The closure is the least glamorous part of a bottle and one of the most consequential. Its single job is to control how much oxygen reaches the wine over the years it spends in glass, and that slow trickle of air is what decides whether a wine ages gracefully, too fast, or not at all. Two closures dominate the argument, and — fittingly for a wine site — both lead back to an oak tree.
Cork: bark from a third oak
Natural cork is the bark of the cork oak, Quercus suber — a third member of the oak genus in the wine story, alongside the Quercus petraea, robur and alba used for barrels. Its cellular structure is a natural marvel: light, compressible, and full of trapped air, it seals a bottle while letting a very small amount of oxygen migrate through over time. The tree regrows its bark, which is stripped roughly once a decade, making cork renewable.
Cork has two well-documented problems. The first is cork taint: the compound 2,4,6-trichloroanisole (TCA), formed when moulds in the cork meet chlorine-based sterilants, produces a musty, wet-cardboard, wet-dog smell that strips a wine of its fruit. Historically it spoiled a few per cent of all bottles — a significant tax on any cellar. The second, subtler problem is variability: no two natural corks admit oxygen at exactly the same rate, so a case of the same wine can age at a dozen slightly different speeds.
Screwcap: oxygen by design
The screwcap solves both problems. Taint from TCA is largely designed out, and — crucially — the small liner inside the cap can be engineered to a specific oxygen transmission rate (OTR), consistently, bottle after bottle. That turns the closure from a variable into a control the winemaker sets deliberately.
A very low OTR means the wine ages slowly and along a reductive path — preserving primary fruit and freshness — which is exactly what an aromatic, acid-driven white wants. The one risk is going too far: starved of oxygen, some wines throw reductive, struck-match or rotten-egg sulfur notes. Modern OTR-tuned liners are formulated to give just enough air to avoid that while keeping the wine fresh.
Why Australia led the switch
Australia became the country most associated with the screwcap for age-worthy whites, and the reason is chemistry meeting style. Its two great cellaring whites — Riesling and Hunter Semillon — are prized for exactly the slow, pure, reductive development a low-OTR seal protects, and cork’s taint and variability were the single biggest threat to a twenty-year-old bottle showing what the maker intended.
In 2000 the Clare Valley’s Riesling producers, led by Grosset, switched over en masse — a coordinated, image-risking move that worked, and spread until the screwcap became the norm for serious Australian and New Zealand whites. The lesson is not that one closure is better than the other, but that the closure’s oxygen behaviour should be matched to the wine’s style — and for Australia’s benchmark whites, a tunable, taint-free, low-oxygen seal is the right tool for the job.