the increased understanding of wine chemistry to Marlborough Sauvignon itself – “New Zealand has put an amazing investment into understanding Sauvignon Blanc, and Sauvignon Blanc has become a model for understanding what makes certain wines distinctive,” he recorded.

Turning the presentation to the detailed science of Sauvignon, Goode explained that thiols are a “big group” of Sulfur-containing organic compounds that are “smelly”.

Importantly, humans are “very good at detecting thiols at low concentrations, and they contribute to aromas such as passionfruit and blackcurrant as well as roasted coffee, popcorn, grilled meat and beer,” said Goode.

One group of thiols are also known as mercaptans, with Sauvignon Blanc containing three of these compounds: 3-MH (grapefruit/passionfruit); 3-MHA (sweat/passionfruit) and 4-MMP (broom/cat’s pee). Importantly, Marlborough Sauvignon, “has particularly high levels of 3-MH and 3-MHA”, according to Goode (see table, bottom).

Thiols are formed by yeasts from precursors present in the grape must, although Goode said that only a small proportion of precursors are converted, and stressed that “the yeast strain only has a limited effect on the level of thiols.”

So what enhances thiol precursors in Marlborough Sauvignon? Goode proposed a number of theories, including the high UV environment in New Zealand, as well as the prevalence of machines to harvest Sauvignon – machine-picked fruit has 10 times more thiols than hand-picked fruit, which, he said, may be due to damage to the Sauvignon, or the creation of aromatic chemicals during machine harvesting as a signal to other plants.

What is vital, said Goode, for the creation of a “high thiol Sauvignon” is to protect the precursors of thiols – called C6 green leaf volatiles – and this can be done by adding Sulphur Dioxide early on.

“Once you’ve got them, you want to preserve them,” said Goode, adding that it was also vital to keep the wine at a low temperature, otherwise the sweaty passion fruit flavours from 3-MHA can quickly degrade.

“3-MHA is transformed to 3-MH by acid hydrolysis, and this is temperature driven, so if you store a wine in a tank or bottle at 10 degrees Celsius, you will lose half the 3-MHA in two years, which is fine, but if you keep it at 20 degrees Celsius, then you will lose the same amount in two months,” said Goode, illustrating starkly the impact of heat on thiol levels.

Consequently, Goode suggested, “Perhaps producers should be storing wine in tank at low temperatures to preserve the desirable characters and then bottle the wine sequentially.”

Nevertheless, he explained that wine flavour chemistry was more complex than the above suggested, noting that there are “wine constituents that don’t have any aromatic character on their own, but influence strongly the way in which various aromatic molecules are perceived.”

In other words, it would be extremely hard to mimic a Marlborough Sauvignon Blanc through manipulating thiol levels alone, something that has been proved by wine reconstitution experiments by Frank Benkwitz and Laura Nicolau, which were cited by Goode.

Concluding, Goode said, “Thiols are important, and you can elevate them, but you need to consider Sauvignon Blanc holistically and the way the compounds work together to create wine aroma.”

Thiols in Sauvignon Blanc:

Thiol (and sensory characteristic)           Perception threshold in wine (ng/litre)      Range found in NZ Sauvignon (ng/litre)
3MH (grapefruit, passionfruit skin/stalk)  60                                                              100-20 000
3MHA (sweet-sweaty passionfruit)          4 (above 100, impact compound)              5-2500
4MMP (broom, cat’s pee)                        0.8                                                              2-50

Table source: Jamie Goode