Closures deemed more important than terroir in avoiding premature oxidation in white wines

n 2008, Alexandre Pons began a long-term study to ascertain the impact of closure permeability on the quality of age-worthy white wines. Every year for ten years, he monitored batches of bottled Sauvignon in three Bordeaux wineries with different closures – natural cork, micro-agglomerated and synthetic closures – or with screwcaps. A research fellow seconded to ISVV by the Oeneo group, he concluded that closures with higher sealant capabilities delayed the onset of markers of oxidative ageing, whilst also maintaining thiols – they also got the thumbs up of wine tasters.

Researchers have been able to continue the work through the PhD programme of Emilie Suhas, funded by Diam and supervised by Alexandre Pons. They have checked whether the OTRs proclaimed by the manufacturers were correct. Until now, no method could do this on old closures – “We have adapted a method called coulometry used by the packaging industry for wine to measure the transmission of oxygen through a plastic bottle for example,” summarised Pons. Using this technique, Sohas cut off the necks of 86 bottles from the same batch of Sauvignon in 2008 and measured the OTR for the various closure types after 12 years’ storage. Her results ranged from less than 0.1 mg of oxygen a year to 90 mg of oxygen a year.

“We observed a significant range of permeability on the cork closures and noticed that with technical closures, the lower the original OTR, the more stable it is over time, particularly with micro-agglomerated corks”, says Pons. Subsequently, the researchers measured the thiols (4-methyl-4-sulfanylpentan-2-one, 3-mercapto-1-hexanol and furfurylthiol), the molecular markers for oxidative faults (methional and phenylacetaldehyde) and the free SO2 and dissolved CO2 levels in 33 of the wines. For some bottles, “the thiol content had virtually not dropped since bottling 12 years before”, comments Pons, for whom the OTR threshold for guaranteeing protection of Sauvignon aromas over time stands at 0.3 mg O2/year. “This was true of three screwcaps, three Diam closures and two natural corks”, he stresses.

Pons believes that “a fine wine cannot reveal itself during the ageing process if the closure is too permeable to oxygen. The closure is vitally important if every effort made beforehand in the vineyard and the winery is not to be wasted”, he concludes.