Cork? Screwtop? Synthetic Cork? Everything you needed to know about wine closures and how they impact premature oxidation and the aging of wine. - The Athan Zafirov Wine Blog
TL;DR at bottom
Last night I had a bottle of premoxed Chardonnay. How did I know it was premoxed? I had another bottle from the same producer, the same vintage, probably came from the same batch. Last night's bottle tasted oxidized, liked an oxidized apple that turned brown. Hey, wasn't pre-mox sorted out a decade ago? Why would a 2018 vintage Chardonnay be affected? Well, since I was drunk and bored, I decided to look into oxidization and closures, and what I found was absolutely fascinating.
First of all, let's start by explaining what premox is. Premox, or premature oxidation refers to a wine oxidizing inside the bottle before the bottle is opened. An issue most commonly associated with Chardonnay from Burgundy (although all sorts of white wines could be impacted, red wines less so), caused by too much air coming into contact with the wine, and thus, the contents of the bottle with pre-maturely oxidize. Wines suffering from premox tend to taste oxidized, and they are browner in color. Interestingly enough, often times different bottles from the same box would oxidize at different levels, and thus, noticing that different bottles in the same batch look and taste different is often the most common way of identifying premature oxidation.
The most common explanation seems to be that cork quality declined in the 90s. As the argument went, cork producers adopted different manufacturing techniques that although might have improved the cork in other ways, reduced the reliability of the cork in its ability to keep air out. Another explanation is that winemakers in the 90s and 2000s were reducing the amount of sulphur dioxide they were using, and thus, the wines were more likely to be oxidized even if the closure did not allow more oxygen to seep in.
Based on the frequency wine magazines and forums discuss this issue, it seems like the premature oxidation problem started becoming frequent in the 90s, and by the mid 2000s premox was terrifying white wine drinkers. Burghound published an article in 2005 bemoaning the "declining reliability of white Burgundy", and the issue was so bad, that the EU was pushed to change their regulations governing organic wine to allow the addition of sulphur dioxide in bottles labeled organic.
The premature oxidation issue seems to have disappeared sometime in the last decade or so, which makes a relatively recent vintage suffering from premox surprising. I'm not going to name the producer, since I picked up the bottles on clearance from an LCBO location that probably didn't kept the bottles on the shelf for a few years, but it did pique my interest. As the issue slowly died down, the number of complaints on the internet seem to have disappeared, which really caught my attention and led me down a long, drunk goose chase.
What caught my attention is that the bottle that tasted premoxed used a Normacorc. That led me to this article on oxidation in wine where the producers of Normacorc explained what was going on. They mentioned two key things:
- Oxidation issues are significantly more noticeable in white wines than red wines, thus explaining why Burgundian Chardonnay was most famously associated with the issue
- Oxidation issues are twofold - Wines that aren't oxidized enough tend to taste meaty and fleshy - decanting will solve the issue. Wines that are overoxidized taste oxidized and turn brown, like an oxidized apple - there isn't anything you can do to fix it.
- The ideal oxidation level for wine at time of drinking is between 6 - 8 ppm, in the bottle it should be under 6. When oxygen levels exceed 8.6 ppm, the wine will taste oxidized.
But how do the different enclosures impact oxidation? Well, before we start, I found this lovely paper explaining oxidation, most importantly, look at this graph from the paper. Let's start with looking at the OG solution, natural cork.
Natural Cork
Cork refers to the bark of the cork oak tree, which is harvested from the tree, and cut into cylinders and pushed into bottle necks to close bottles. You can watch a good video explaining how they are made here. A lot of people like to talk about how natural corks are the best for aging, as it allows some oxygen exchange. Oxygen exchange does happen with a natural cork but look at that oxygenation graph up there - The majority of oxygenation occurs in the first 6 months, afterwards, the closure is practically airtight.
Natural cork does have many downsides, including cork taint, seepage, and crumbling. But the fact that natural cork is effectively airtight after the first 6 months means that it has effectively an infinite service life. Because no more air enters the bottle, if the cork is functioning properly a wine sealed with natural cork should never oxidize.
Natural cork is graded A, B, and C, (or 1st, 2nd, and 3rd) based on a set of criteria from the cork quality council, although cork performance does vary even within the same grade. Corks are a natural product, so it is difficult to predict consistent performance.
Synthetic Cork
This category refers to all enclosures that look like cork but doesn't use the bark of the cork oak tree. There are numerous different brands competing in this segment, of which Nomacorc is the biggest one. Synthetic corks were introduced to eliminate the possibility of cork taint and cork disintegration, but they do have their own downsides such as absorbing certain volatile compounds from the wine.
Traditional synthetic corks were created by injecting plastic into a cylindrical mold to a solid core, that core is then bonded with a slightly elastic skin to create cork like closures that could be squeezed into bottle necks. Of course, the synthetic cork industry is competitive, and companies are constantly introducing new products. Nomacorc is now promoting a new product line called "Nomacorc green", a sugarcane based, recyclable synthetic cork.
In the paper, the researchers used Nomacorc Green as their synthetic cork entry, but annoyingly, they didn't mention the specific model used. If you look at the Nomacorc brochure, Nomacorc mentions that oxygen ingress after 5 years is lower with Nomacorc than with B grade natural corks, and some of their models allow even less ingress than A grade and even extra grade natural corks.
But looking at the Nomacorc brochure and the graph linked above, we can quickly see that oxygen ingress with synthetic corks is completely unlike oxygen ingress with natural corks. Whereas with natural cork, a lot of oxygen ingress occurs initially, after which there is little further ingress. On the other hand with Nomacorc, oxygen ingress is linear after the first year, from 0.6mg of O2 in the Nomacorc Riserva, to 3mg of O2 in the Nomacorc Smart Low Green.
