Cork is the quintessential material for sealing wine bottles. Today, cases of using alternative closures such as screwcaps to seal wine bottles are increasing. Winemakers have come to freely choose from multiple options based on their own philosophy. Against this backdrop, the usage rate of screwcaps and similar closures has been growing. Nevertheless, many bottles around the world continue to be sealed with cork.
Cork has been used to seal amphorae since ancient Egyptian and Greek times. While cork has been used historically as a sealing material, its widespread adoption for wine bottle closures is a relatively recent development. According to one theory, this practice became popular in the 17th century when Dom Pierre Pérignon, a French monk and winemaker, promoted cork as the ideal material for wine bottle stoppers.
As an ideal material for bottle closure—this may indeed have been true in the 17th century. However, in modern times, alternative materials to cork have been developed, expanding the available options. Is cork still the ideal material for sealing wine bottles in the contemporary era? This article examines this question by focusing on cork's impact on wine flavor and aroma from a perspective different from conventionally discussed factors.
Characteristics of Cork
Cork is made from the bark of trees called cork oak, belonging to the beech family (Fagaceae) and oak genus (Quercus). Occasionally, texts mention mizunara (Japanese oak) bark, but while cork oak has the scientific name Quercus suber, mizunara, though belonging to the same beech family and oak genus, has the scientific name Quercus crispula. They are taxonomically different species. Incidentally, while cork oak is an evergreen tree, mizunara is deciduous, making them fundamentally different trees.
The reason cork is considered ideal as a wine bottle stopper lies in its mechanical and physical properties. Cork is lightweight as a material, has low thermal conductivity, and extremely low liquid permeability. Additionally, it possesses compressibility while maintaining excellent elastic memory, allowing it to conform well to the bottle interface when used as a stopper. "Conforming well to the bottle interface" means that the contact area between the stopper and the bottle's inner wall creates a seamless seal, making liquid leakage extremely unlikely.
While the cork oak's native habitat is the Mediterranean coastal region, Portugal and Spain have become the centers of the cork industry. Cork applications are diverse, with demand for wine stoppers representing less than 20% of total production volume. However, in terms of sales revenue, this application accounts for nearly 70% of the total, demonstrating that the cork industry operates as an extremely high-value business.
Incidentally, in wine applications, prices vary significantly depending on product grade, length, and the presence or method of treatment. High-end corks can sell for over one euro per piece.
Relationship with Wine Aging and Off-Flavors
A well-known significance of using cork stoppers is the oxygen transmission rate (OTR).
Screwcaps and glass stoppers like Vino-Lok have high material density and do not allow oxygen passage through the material itself. In contrast, cork possesses oxygen permeability as a material property. It is said that minute amounts of oxygen are supplied into the bottle during storage. Contact with this oxygen promotes oxidation of various components in the wine, allowing the wine to age slowly. Many readers may have heard the phrase "wine breathes through cork."
Cork is also known as a potential source of cork taint, a wine defect (off-flavor) called "bouchonné." This off-flavor results from TCA (trichloroanisole: 2,4,6-Trichloroanisol), a chemical compound formed in cork through contact with chlorine, transferring to wine upon contact. Strictly speaking, cork is not the sole cause, but since most occurrences result from contact with TCA-contaminated cork, it is also called "cork odor."
The commonly known impacts of cork on wine are these two: aging through OTR and contamination by TCA. However, there is another impact that winemakers should understand when using cork as a closure: extraction from cork.
Does Cork Change Wine Flavor?
Different bottle closures change wine flavor. Upon hearing this, one might immediately think of the aging discussion mentioned earlier, but that's not the case. Cork can change wine flavor in a much more direct way.
Cork, made from natural plant material, contains various components within the material itself. Some of the components contained in cork transfer to the target substance through contact with wine and other liquids, similar to the case with TCA. This might seem like a phenomenon unique to cork, but that is not the case. In winemaking processes, such techniques called "extraction" are very common.
