“There is a shock factor in approaching a table in a dining room like The Modern with a bottle with a screw cap,” explains Isabel Kardon, a sommelier at the two-Michelin-starred restaurant in New York City. “I think that it often rubs guests of a certain age range the wrong way.” That’s likely due to the limited presence of screw caps in the U.S. market over the last several decades. In a 2021 survey of North American wineries, 70 percent of respondents said that cork was the preferred closure type.
But these assumptions about screw caps and wine quality are just that—assumptions. In the 66 years since Stelvin (then known as Stelcap-Vin) first introduced screw caps for wine bottles, a lot has changed, and a lot more is known about how a wine is impacted when bottled under a screw cap versus a cork—both in the short term, and now, the long term.
This greater understanding of screw caps and their impact on wine is perhaps one reason that the tide is turning for screw caps in the U.S. In the same 2021 survey, screw cap usage by wineries jumped to 52 percent of respondents, up from an average of 30 to 40 percent over the past decade.
“[When] you taste the quality of what’s in the bottle, you start to understand that screw caps aren’t an indicator of quality,” adds Kardon. Indeed, just as with corks, a screw cap says little about the quality of the wine inside the bottle. But screw caps differ from corks in important ways, which has implications for how a wine ages.
With that in mind, SevenFifty Daily spoke with producers and researchers around the world to understand the science behind aging a wine under a screw cap, and what it means for the liquid when it eventually hits your glass five, 10, or even 25 years after bottling.
Corks and the Oxygen Transmission Rate
It’s impossible to talk about screw caps without first talking about corks. Corks have been used to seal wine bottles for centuries, so our understanding of how a wine should age over a given time period is based on how it develops under cork. While some might argue that cork is not the best way to seal and age a wine, it is undeniably the standard against which we judge all other methods.
When most people judge the quality of a wine aged under something other than a cork, what they’re asking is, “does it taste like a wine that has been in a glass bottle with a cork for that period of time,” says Andrew Waterhouse, Ph.D, a professor of wine chemistry at UC Davis.
At the center of this question is a measurement known as the oxygen transmission rate (OTR), or how much oxygen passes through a material over a given period of time. It’s an important measurement because oxygen can improve or deteriorate a wine depending on the timing and amount of exposure. During aging, for example, micro-oxygenation can help stabilize color and soften tannins.
“The key performance criteria for any closure is how much oxygen goes through it,” confirms Dr. Waterhouse. “For natural cork—although natural corks are highly variable—on average about one milligram of oxygen goes through the cork [per year] when it’s new. What you see in the market is a lot of other closures trying to mimic that performance.”
That’s because many have come to view a cork’s average OTR as the ideal, or at least expected, rate for a wine to achieve gradual maturation. “You need some oxygen early on to develop color and produce what are called polymer pigments,” says Waterhouse. This process helps the wine develop a softer mouthfeel and stabilizes the color. “But then it’s got to slow down because if all that oxygen keeps coming in every year, then the wine in 10 or 20 years is going to be shot.” Thanks to the porosity of cork, it gradually becomes saturated with wine and less oxygen enters over time. These qualities have enabled the gradual aging of wine that we’ve come to expect.
But natural corks are highly variable, and therefore the OTR is not consistent from one bottle to the next, leading to high bottle variation. “For corks, the OTR rates will distribute over two orders of magnitude, so the notion of there being an average when your distribution is so wide is kind of [misleading],” says Tim Keller, the founder of the now-closed VinPerfect, which developed screw caps with specific OTRs. “The ‘average’ cork is 20 percent of all corks.”
Plus, corks have long come with another significant downside: 2,4,6-trichloroanisole (TCA) or cork taint. While more rigorous testing has lowered the occurrence of tainted corks, the previous standard—three percent of all corks on average—drove many producers to seek alternative closures, most notably, the screw cap.
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What Distinguishes a Screw Cap?
While usually positioned as a single closure type, a screw cap can take many forms. “There are several variations, largely differentiated by the liner inside the cap,” explains Dane Narbaitz, the president of Nine Hats Wines and Long Shadows in Washington State. “Some are designed for near-zero oxygen ingress, while others allow for a controlled, very small amount of oxygen exchange.”
Michael Brajkovich, MW, the third-generation winemaker for Kumeu River and an early champion of screw caps in New Zealand, breaks down screw caps further, describing two different elements, the outer shell and the liner. “[The outer part] is the aluminium shell that holds the liner and mechanically attaches it to the bottle rim,” he says. “[The liner] consists of the facing that comes in contact with the wine (and the bottle rim), and the wad, which is elastic and maintains the pressure of the facing onto the bottle rim.”
While the outer shell comes in different designs, the liner that comes in contact with the wine is arguably the most important element. Again, there are different options when it comes to the type of liner, but according to Brajkovich, the most important two are Stelvin’s Saran Tin and Saranex. Both have a layer of polyvinylidene chloride (PVDC), a thermoplastic polymer known to be a barrier against oxygen, but they different in one key way: OTR.
“[PVDC] has some oxygen-barrier properties, but not enough to prevent oxidation over the mid-to-long term,” says Brajkovich. “Saran Tin has a thin layer of tin that is fused to the back of the saran layer. This has absolute oxygen impermeability, but oxygen can still leak into the wine through the exterior edge of the PVDC (saran) film.” The Saranex liner, which does not have a layer of tin, aims to mimic the OTR of cork.
