Copper fining kills reductive aromas but also wine character

Eliminating unpleasant reductive aromas in wine starts with practices in the vineyard and should not be corrected using copper fining or injecting oxygen according to Dr Pascal Chatonnet.

The winemaker and oenologist, speaking at Amorim’s wine faults workshop during the London International Wine Fair (LIWF) yesterday, said, “Don’t try to manage the risk of post-bottling negative reductive evolution by increasing the oxidative status of the wine, or worst, treating it with copper.”

The remark followed a tasting of a Vin de Pay D’Oc Sauvignon Blanc deliberately tainted with different volatile sulphur compounds (VSCs).

Smelling the wines showed the influence of not only various VSCs but also the same compounds at different concentrations, and highlighted a range of aromas, from the unpleasant, such as burnt rubber, to the pleasing, such as passion fruit.

For example, speaking of the different concentrations of VSCs, Chatonnet pointed out that the mercaptan (Thiol) called 4-methyl-mercaptopentanone brings the boxwood aroma typical of Sauvignon Blanc, but, in excessive amounts, produces a “cat’s pee” smell.

Then, pointing out that some mercaptans can bring attractive aromas to a wine, he isolated mercaptan 3-Mercaptohexanol, which produces the passion fruit smell mentioned above.

Notably, he said that this molecule is present in several grapes, such as Gewurztraminer, Riesling, Muscat, Petit Manseng, Colombard and certain red varieties, and, should one inject gas into the wine, 3-Mercaptohexanol combines with the oxygen to produce a molecule with no aroma.

Similarly, adding copper sulphate to wine to remove negative reductive aromas will also strip the wine of the beneficial aromas from this mercaptan.

As Chatonnet said, “If you add copper sulphate to the wine [referring to the Sauvignon Blanc sample with excessive VSCs] it will eliminate the negative reductive aromas but also the aromas of Sauvignon Blanc – so you have found out how to eliminate reduction, but also produce a basic Chardonnay from a Sauvignon.”

Hence, to attempt to control the production of excessive concentrations of VSCs in wine, Chatonnet noted the importance of vineyard practices, as, among other sources, pesticides are “promoters of VSCs”.

“The VSCs come from the vineyard, and depending on the grape composition, you have more or less potential of sulphur compounds,” he said.

Sulphur containing compounds in the vineyard become sulphides in the grape, which then can become Hydrogen Sulfide during winemaking, a compound responsible for a rotten egg or sewage-like smell.

Chatonnet also considered the role of ageing processes, picking out the need to balance the reductive influence of ageing white wines on their lees with battonage (lees stirring), which introduces oxygen to the wine.

An adequate nitrogen supply in the must for yeasts is also important to avoid the excessive production of smelly sulphur compounds.

Furthermore, Chatonnet stressed the necessity for a closure with consistency but also oxygen permeability between the very low levels of a saratin lined screwcap to the high rate from a plastic stopper.

However, he said, “What is the perfect closure? I don’t know and I’m not sure it exists.”

Nevertheless he schooled winemakers to carefully consider the closure choice according to the nature of the wine.

“If a wine is sensitive to ageing quickly then use a closure with a low OTR [Oxygen Transmission Rate] but if it is rich in antioxidants (such as tannins) then using a closure with a low OTR will mean the ageing is delayed and there is a risk of a reductive, rather than a positive evolution.”

Implying that cork may be the best solution, particularly high quality natural cork or agglomerated corks – to avoid the problem of pore size variation in lesser quality natural corks – he expressed some drawbacks of screwcaps.

Speaking of the challenges of producing and applying this type of closure he said, “It’s not just the choice of liner and the process of making the cap,” he began, “but also the quality of the seal.”

Concluding on this topic he said, “And that is not easy because you are combining glass, metal and plastic – three materials – and you can find variability from one cap to another.”

Finally, summarising the session, he said, “Be careful of your solution [to reduction], because you don’t want a wine with no identity.”