Guide to Natural Wine Corks

There’s more than one option for sealing your premier beverage from screw caps to plastic stoppers; however, the long standing tradition of using cork to finish your wine remains one of the most popular and reliable methods for many winemakers. It can be confusing when comparing seemingly similar wine corks that have huge price and quality differences. This is particularly true when it comes to natural cork. We’ve compiled some frequently asked questions to help simplify your shopping experience with this Guide to Natural Wine Corks.

What is natural cork?

Natural cork comes from the bark of the Quercus suber, or the cork oak. While this tree can grow in many climates, a majority of the cork forest is in Western Europe and Northern Africa where it makes up the dominant ecosystem. While the cork forest is expanding due to good husbandry practices, most natural cork closures are harvested in Portugal.

Natural cork can be costly as the cork tree can be first harvested only once it reaches 25 years of maturity. The first harvest is known as the debosia, and the produced cork is known as virgin cork. The second harvest is usually extracted 9 to 12 years later. This harvest is known as the secundeira harvest. Cork produced from the first two harvests cannot be used for cork closures. This is because the resulting cork is hard and has a less regular structure than subsequent harvests. Common applications for this cork include thermal and acoustic insulation, shoe constituents, flooring, etc.

The cork tree can be subsequently harvested at nine year intervals. Calendar years are typically painted on the new bark to indicate harvest date and prevent early stripping. High quality cork, known as amadia, only comes after the third harvest when the tree is over 40 years old. These trees have an average lifespan of over 200 years. One cork tree has been known to produce over one hundred thousand cork stoppers from a single harvest.

Natural cork is harvested in a process called stripping when the tree is in a period of active growth, during the spring or summer. This process is accomplished by hand with traditional tools. There is no mechanized process that can compare to the traditional harvesting techniques of professional harvesters, called descortiçadores.

After the cork has been stripped from the cork tree, the cork must stabilize and remain outside for a minimum of six months. It is then boiled and then stabilized again before the cork planks go through a selection process to determine quality.

How are natural cork stoppers classified?

As cork comes from an organic resource, natural cork does not have the same uniform appearance as synthetic cork stoppers. It instead has pores – holes called lenticels – throughout as well as cracks, worm holes, hardwood, belly spots and greenwood. These are considered visual flaws, and corks with fewer flaws are sought after.

Cork producers created a system to classify cork stoppers so that customers could purchase corks with a higher quality visual appearance at a premium price. High grade natural cork also is classified based on density, humidity, surface treatment, extraction force, etc. Unfortunately, there is no standardized system to classify natural cork and even cork producers who use the same classifications might use different qualifications for each quality of natural cork.

Traditional categories of natural cork include: Flor or Flower grade, Extra, Super or Superior, 1st, 2nd, 3rd, 4th and 5th. The highest grade is Flor with the smallest quantity and size of pores. This traditional classification is also sometimes expressed in numbers with Flor as Grade 1, Extra as Grade 2, etc.

Another common classification divides cork stoppers into three categories: A, B and C. The United States Cork Quality Council has set parameters for this visual grading system as follows:

Flor and Extra quality natural cork typically fall into the Grade A or top grade category, where Super and 1st are often considered Grade B corks, though some 1st quality corks may be considered commercial grade or Grade C corks.

High grade corks are typically used for bottling quality wines for longer periods of cellar aging. This is because the higher density corks create better seal and offer a smooth aesthetic. However, commercial grade corks still provide a high quality performance at a cost effective price.

What are the benefits of using natural cork closures?

Natural cork has a molecular structure with a regular honeycomb arrangement: closed cells and air-filled interiors. This creates a lightweight raw material that is low density and buoyant without being very permeable to fluids. With good elasticity and the ability to deform without fracturing, cork has good durability and resistance to environmental changes.

Highly compressible, natural cork is the only material that can adapt to the internal irregularities of the neck of a wine bottle ensuring proper sealing even if the glass expands or contracts during storage or shipping.

The maturation of wine finished with a natural cork is predictable as oxygen ingress with natural corks is largely a result from diffusion. Additionally, the physical properties of natural cork interact with wine during the maturation process by developing its character and impacting the creation of the bottle bouquet – the aroma developed during aging.

While there have been rapid developments in synthetic cork materials and these improvements have addressed previous shortcomings associated with synthetic cork, natural cork is the oldest and most proven material for use as a wine sealant.

What size natural wine cork do I need?

Choosing the right size natural cork for your bottle and maturation process is essential for ensuring the ideal evolution of the bottled wine. Cork measurements are typically expressed in millimeters with the length of the cork first and the diameter second.

Diameter of a Cork

The diameter of the cork is the most important measurement for creating the proper seal. A natural cork stopper should be at least 6 mm wider than the smallest diameter of the neck of the bottle. However, if the cork is compressed by more than 33% of its total diameter when inserted into the bottle, the honeycomb-like cellular structure of the cork may become damaged and impact the quality of sealing. 24 mm is the most common natural cork diameter and it is the size most commercial wineries use. This size fits most industry-standard 750 ml wine bottles.

Natural wine corks 24 mm in diameter are sometimes referred to as #9 wine corks. Wine corks are also available in size #8 corks and size #7 corks. Size #8 wine corks are typically 22 mm in diameter while size #7 corks are 21 mm in diameter. When selecting the diameter of the cork, it is not only important to choose the best cork for your bottle, but also your bottle corking procedure.

22 mm and 24 mm corks need to be inserted with a wine bottle corker. Almost all corkers can handle a 22 mm wine cork, but not all wine bottle corkers can handle a 24 mm wine cork as it is wider. Corkers that use a funnel-design to compress the cork might fray the 24 mm cork when you try to insert the closure.

