Closures are an essential consideration for any wine. A bottle’s closure contributes to how the wine tastes and how it ages. When choosing a closure, winemakers must evaluate how the closure might affect oxygen ingress, flavor, and bottle consistency. For Champagne, however, there are unique factors that impact producers’ decisions around closures. This article will explore the history of Champagne closures, the most-used types of cork, and the development of cork technology in Champagne.
For any type of wine, a cork must be compressed from all sides to fit inside the neck of a bottle, and it is the natural elasticity of the cork trying to resume its normal size that forces it to press evenly against the inner surface of the neck, creating its hermetic seal. In a still wine, this pressure against the inside of the neck will be sufficient to keep the wine from being exposed to the air outside. For Champagne, however, there is also pressure from underneath the cork, trying to push it out. At six atmospheres, the equivalent force of a double-decker bus tire, this is a significant amount of pressure, which is why one-third of the cork is left exposed, outside the bottle. When the metal capsule, called a plaque, is pushed onto the top of the protruding portion, it molds the cork into its iconic mushroom shape. The plaque is necessary to protect the cork from the wire cage (muselet or, in full, muselet de fil de fer) that is used to secure the cork against the internal pressure of the bottle.
What is perceived today as the classic Champagne cork consists of an agglomerate body faced with one, two, or (occasionally) three pure cork disks (also known as mirrors or rondelles).
Champagne corks used to be solid cork, but as the general demand for cork rose during the 19th century, the interval between cork harvests dropped to increase supply, which led to a reduction in the thickness and quality of the cork bark stripped. Since the bark required for a Champagne cork (31 millimeters, or 1.2 inches) must be 30% thicker than that used for a still-wine cork (24 millimeters, or 0.95 inches), the thicker, higher-quality cork bark required by Champagne became too rare and costly to sustain the entire industry.
The initial alternative was made by gluing halves together vertically, then cutting the corks shorter and adding disks made from remnants that would not normally be of any use. It was at this time that such disks first appeared, on the end of solid corks, not agglomerates. Interestingly, and rarely discussed, this was the only time when the disks were cut correctly (parallel to the bark, not at a right angle). Eventually, thick cork bark became so scarce that the main body was replaced by agglomerate cork, a product that had been available for other uses since the end of the 19th century.
For economy, the disks were stamped perpendicular to the bark—rendering this closure fatally flawed. The shaft of a regular 24-millimeter cork is cut parallel to the bark to ensure the lenticels (raised pores in woody plants that allow for gas exchange) are kept horizontal to the width of the cork, not its length. The purpose of lenticular channels is to permit both an ingress of O2 through the cork bark to the tree’s interior and an egress of CO2.
Even the highest-quality corks are peppered with lenticels. With lenticels horizontal to the width, a regular still-wine cork is almost completely impermeable. Yet in sparkling wine disks, the lenticular airways form a network running from one end of a cork to the other, almost guaranteeing its permeability.
The more obvious and prolific the lenticels in a Champagne cork disk, the shorter its life. The wetter and more pinched the disk, or disks, of a classic agglomerate cork becomes, the less effective the cork is as a closure. Over time, the wine may come into direct contact with the agglomerate, and the danger of loss in pressure, oxygen ingress, and TCA increases.
Strictly speaking, any cork that is not a single piece of natural cork might be classified as a technical cork. But since oxygen ingress for an agglomerate cork can vary by as much as 300-fold (3,000%), the agglomerate cork’s efficiency does not differ significantly from that of the cork closures used by the ancient Romans and Egyptians. A more appropriate definition of a technical cork is that, at the very minimum, it ensures a consistent and effective level of oxygen ingress.
Among the dozens of technical corks, Mytik Diam is the gold standard in Champagne. It is made from microagglomerated cork that has been subjected to a supercritical CO2 extraction process to guarantee that it is TCA free, with uniform CO2 retention and oxygen ingress. This patented Diam process is called Diamant. In this process, as for all microagglomerate closures, the cork is crushed to reveal and remove the woody parts, lenticels, and other organic materials. The remaining cork is then ground into tiny particles so that all traces of TCA, its precursors, and other volatile compounds can be removed. The particles are sifted to remove cork dust, leaving so-called cork flour. This looks nothing like flour but is more granular than cork dust. The cork flour is placed in an autoclave, which is injected with compressed supercritical CO2. As matter moves between phases, it passes through an intermolecular space between states; supercritical CO2 occupies the space between gas and liquid. Uniquely, it possesses both the penetrative behavior of gas and the extractive property of liquid.
