Since wineries began to focus on the losses that they incur from bad corks, probably between five and ten percent spoilage, the race has been on for good natural cork substitutes. Normally, a bad cork refers to those tainted with TCA, which initially makes wines taste flat and in more advanced stages moldy. To mitigate the problem, certain wineries have tried to promote metal screw caps, the esthetic equivalent of Cary Grant in a baseball cap, in other words, not good. More commonly, wineries use plastic corks although almost all of them are annoying. Plastic corks can be difficult to extract, nearly impossible to reinsert, and esthetically unappealing. While not yet widely used, the Zork seems to be the best plastic cap. It even pops like a natural cork. But, other than natural cork, I think the closure that best meets both esthetic and use tests is the glass stopper. You’ll find it on the Monticello rosé that we are sending this month. If you reuse the bottle for water, it’ll look great on your table.
In New Zealand and Australia, more than 30 percent of modestly priced wines, not meant for aging, are sealed with aluminum screw caps. But Plump-Jack Winery in Napa was the first in the U.S. to promote screw caps for its ultra premium Reserve Cabernet Sauvignon. On the website, the winery prices the 2005 at $400 for a two pack and has posted a “sold out” sign for the wine. With all due respect to the winery owners, the philanthropic Getty family and San Francisco Mayor Gavin Newsome, much beloved by his constituency, I wouldn’t want to see a screw cap on my $200 Cabernet, if I owned one. At their new CADE winery, only the whites have screw caps. Despite the winery’s enthusiasm for the metal cap, it hedges its bets and lets the consumer know whether the bottle is topped with a natural cork or an aluminum cap so that buyers can take their pick. Mine would be easy.
Although the French already did it, PlumpJack is conducting an interesting experiment. In 2000 when it released its first Reserve Cabernet with metal caps, it also closed a third of the 300-case production with natural cork. In 2010, the winery will open the bottles that it has set aside to see which have aged the best after ten years in bottle. The test is interesting because the extremely slight air penetration that corks permit over time contributes to proper aging, and a metal cap creates an air tight seal. Some experts have warned that wine making techniques have evolved to include the slight oxidation that corks provide and that an air tight seal would require different techniques, for example much less of the preservative sulfur dioxide. While corks allow a slow oxidation, an opposite process called reduction can take place within an air-tight bottle, especially in some wines like Sauvignon Blanc. Reduction can make a wine taste dirty and smell of sulfur compounds. The French experiment found that the wine was fine for ten years, but after that it began to develop a metallic taste. Ultimately, the best use of aluminum screw tops might be for inexpensive, fresh wines, instead of those meant for aging.
Like the screw cap, the plastic Zork does not require a cork screw, but it has the dignity to pop upon extraction and can be easily removed and reinserted, even reinserted into most any 750 ml bottle. Invented in Australia, the cap has three parts, an inner plunger that creates the “pop” after extraction, an outer cap that hugs the outside of the bottle, and a strip that extends from the bottom of the cap and winds further down around the neck of the bottle. In a word, it works and works well. But along with any other kind of plastic stopper, it does not quickly bio-degrade as natural cork does. If you’d like to check out the Zork, you’ll find one on the Don Sebastiani & Sons, Dry Creek Valley, Plungerhead, a big, raisiny Zinfandel at about $15.
On an esthetic level, the glass stopper is my clear favorite, pun intended, and I’m also happier that my wine is coming in contact with glass, not aluminum or plastic, from which it might leach out compounds that I’d rather not consume. Various types of glass stoppers exist with different shapes. The simple one that you’ll see on the Monticello rosé has a flat top and inert, acrylic, o-rings around the stopper that provide an air-tight, flexible, seal. Other designs are more elaborate and similar to decanter stoppers, and most are covered with foil like a natural cork stopper would be. The Monticello rosé is a crisp, fresh wine, meant to be consumed immediately, so it runs no risk of reduction problems, which could occur with wines that spend years in the bottle. And last, the stopper is not only reusable but recyclable. Wine may be a temporary pleasure but the bottle with a glass stopper will continue to be useful or at least can be entirely recycled.
What a Closure Is Actually Asked to Do
Every closure is judged on a handful of jobs, and the arguments about materials usually come down to which job is being prioritized. It must hold liquid in, keep spoilage organisms out, and admit oxygen at a rate suited to the wine’s intended life. That last requirement is the one that divides the field, because the ideal rate for a wine meant to be enjoyed young is not the ideal rate for one meant to age.
Closure makers describe this as oxygen transmission rate, and it varies not only between materials but between individual units of the same material. Natural cork is a biological product with natural variation, which is both its advantage and the source of its inconsistency.
The Options Compared
| Closure | Oxygen behavior | Practical notes |
|---|---|---|
| Natural cork | Slow ingress, varies piece to piece | Renewable and biodegradable; carries taint risk |
| Technical or agglomerated cork | More consistent than whole cork | Built from cork granules and binder |
| Synthetic stopper | Generally higher ingress over time | No taint risk; often hard to reinsert |
| Screw cap with liner | Very low, set by the liner chosen | Reseals easily; reduction risk in some wines |
| Glass stopper with seal ring | Very low | Reusable and recyclable; higher unit cost |
Cork itself is bark, harvested from a living tree on a cycle of several years and regrown without felling, which is why it holds a strong environmental position among the options. The alternatives are judged differently, on whether the material re-enters a recycling stream, and the Environmental Protection Agency’s guidance on recycling sets out how glass, aluminum, and mixed plastics are handled once a bottle is emptied.
FAQ: Corks and Caps
Does a screw cap signal an inexpensive wine?
It did once, and the association lingers, but the technical case has always been separate from the price case. Liner choice lets a producer target a specific oxygen rate, which is a level of control whole cork cannot offer.
What is reduction, and why does it come up here?
Reduction is the opposite problem from oxidation: with too little oxygen, certain sulfur compounds persist and give a struck or eggy character. Very low-transmission closures make it more likely, so winemakers adjust their handling accordingly.
Should bottles with alternative closures still rest on their side?
Only cork-sealed bottles need it, since the purpose is keeping the cork moist. Screw caps and glass stoppers can stand upright indefinitely, a detail covered further in our wine skills and knowledge collection and worth knowing before setting bottles aside for one of the occasions in the wine for celebrations guide.