Cork is essentially living wood. When thinking of living wood and wine, many readers might recall oak barrels and chips. Indeed, some components that can potentially transfer from cork to wine are identical to those extracted from materials used in winemaking specifically to influence wine flavor and aroma. These components transfer to wine in the same manner. In other words, polyphenols, vanillin, and other non-volatile compounds may be supplied to wine through cork. Furthermore, wines that did not use barrels during the winemaking process might even develop barrel-like nuances through cork usage.
High-Impact and Low-Impact Corks
However, when uncorking a wine bottle, if cork is present, this doesn't mean all wines are equally affected by cork influence. Cork's impact patterns involve both certain degrees of predictability and extremely high randomness. The combination of these two factors determines the magnitude of impact on wine.
When cork is used as a bottle closure, the degree of impact on the bottle's contents is somewhat determined by the specific cork used. In other words, there are corks that tend to have greater impact on wine and those that have less influence. These can be judged based on certain conditions. This represents the predictable aspect.
Laws of Impact Based on Type and Grade
Two major factors determine the degree of impact a cork will have on wine: cork type and product grade. Surprisingly, factors such as cork origin do not significantly influence the impact.
Cork type simply refers to whether it is natural cork or agglomerated cork. While we speak of "cork" as a single entity, there are actually several varieties.
Corks Divided into Two Categories
Some products called synthetic corks are made from 100% synthetic resin without using cork oak as raw material, yet are still called corks. In contrast, when limited to products using cork oak as raw material, the product categories are broadly divided into two types: natural cork and agglomerated cork.
Natural cork refers to single-piece materials created by punching out cork oak bark. This is probably the most familiar product that first comes to mind for most people when they hear "wine cork."
In contrast, agglomerated cork is made by first crushing cork oak bark into granules of a certain size or smaller, then reforming it using food-grade binders and similar materials. In recent years, products from France's Diam Bouchage company have become famous, and many readers may recognize this product by the name DIAM.
This product is called "micro granulated cork stopper" or "microagglomerated cork stopper" in English, but in Japanese, it's referred to by various names such as compressed cork, crushed cork, or pressed cork. For convenience, this article adopts the term "agglomerated cork."
When examining the relationship between these cork type differences and their impact magnitude on wine, natural cork clearly has an overwhelmingly greater impact on wine. This means natural cork results in both more types and greater quantities of components being extracted into wine. One case study reported differences as large as 25-fold.
Such differences arise from fundamental manufacturing process differences. While one maintains its natural structure, the other is crushed and reformed, resulting in dramatically different characteristics including porosity. However, natural cork doesn't necessarily always have a major impact on wine. This relates to product grade classification through selection and quality improvement through processing.
Product Grades and Quality Enhancement Processing
Like many products, cork has established product grades. When discussing cork grades, one might think this refers to length, but that is separate.
For example, natural cork fundamentally operates on a nine-grade system, though some manufacturers use a simplified three-grade system. These grades are determined primarily based on compositional aspects centered on cork surface conditions. Simply put, products with higher surface uniformity and fewer voids receive higher grades, while more porous products within acceptable quality ranges receive lower grades.
If cork surfaces have many voids—gaps from grooves and similar imperfections—the likelihood of liquid leakage through such gaps increases accordingly. The specific surface area also increases. Furthermore, the volume of air trapped in grooves and pores also increases. Considering these factors, porosity becomes an important indicator for cork quality control.
While product grades are determined by such cork conditions, selecting cork for actual use involves additional choices including washing methods, coating, inspection methods, and the presence or methods of various treatments. For example, agglomerated cork that undergoes special washing processes during manufacturing to prevent TCA is treated as higher-grade products. The same applies to natural cork products that undergo comprehensive inspection processes.
Generally, higher-grade products are known to have smaller impacts on wine even when in contact. Furthermore, adding various treatments and inspection items enhances such safety. Cork grades and various treatments are often received as standards corresponding to TCA content levels and OTR degrees, but they serve as meaningful standards beyond these aspects as well.
Randomness and Potential for Bottle Variation
By selecting low-porosity types and grades and applying as much processing as possible, the degree of cork impact on wine can be significantly reduced. This follows a certain law: the more items added, the higher the safety; the more omitted, the lower the safety. This relationship is built on very clear cause-and-effect connections.