21 mm corks can be inserted by hand but are not ideal for long term storage. The resulting seal is not as secure, and it is prone to leaking. Wine bottles closed with a 21 mm cork should not be stored on their side as the wine may seep from the bottle.

Length of a Cork

Once you determine the proper cork diameter for your bottle and corker, you must choose the proper length. The second dimension, the length of the cork, also affects the quality of the seal. Typically, longer cork stoppers are associated with longer bottle maturing. The most common length for wine corks is 44 mm, and this length will fit most industry standard wine bottles.

When selecting the cork length, it is important to know the total bottle neck length and the fill point of the wine. The fill point suggested from most glass manufacturers is typically for wine at 68 degrees Fahrenheit. It is measured from the top of the bottle to the correct wine level. On most standard 750 ml bottles, the fill point will be about 64 mm from the top of the bottle at 68 degrees.

In general, there should be about 20 mm between the bottom of the cork and the wine level in the bottle to ensure the best seal. Typically, traditional 750 ml bottles can handle corks ranging from 38 mm to 54 mm in length whereas traditional 375 ml wines bottles are usually compatible with 38 mm long corks.

When choosing the proper length and diameter for your wine cork, it is essential to look at the specific bottle you are using as well as your corking procedure and desired wine maturation.

Is there more than one type of natural cork stopper?

While there are many types of cork stoppers that use natural cork as an integral component of the closure, natural cork stoppers are a single piece of treated natural cork. Natural cork, because of its significant harvesting requirements and limited output, can be an expensive method of sealing a wine bottle. Cork producers have developed a range of alternative cork stoppers using natural cork that can offer excellent mechanical properties and sealing.

What about synthetic cork – a natural cork alternative?

Synthetic cork is an alternative to natural cork designed to offer the traditional bottle opening experience without the risk of crumbling or breakage. Synthetic corks are typically made from either low density polyethylene (LDPE) plastic or plant-based plastic derived from raw materials such as sugarcane. Many of The Cary Company's synthetic corks are derived from sustainable, renewable resources.

Our synthetic wine corks are based on an advanced co-extrusion manufacturing technology which avoids the common pitfalls associated with synthetic wine corks. Controlled oxygen ingress allows for predicable results in wine maturation. Our Nomacorc Green Reserva line is capable of wine preservation up to 25 years.

Synthetic corks are also Trichloroanisole (TCA) free which is a chemical compound associated with cork taint. With their consistent bottle-to-bottle performance and soft feel, synthetic wine closures are an economic natural cork alternative for a range of quality wines.

What is cork taint?

Cork taint is a commonly used term to describe a wine fault attributed to the cork. Wines with cork taint often feature undesirable aromas and flavors described as moldy, musty or damp. Although the cork is often thought of as the cause for these faults, cork taint can be the result of a variety of factors including the storage conditions, the transport of the corks and wines, etc.

One of the leading causes of cork taint, however, is the presence of 2,4,6-trichloroanisole (TCA) . TCA is a chemical that even in trace amounts can produce overpowering smells and tastes in a wine.

While there are no health concerns associated with TCA, small amounts of TCA can leave a wine unpleasant or dull while high levels of TCA can cause the wine to be unpalatable. While the frequency of cork taint is low, around 1% to 7% depending on who you ask, there are alternatives to reduce or even eliminate the occurrence of cork taint due to TCA levels in the cork.

Synthetic corks are naturally TCA free. The Cary Company also sells TCA free agglomerated corks that have gone through a super critical CO2 extraction process to remove TCA to below detectable levels as well as other non-desirable aromatic compounds.

How is oxygen ingress affected by closure type?

Wine, just like the fruit it is derived from, is affected by oxygen. Just as an avocado or apple turns brown and changes flavor when left out on the counter, the aroma and flavor of a wine is altered by the introduction of oxygen.

Low levels of oxygen ingress are typically associated with fresh fruit or crisp flavors. Wines with less oxygen introduced typically lack toasty or honey attributes. When a wine is reduced – not exposed to enough oxygen – flavors are often described as skunky and undesirable. High levels of oxygenation will reduce the fresh fruit flavor of a wine and prevent reduction in the wine, but too much oxygen can result in oxidized characteristics which are equally unpleasant.

The type of closure used in bottling can affect the oxygen ingress during storage, therefore the oxygen transmission rate (OTR) should be considered when selecting the proper closure for your wine.

Natural cork introduces oxygen to the wine primarily through diffusion. This means that all the oxygen that enters the wine originates from within the cork and not from the outside. A typical 44 mm cork has about 3.5 ml of oxygen before being inserted. When the cork is compressed into the bottle, the internal air pressure increases. During the first 6 to 9 months of aging, oxygenation rates of wines with a natural cork stopper are higher due to the equalization of gas pressure in the cork and the headspace.

The amount of oxygen varies likely due to the variation in cellular structure of the cork. However, after the initial period of pressure equalization, oxygen ingress is no longer significant. The controlled process of oxygenation in natural cork can prevent premature oxidation without causing reduction.

Most wine closures outside of natural cork allow oxygen ingress through permeation rather than diffusion. The primary difference between diffusion and permeation is that with permeation, oxygen will continue to enter the bottle throughout the duration of wine storage. Where natural corks largely prevent oxygen ingress after the initial equalization period, many artificial closures will continue to provide oxygen into the bottle throughout bottle storage.

Many synthetic wine cork manufacturers have reduced the permeation rate and can provide data about how much will be introduced over time, allowing these closures to be highly predicable and great for wine makers who know the exact amount of oxidation a wine will need throughout its bottle storage.