Joy Ting

November 1, 2023

Introduction Closure Considerations: Things to think about when choosing a closure Components of total Package Oxygen at Bottling Closure contributions to total package oxygen Effect of Closure Type on Wine Aromatics: A review Experimental Results Virtual Sensory Session: Pre-bottling decisions impacting post bottling wine quality


As harvest comes to a close and malolacic fermentations are well underway, its time to turn attention to the bottling schedule. In addition to blending and stabilization of the wine, part of the backtrack of tasks before bottling includes ordering supplies, including closures. 

The primary function of the closure is to simply seal the bottle. Cork was the primary closure for centuries, but in recent years, many more options for wine closure have become readily available for winemakers to choose from. Deciding which type of closure to use incudes considerations of cost, brand management, carbon footprint, and the target customer. But its important to keep in mind that the type of closure you choose may also have an impact on wine quality. Like many winemaking decisions, there is no single correct answer to which closure is "best". However, there are several things to keep in mind as you place your orders. These include how the wine has been made, which types of aromatics predominate, how long you will be aging the wine, what kind of bottling line you are using, and many more.

This learn module includes:

  • Things to think about when choosing a closure, including a table of pros and cons for different closure types
  • An brief explanation of total package oxygen
  • More details on how the type of closure you choose affects TPO
  • A review of practical experiments focused on how the type of closure affects wine aromatics 
  • Results from WRE experiments evaluated at a sensory session in 2023

Effect of different types of cork on chemical and sensory properties of reserve Chardonnnay (Rappahannock Cellars)

Comparison of Chardonnay and Viognier wines aged wth screwcaps of different oxygen transmission rates (Blenheim Vineyards)

Closure considerations: Things to think about when choosing a closure


Cork was one of the first living things observed under a microscope. The regulated structure of small compartments reminded Robert Hooke of the rooms, called cells, occupied by monks in the monastery. This is how cells, the smallest unit of life, got their name. 

Cork has been the predominant wine closure for hundreds of years1,2. This material has many properties that make it a great way to seal a glass wine bottle, including its ability to contract enough to be inserted into a bottle, while quickly expanding to form a seal between the cork and the glass. Unfortunately, as a natural product, it is also sometimes inconsistent, and carries the risk of various taints. Issues with manufacturing practices in the mid 20th century2 increased these risks, opening up market opportunities for alternative closures. A full discussion of the history, pros and cons of different wine bottle closures is beyond the scope of this review, but Table 1 gives a brief summary of some practical considerations when choosing which cork to use.

Table 1: Several different closure types are currently used to seal commercial wine bottles. Each has its benefits and drawbacks.

Changes in wine aromatics during aging

Each types of wine closure has the ability to seal the bottle without leaking. Beyond that, closures can have several effects on the chemical changes in wine during aging that have profound impacts on its sensory characteristics. When considering wine aging in bottle, the primary mechanisms of change are oxidation, adsorption onto the closure and desorption from the closure. 

Oxidation of wine in the bottle depends on the total package oxygen at the time of bottling, as well as the oxygen transfer rate through the closures. Oxidation reactions will occur faster with higher concentrations of oxygen, warmer storage temperatures, and if light is present3. Metal catalysts such as iron in the wine will also speed up oxidation reactions3

There are many detrimental effects of oxidation in wine. Oxidation of varietal thiols can quench the sweet citrus and boxwood notes of Sauvignon Blanc and Rosé as well as berry flavors of red varieties4. Oxidation can also alter the expression of fruity/flower esters and bring on “grilled” characteristics to the wine3,4. Some C13 norisoprenoids such as Beta damascone and alpha ionone are also decreased with oxidation4. Oxidation can lead to loss of phenolics and browning3, lower free SO2, and increased acetaldehyde (bruised apple)4

Not all oxidation of wine is bad, however. A little bit of oxygen during bottle aging can lead to reduction in H2S, methanethiol, and thioacetates4 that can form a “reductive mask” that diminishes perception of other aromatics5. Beta damascone, a fruit enhancer, is often bound by SO2 and rendered odorless. A bit of oxygen will coax the SO2 from the Beta damascone, resulting in greater expression of fruity esters. Oxygen is also needed for the formation of TDN, the compound that brings petrol to aged Riesling, from its precursors5

During bottle aging, odor active compounds can adsorb onto the closure itself. (Adsorption is the adhesion of particles onto the surface of a substance.) Synthetic closures are more prone to adsorption than natural corks, while none of these effects has been reported for screwcap closures. Though normally blamed for desorption of TCA (more on that later), both cork and synthetic closures can also adsorb TCA, taking it out of the wine. In addition, closures canadsorb volatile phenolics (4-EP, eugenol) and methoxypyrazines. Adsorption of other odor active compounds can have positive or negative effects depending on the concentration of the compounds themselves. Esters such as ethyl octanoate and ethyl decanoate can contribute positively to wine aromas, producing sweet/floral smells, but in excess, they can be perceived as soapy. Likewise, TDN contributes the desired petrol aroma of aged Riesling, but can be unpleasant at high concentrations. Each of these can be adsorbed by cork, and even more by synthetic closures. Most often, however, adsorption of fruity and floral esters, organic compounds and pigments, reduces the aromatic intensity and fruitiness of wine4