Although the Nomacorc brochure doesn't mention an effective lifespan, I found a Nomacorc retailer that does list effective lifespans, from 3 years all the way to 25 years. After all, if oxygen ingress is constant and linear, then it does make sense that depending on the model, after a certain amount of time too much oxygen has entered the bottle. Because it is oxygen that oxidizes your wine (Nomacorc says 8.6 ppm), Nomacorc lifespans are effectively the amount of years that your wine will remain under that threshold.
Microagglomerated cork
The most famous entry in this category is probably the DIAM closure, so I'll focus on them, but there's also other popular competitors like Neutrocork.
So what is a microagglomerated cork? This refers to enclosures made from real cork bark, but instead of cutting cylinders out of the bark, microagglomerated cork manufacturers shred the cork into tiny pieces, then they reconstruct the pieces back together into a cylinder with an adhesive. Originally designed to eliminate cork taint, by shredding the bark into tiny pieces and then treating it, microagglomerated cork producers guarantee no cork taint.
When it comes to oxygen ingress, microagglomerated corks behave similarly to natural corks - high initial ingress, but after the first year, ingress is very low. DIAM guarantees less than or equal to 0.6mg/year for their "medium" permeability closures, and less than 0.3mg/year for their "very low" models. Notice that the most permeable "medium" DIAM closures still guarantee less than or equal to the Nomacorc Riserva.
Looking at DIAM's catalogue, they offer closures ranging from Diam 1 to Diam 30. But what this number refers to is their guarantee against mechanical failure. A Diam 5 is designed to not mechanically fail within 5 years, but if it doesn't mechanically fail, a wine enclosed with a Diam should still be fine to drink, since oxygen ingress is so low and similar to natural cork.
DIAM closures are very popular in Burgundy. Over 75% of white grand crus in Burgundy are sealed with a DIAM closure. Interestingly enough, William Fevre revealed that their cheaper bottles use Diam 5, while their Grand Crus use Diam 10. Does that mean that William Fevre's have a lifespan of only 5 years (10 for grand cru)? Not exactly. It simply means that the likelihood of mechanical failure goes up after 5 years, 10 for grand cru. If the closure did not mechanically fail, the amount of oxygen egress is so low that oxidation shouldn't be a worry. Now interestingly enough, as of 2017, William Fevre doesn't use Diam 30, but I guess nobody keeps Chablis for 30 years, and besides, if you do, as long as the Diam 10 didn't fail, the wine should be fine.
Screwcaps (Stelvin Closures)
I'm going to focus on the Stelvin here, since they're the original and market leader, but there are tons of imitators, most of which behave in similar ways.
There's a common belief that wines sealed under screwcaps don't age, because the seal is too tight and no oxygen gets in. That's not exactly true, for two reasons. But let's talk about the two types of Stelvin Closures first.
Stelvin produces two models of closures, the original Saran Tin, and the newer Seranx. You can see the difference between the two models on page 6 of this brochure. Seran Tin is designed to keep oxygen out, while Seranx is designed to allow oxygen ingress.
The Seran Tin behaves similarly over time to natural and microagglomerated corks - There is very little oxygen ingress over time. But unlike those two, there is no higher level of initial oxygen ingress, thus, a bottle sealed under Seran Tin typically has the least amount of oxygen in it after 5 years. As long as it doesn't mechanically fail, a Seran Tin closure will probably never let in sufficient oxygen to premox your wine.
Seranx on the other hand, performs differently. Similar to Seran Tin, there is very little initial oxygen ingress immediately after bottling, but unlike Seran Tin and similar to Nomacorc, there is constant oxygen ingress at a steady level. So unlike Seran Tin, wines sealed with Seran X have a lifespan dependent on oxygen ingress.
Conclusion and TL;DR
Now I totally understand the premature oxidation issues with my 2018 Chardonnay. If the producer intended the wine to be drunk young, and thus, used a Nomacorc 3, oxygen ingress would have continued at a higher rate. For a 2018 vintage wine bottled in early 2019, the wine would have oxidized by early 2022. So by the time I grabbed the dusty bottle off the shelf last month, it made sense that the bottle was oxidized.
Let's end with some quick generalizations about how different closures impact the ability of wine to age, and the likelihood of premature oxidation:
Natural cork, microagglomerated cork, and Seran Tin closures does not allow a large amount of oxygen ingress over time, thus their lifespans are indefinite until mechanical failure.
Seranx and synthetic corks do allow constant oxygen ingress, and thus, wines sealed with these closures do have an effective shelf life. However, what this shelf life actually is depends on a number of factors like storage conditions and bottle size.
Is there a way to tell what kind of closure your bottle used? Hmm, I don't think there's a non-destructive way to do so. Yes, it is possible to visually differentiate between natural cork, microagglomerated cork and synthetic cork, but you have to pop the cork or at least rip off the foil wrapper. Considering that most producers brand their closures, it is then impossible to tell exactly which type of microagglomerated cork or synthetic cork was used. As for Seranx versus Seran Tin, well, I don't think it is easy to tell without cutting the cap open.
Hence why I always go back to the same old lazy response of "how can I tell if this wine will last? Check with the manufacturer".
PS: Yes, the different closures do have some impact on the way a wine will taste, as various different compounds do leech off the closure into the wine. But that debate is mostly only relevant to the producer, as once the producer has chosen the closure and bottling technique, all the bottles will be impacted in more or less the same way.
For 15 years, Athan Zafirov has traveled the vineyards around the world and worked with some of the greatest chefs including Francois Duc and Alan Brown.
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