For a quarter of a century, the wine industry's most tedious dinner-party argument has been whether a bottle should end in a pop or a twist. The Australian Wine Research Institute has now provided what may be the definitive answer — and it is, characteristically, more nuanced than the partisans on either side would prefer.
The AWRI's closure trial programme, initiated in 1999 under the direction of group manager Peter Godden, has tracked eight major trials across five white wines, two reds, and one sparkling wine, sealed under fourteen different closure types. The results, presented at the most recent Australian Wine Industry Technical Conference, are unequivocal on one point: for consistency and long-term preservation of primary fruit character, the Stelvin screw cap with a saran-tin liner — permitting a near-negligible 0.05 mg of oxygen ingress per year — outperformed every other closure tested.
The Numbers That Matter
After ten years of storage, wines sealed under screw cap retained what Godden described as 'appealing secondary aged characters and freshness.' Most wines sealed with natural cork or synthetic closures were, in his words, 'completely undrinkable.' The fault rate told a similar story: roughly 1.5% for Stelvin closures versus 4.5% for natural cork, the latter driven primarily by TCA contamination — the musty, wet-cardboard taint that has plagued cellars for generations.
Independent data corroborates the scale of the problem. Professional tasting panels between 2020 and 2023 flagged TCA or mould contamination in 2.6% to 3.05% of cork-sealed wines. The cork industry's own estimate sits at a more charitable 1-2%, though detection thresholds vary wildly between individuals, which partly explains the gap.
Cork Fights Back — With Science
Yet declaring screw cap the outright winner ignores a critical development. France's DIAM Bouchage, a subsidiary of Oeneo, now sells over one billion micro-agglomerated cork closures annually, processed using a patented supercritical CO₂ treatment that strips releasable TCA to below 0.3 nanograms per litre — effectively undetectable. DIAM has captured 35% of the Burgundy Grand Cru market, a region where tradition is not surrendered lightly. These technical corks deliver an oxygen transfer rate of approximately 1.0 mg per year, sitting neatly between the impermeability of screw cap and the variable porosity of natural cork, which can range from 0.5 to a staggering 23 mg per year.
Meanwhile, Corticeira Amorim — the Portuguese giant producing over three billion closures annually — has invested heavily in its own TCA-mitigation technologies. The company reported consolidated sales of €234.7 million in Q1 2024, though that figure was down 9.7% year-on-year, reflecting broader market softness rather than a closure-specific retreat.
A Market Divided by Geography
The global picture remains stubbornly split. Cork commands roughly 47% of the worldwide market, screw cap 35%, with synthetics taking the balance. But regional variation is dramatic: New Zealand jumped from 1% screw cap adoption in 2001 to approximately 99% today, while Australia sits at around 80%. In France, Italy, Spain, and Portugal, cork retains near-total dominance of the premium segment — 98% of top selections from those countries in Wine Spectator's annual survey were cork-sealed.
The commercial data adds a wrinkle that should give screw-cap evangelists pause. In the United States, premium cork-finished wine case sales grew 97% between 2010 and 2020, compared with just 6% for alternative closures. Cork's share of the American premium market climbed from 47% to 67.6% in that decade. Consumer perception, it turns out, remains a formidable market force.
The Verdict
The science is settled: screw caps preserve wine more reliably and consistently than natural cork. But the market has not followed the science, and in certain segments it is moving in the opposite direction. What DIAM and Amorim have demonstrated is that technological innovation can close the quality gap while retaining the ritual of the corkscrew. The closure debate, it seems, was never really about closures at all. It was about what we believe a bottle of wine should be.