In other words, a winemaker can choose the type of liner to use in their screw caps, which gives them precise control over the amount of oxygen that enters the bottle as it ages. “The screw cap is the only closure that allows the winemaker to decide, do I want air or do I not want air?” says Blake DeBerry, the owner of Torr Na Lochs Vineyard & Winery in Texas.
The Research on Screw Caps and Wine Aging
So what does the OTR really mean when it comes to aging wine? Decades of research have been dedicated to answering that question.
In a 2009 study, researchers evaluated how different closures impacted the aromas, color, and flavor of a Sauvignon Blanc from Bordeaux over two years of aging, and meaningful differences were found between closures with different levels of oxygen ingress. The researchers found that wines sealed under a Saran Tin screw cap, which is almost airtight, had the slowest rate of browning (a proxy for oxidation), the greatest levels of antioxidants and thiols, but also high levels of hydrogen sulfide, which resulted in a reduced character in the wines. Wines sealed with both cork and Saranex screw caps showed negligible reduced or oxidized characters. Synthetic corks, which have the highest OTR, resulted in oxidized wine.
A 2021 review of several studies that investigated the impact of different closures on different wines over periods of two to four years, found that the results discussed above are fairly consistent. In general, Saran Tin liners have the lowest OTR, highest antioxidants, and a tendency towards reduction; synthetic corks have the highest OTR, lowest antioxidants, and a tendency towards oxidation; and corks and Saranex liners fall in the middle, though corks have a very high level of variability when it comes to OTR.
While these differences were repeatedly demonstrated, the perception of those differences seems to depend on the audience. In a study that looked at the effects of temperature and packaging type, including different types of closures, on the sensory properties of Cabernet Sauvignon, “a trained sensory panel was able to detect significant changes in aroma, flavor, taste, mouthfeel, and color attributes among the samples,” according to the authors. A second study on Chardonnay demonstrated a similar result.
However, another study conducted by Waterhouse and Cade Estate Winery showed that the perception of sensory differences changes with the audience. This study tested if consumers could detect the differences in sensory characteristics of a Sauvignon Blanc that was bottled with closures with different OTRs after 2.5 years of aging. “We got the brownest and the lightest bottles of each closure and had people taste them,” says Waterhouse. “Winemakers would have easily picked up [the differences], but we wanted to know about consumers. The consumers could not tell the difference, and it was surprising because just visually they could see the difference.”
Screw Caps in Practice
Beyond the scientific research, winemakers around the world have been experimenting with closures for decades and have plenty of anecdotal evidence about the impact of screw caps on wine aging—and plenty of opinions to go along with it.
While the research might suggest that the more cork-like Saranex liners would be the most popular choice of liner, that’s far from true. The Australian Wine Research Institute, which has been conducting closure trials since 1999, points out that different markets have different preferences when it comes to closures, and Brajkovich confirms this. “Just about all wineries in New Zealand and Australia that use screw caps will use Saran Tin because it offers the best seal against oxygen ingress,” he says.
David LeMire, MW, the head of sales and marketing and the joint CEO of Shaw + Smith in Australia, confirms that they prefer Saran Tin liners for their wines. “We find that wines under [Saran Tin] screw caps age very well, approximately at a similar rate or slightly slower than a high-performing cork,” says LeMire. “If there is excessive reduction that is not managed properly before bottling, screw caps can capture that problem and show it clearly, but that’s more a winemaking issue than a screw cap issue.”
Juan Pablo Solis, the winemaker at Kaiken in Argentina, further argues this point. “An excessively anaerobic environment can lead to unwanted sulfidic notes,” he says. “However, this is not a flaw of the closure itself but an advanced winemaking challenge. We actively mitigate it by controlling dissolved oxygen and SO₂ at bottling.” Therefore, Saran Tin’s tendency towards reduction is not necessarily an issue if the winemaking is adapted in a certain way.
Pointing to research carried out by the late French scientist Jean Ribéreau-Gayon, Brajkovich further explains that positive aging can occur even if a closure’s OTR is much lower than an average cork. “[Saran Tin liners] encourage the correct development of bottle bouquet through reductive chemical reactions, rather than through oxidation.”
Brajkovich has further confirmed this through comparative tastings going back 25 years. “Recently we tried Chardonnay wines from 2001 under both screw cap and cork, and the screw cap was always superior. The cork-sealed wines tend to show high levels of oxidation as opposed to the correct bottle bouquet. We also tried some 2000 vintage Merlot wines under both screw cap and cork. In this case, the screw cap was very similar to the very best cork. However, many of the cork-sealed wines were oxidised.”
In the U.S. and Europe, the preferences are much more varied, with many wineries choosing to use a mix of different liners, and plenty of cork. Narbaitz prefers Saranex liners, as does Chad Johnson, the owner of Dusted Valley in Washington State. “We are using a Saranex lined screw cap that has a very low OTR, but enough for our desired aging curve and potential,” says Johnson. In Texas, while DeBerry likes the Saran Tin liners for his whites and rosés to preserve freshness, “the reds have a Saranex liner to allow for oxygen transmission for aging,” he says.
“Corks are so unreliable that we’ve developed a ceremony around it,” says Keller, pointing to a sommelier’s job to present the cork and a taste of the wine for the customer to inspect. With screw caps, both the inconsistency and the need for ceremony are eliminated, which is part of why they are controversial. Proponents of screw caps argue that they are “an investment in consistent quality,” as Solis puts it, which for many winemakers is the obvious choice—and the science generally supports this argument. But wine is often as much about culture as it is about science. For these traditionalists—including many U.S. consumers—natural cork is still paramount, thanks to “its cultural and emotional weight as a symbol of prestige and long aging,” adds Solis.