However, as long as cork—especially natural cork—is used, the impact cork brings cannot be completely eliminated. No matter how many preventive measures are applied to the predictable aspects, such impact will never reach zero. This occurs because the randomness inherent in cork as a natural material sometimes exceeds the realm of systematic management.
The Reality of No Quality Reproducibility
Consider vegetables on our daily dining tables. Even with the same daikon radish or spinach, no two are exactly identical in the strict sense. Taste and texture differ slightly. While cultivation management and pre-shipment inspection can achieve some uniformity, nothing becomes completely identical. This is natural for anything derived from nature. Natural materials lack strict reproducibility.
Cork is the same. Every cork oak tree differs, and bark conditions vary by position. Furthermore, even bark stripped from the same position shows different scratch and groove patterns with differences of mere centimeters or millimeters.
Cork characteristics occur at the cellular level. Considering cell size, distances measured in millimeters or centimeters represent vast spaces with more than sufficient room for generating various changes. Product grade determination for cork is established while encompassing such enormous margins of error.
Cork as an Entity Containing Much Variation
As an industrial product, cork has established product grades with quality control based on those standards. However, such control standards operate only within certain ranges. This is more accurately described as something that "cannot be done" rather than simply not being done.
Against this background, cork properties exhibit very large lot-to-lot variations. This refers not to variation between product grades, but to lot variation within the same grade. Furthermore, investigations examining the degree of variation within identical lots have reported statistically significant differences between individual products even within the same product grade and lot.
The variation referred to here concerns the types and quantities of components contained in cork—specifically, variation in the presence of components that might be extracted into wine when using that cork. Contained components are not necessarily extracted into wine, and even when extracted, quantities are not consistent. Extremely high randomness exists here. Such facts simultaneously mean that cork usage can become a source of bottle variation in wine.
While focusing on the perspective of cork extraction here, cork's randomness similarly affects OTR and TCA possibilities. This relates to aging tendencies and the presence or absence of off-flavors. In this sense, the scope of cork's influence on bottle variation can be quite extensive.
Summary | Clarifying Why Cork is Used
Generally discussed cork impacts mostly relate to OTR and TCA. Indeed, the scope of impact these bring is relatively large and cannot be ignored. However, this article has focused on cork extraction possibilities, which rarely receive attention.
Various compounds can potentially be extracted from cork into wine, with polyphenols being representative. When such components transfer into wine, not only can gustatory impacts like bitterness and astringency occur, but visual impacts such as browning and cloudiness are also suggested as possibilities.
Generally, the perception exists that natural cork is superior to agglomerated cork, with agglomerated cork being used for inexpensive wines. Indeed, focusing solely on material costs, this is accurate—even the highest quality agglomerated cork can be procured at considerably lower prices than the highest quality natural cork. However, from the perspective of quality stability in final products, this relationship reverses. Inexpensive agglomerated cork has less impact on wine, while expensive natural cork can pose risks to wine.
This relationship also appears between cork and screwcaps. Screwcaps are often less expensive than agglomerated cork, yet their impact on wine is even smaller. From the standpoint of protecting and not altering wine's original quality, less expensive materials prove superior.
In general discussions, wine bottle closures are strongly associated with impacts on wine aging. Sometimes such relational aspects carry impact that surpasses more essential considerations, which is not particularly uncommon. However, such trends have begun changing gradually in recent years.
Using cork to seal bottles is not inherently wrong. Even today, nearly 80% of consumers worldwide reportedly prefer bottles sealed with cork. Cork carries significance not only in manufacturing and quality aspects, but also in marketing and preference. Recently, perspectives discussing cork usage from SDGs standpoints have emerged.
What matters is that winemakers themselves understand what potential impacts using cork might have on their wines.
Even with such knowledge, complete control over all these factors is impossible. However, knowing allows preparation for when something occurs. At minimum, maintaining the understanding that cork is a material that can potentially impact wine flavor and appearance—even this alone will surely make a difference in wine.
[Detailed Analysis Article] How Does Cork Change Wine Flavor?
https://note.com/nagiswine/n/nc7cd6d397a59
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