DIAM closure (top) compared to natural cork.

In recent years there has been a dramatic increase in the use of DIAM and related closures in the Pacific Northwest. Cork taint, meanwhile, has remained stubbornly high. Below, I look at each from my professional tastings in 2023.


I have been tracking cork tainted bottles I open professionally for review since 2015. (Read everything you ever wanted to know about cork taint.) Starting in 2016, I also started tracking closure type.

I break down closure type into natural cork, TCA-free micro-agglomerated corks, micro-agglomerated corks, screwcaps, and other (glass stoppers, synthetic corks, crown caps, etc.). You can read more about this here. I define natural cork as whole cork, twin top corks, and micro-agglomerated corks that are not certified to be trichloroanisole (TCA) free.

TCA-free micro-agglomerated corks are predominantly closures from DIAM (pronounced DEE-yam), though there are competitors. These are ground up natural cork pieces held together by a binder. (See picture at top.) They go through a process to remove contaminants such as TCA, the principle cause of cork taint.

Bottles I note to be contaminated by TCA or another moldy contaminant are identified by sensory analysis only. However, I am in all cases having a second person confirm the presence of the taint.

The data below are based on over 1,250 wines sampled.

DIAM usage has surged

In 2016, over three-quarters (77%) of the wines I sampled for review were closed by natural cork. By 2023, that number had dropped to well below half (42.8%). All of that change has been driven by the increased use of DIAM and related closures. In 2016, DIAM and related closures made up 5.7% of the wines I opened for review. In 2023, they made up nearly 40%.

This is by far the largest, fastest change I have ever seen in the wine industry.

In contrast, use of screwcaps and other closures has remained static. Screwcaps account for approximately 13% of closures in bottles I opened. Other closures made up approximately 5%.

As a result of these changes, I expect that DIAM and related technical closures will overtake natural cork closures in my tastings in 2024. Time will tell.

There is an important caveat. I consider each bottle as a single sample. I do not take case production into account. If I did, the vast majority of wines on the market from the Pacific Northwest are still closed by natural cork. That said, the increased use of DIAM and related closures is undeniable.

Cork taint remains stubbornly high, around 3%

In my 2023 tastings, 2.6% of wines closed by natural cork presented as contaminated by TCA or another moldy contaminant. This is very close to the number I saw in 2020 (2.8%) and 2021 (3.05%). (My 2022 data were lost in a hard drive crash.)

I wrote last year that cork taint has been decreasing in recent years and speculated that perhaps it was about to go down a whole lot more. Unfortunately, that was not what I saw in 2023.

In a February presentation at WineVit, the Washington Winegrowers conference, I expressed hope that the change I anticipated seeing last year might appear this year. However, based on my tastings in the first four months of 2024, I am much more sanguine about that.

How bad is 3% tainted wines? It’s terrible.

Some might think a roughly 3% contamination rate isn’t that bad. It’s actually terrible.

Depending on a winery’s production size, that leads from hundreds to thousands of unhappy customers every year. This table tells the story, assuming a 3% taint rate.

Research shows that TCA and other moldy contaminants decrease the enjoyment of wine, even at levels at which individuals do not detect the taint. People presented with TCA-contaminated wine will often not identify it as tainted. Rather, they will think the wine is just not that good.

Overall, while progress has been made, cork taint remains a stubborn issue. Cork suppliers need to do a whole lot better than they are doing right now.

The absolute number of tainted wines has gone down dramatically

In 2015 cork tainted wines made up 6.8% of all of the wines I sampled for review. In 2023, they made up only 1.1%.

This is a dramatic change. However, that decrease is largely due to a surge in the use of DIAM and related closures and the commensurate drop in the percentage of wines using natural cork closures.

Overall, cork taint has gone down significantly from eight years ago. The differences from four years ago are negligible.

Why has cork taint gone down from 8 years ago?

My belief – and it’s just that – is that much of the change in cork taint from eight years ago came from the elimination of taint in two sources. The first is in twin-top corks.

Previously, the agglomerated middle in these closures was a major source of contamination. (Note that this was despite manufacturer’s claims to the contrary.) That problem is now solved, and the issue is solely in the natural cork ends.

Similarly, micro-agglomerated corks that were not certified to be TCA free also had a very high contamination rate. (Again, despite manufacturer claims to the contrary.) This issue has also been addressed. I have not seen contamination in one of these closures since mid-2018.

Note that I currently include micro-agglomerated corks that are not certified to be TCA-free in my natural cork bucket, as historically they were a major cause of taint. This is no longer the case. If I move these closures to a different bucket, the percentage of tainted natural cork wines would go up slightly.

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