The supercritical CO2 in the autoclave, now contaminated with TCA and other volatile compounds, is piped through a cooling system, in which CO2 reverts to gas form. The rapid change in temperature causes TCA and all other volatile contaminants to fall out of solution. The CO2 is then filtered back to a pure state to be reused. Meanwhile, the odorless cork granules are mixed with a food-grade binding agent and food-grade polymer microspheres, molded, and baked. In their dry state, before mixing, the microspheres have the consistency of talcum powder, but when baked they swell to fill the gaps between the cork granules.
There are two basic, commercial Mytik Diam formats, each visibly recognizable by its size of granule: Mytik Diam 5 (formerly Classic), which has the tiniest granules and offers the lowest oxygen ingress; and Mytik Diam 3 (formerly Access), with slightly coarser granules (although much finer than those found in classic agglomerate closures) for a more open permeability. The numbers in the closures’ names refer to Diam’s aging guarantee. The size of the granules alone is not the determining factor for oxygen ingress, as microspheres possess great elasticity and are impervious to oxygen. The ratio of microspheres to cork affects oxygen ingress, as does the proportion of food-grade glue. By tweaking all these elements, Diam can create bespoke Mytik closures to suit the specific requirements of any producer. Note that MDC and MDA are sometimes printed on these closures to denote Mytik Diam Classic and Mytik Diam Access, respectively.
There is also a newer closure in the Mytik range, Mytik Diam 10, released in 2022. Diam claims this cork has improved oxygen management and mechanical performance, and aging is guaranteed for 10 years.
Not all Mytik is gold standard. Revtech is a technology used by Diam and many other producers of cork closures. Compared with the Diamant process, Revtech is an older, cheaper, steam-cleaning technology that removes only 75% to 85% of TCA. Any remaining TCA in a specific batch will be equally divided among all the corks before baking, creating a low-level spread of TCA. While TCA aromas might not be detected in these corks, they are capable of tainting wines. Revtech corks are rarely seen in Champagne. They are primarily used (together with Diamant Mytik closures) for the sparkling wines of the Loire Valley and Cava.
Amorim’s NDtech, a nondestructive gas chromatography detection technology, offers “a non-detectable TCA guarantee” for natural cork, with the threshold for detection defined as less than 0.5 parts per trillion. NDtech represents a giant technological leap from in-house testing performed by some Champagne producers, in which a sample number of corks from each batch received are evaluated by using them to seal bottled water and then sniffing the water after a few weeks. But gas chromatography effectively measures only surface-released volatiles and cannot penetrate inside the cork. This means that only the cork’s surface, not its core, is guaranteed to be free of TCA.
The name and manufacturer of a specific technical cork are usually impossible to ascertain, since such a closure is seldom marked. Even when Diam closures are identifiable, the distinction between the all-important Diamant and Revtech processes might not be. This is not the manufacturer’s fault; Champagne producers typically want only their own brands on the corks. Below are several other manufacturers of technical corks and the names of their most popular closures.
Ganau: The family-run, Sardinian cork manufacturer Ganau produces microagglomerate technical corks using Ganau Revolution. This is a custom-designed high-pressure autoclave, with a replenishable water- and steam-cleaning system that decreases TCA levels to less than one part per trillion. Ganau’s top sparkling wine corks include the Phénix, Primo Plus, and Uniq lines.
Germond: The French cork manufacturer Germond makes microagglomerate technical corks using steam extraction to reduce TCA content. Germond features this process in its Finecork and Tradi Plus corks.
Mundial: Mundial, a Catalan cork manufacturer, produces microagglomerate technical corks using steam extraction to reduce TCA content. Some of Mundial’s popular corks are Dynamic, Dynamic-plus, Energic, and Mixte.
Piedade: The Portuguese cork manufacturer Piedade has been part of the Oeneo Group (which owns Diam) since 2014 and produces various steam-cleaned, TCA-reduced (but not TCA-free) microagglomerate closures under the Pietec brand.
Trefinos: Trefinos is a Spanish cork manufacturer based out of Girona and owned by Amorim. Using the supercritical CO2 facility owned by AINIA, Trefinos produces guaranteed TCA-free closures. Its top models are Compac 2, Compac 3, and Compac Mixte.
This article is an expanded version of the section on cork closures found in the Champagne Viticulture and Winemaking Expert Guide, developed by Tom Stevenson.
Champagne Viticulture and Winemaking Expert Guide [Members only]Introduction to Champagne Expert Guide [Members only]History of Champagne Expert Guide [Members only]