In addition to adsorption, volatile compounds from the closure can desorb into the wine. For example, terpenes like L-camphor and alpha terpineol that lend sweet, herbal, citrus, and woody aromas can move from natural cork into wine. Other desorption products have been described as sweet/matured fruit, toasted, and sweet wood4. Unfortunately, unpleasant aromas also come from cork. Oxidative degradation of fatty acids in the wax and suberin cell wells can produce unpleasant aldehydes and ketones4. Corks can also desorb TCA (moldy/musty), geosmin (earthy), 2-methylisoborneol (musty/muddy), IPMP (green bell pepper), and IBMP (vegetative/green). Synthetic corks don’t contain these substances, but can desorb monomers from poorly polymerized plastic that contribute candle, stuffy, musty, soapy, rancid, rubber, or mushroom notes as well as potentially releasing microplastics into the wine itself4.


(1) Waterhouse, A. L. A chemist explains why corks matter when storing wine. Wine Folly.

(2) Taber, G. M. To Cork or Not To Cork, 1st ed.; Scribner: New York, 2009.

(3) Díaz-Maroto, M. C.; López Viñas, M.; Marchante, L.; Alañón, M. E.; Díaz-Maroto, I. J.; Pérez-Coello, M. S. Evaluation of the Storage Conditions and Type of Cork Stopper on the Quality of Bottled White Wines. Molecules 202126 (1).

(4) Furtado, I.; Lopes, P.; Oliveira, A. S.; Amaro, F.; Bastos, M. de L.; Cabral, M.; Guedes de Pinho, P.; Pinto, J. The Impact of Different Closures on the Flavor Composition of Wines during Bottle Aging. Foods202110 (9).

(5) How to Modulate the Wine Aromatic Evolution by Closure Oxygen Ingress; InfoWine; 2021. (accessed 2021-11-02).

(6) Pickard, C. The Pros & Cons of Different Wine Closures. Wine Enthusiast 2019.

(7) Lasky, M. TCA Guaranteed Natural Cork Gains Winemakers Trust, but Screw Cap Adoption Soars as Well. Wine Business Monthly. 2021, pp 50–56.

(8) Chevalier, V.; Loisel, C. Cork and Oxygen Transfer (Diam), 2018.

(9) Goode, J. The New Zealand Screwcap Initiative. Wine Anorak.

(10) Meistermann, E.; Diéval, J. B. Impact of Closure Oxygen Permeability on the Conservation and Ageing of White Wines in Bottles. InfoWine 2022, 7.

(11) Wong, D. P. Taking Control of Total Package Oxygen. Wine Business Monthly 2020, No. March 2020.



Components of Total Package Oxygen at Bottling


OIR = Oxygen Initial Release from the closure due to compression during bottling

see Closure contributions to total package oxygen includes initial release as well as transmission over time for more details

HSO = Headspace Oxygen

A bottling audit conducted by the AWRI found 60% of TPO is from HSO1

Depends on ullage (volume of fill) and bottle shape/size

Reported HSO values ranged from 0.85 – 2.5 mg depending on the volume of the wine2

Vacuum fillers remove 60-80% of the oxygen from headspace2,3

Cylindrical closures applied without vacuum increases HSO by 1 mg/L1

Use of inert gas in all filling tanks, hoses, etc… decreases HSO

Corks take up more space that screwcaps, leaving less headspace, and therefore contribute less HSO.

DO = Dissolved Oxygen

During pre-bottling activities (cold stabilization, racking/transfer, filtration) and bottling processes themselves, oxygen becomes dissolved in the wine.3

DO is highest at the top of the tank (where the wine interfaces with air) and lowest at the bottom of the tank.3

Best practices keep DO in the tank below 1.0 mg/L for red wines and 0.5 mg/L for white wines before bottling

Sparging the wine with inert gas (nitrogen, CO2) removes dissolved oxgyen1

Within bottling run, DO in the bottle shows a U-shaped curve, with the highest DO at beginning and end and the least DO in the middle2 of the bottling run. Small batches of wine have less wine in the middle of the run, and are more likely to have high DO.

Slow bottling and interruptions during the bottle run increase DO pickup.2,4


(1) Oxygen pick-up during packaging - understanding total package oxygen. AWRI.

(2) Letaif, H. Key Points of the Bottling Process. Wine and Vines 2016, No. May 2016.

(3) Wong, D. P. Taking Control of Total Package Oxygen. Wine Business Monthly 2020, No. March 2020.

(4) Understanding Total Package Oxygen; Fact Sheet: closures and packaging; AWRI, 2019.

Closure contributions to total package oxygen include initial release as well as transmission over time

Figure 1: Introduction of oxygen to bottled wine over time based on closure type (From: Wong 20201)

Oxidation of wine during aging, and evolution of wine in the bottle, depends on the total package oxygen at the time of bottling as well as the oxygen transmission into the bottle after bottling. Different closure types have different initial oxygen release, which contributes to total package oxygen, as well as the oxygen transmission rate during aging. 

  1. Oxygen initial release (OIR) is a major contributor to total package oxygen. Cylindrical closures (natural cork, microaglomerate cork, and synthetic cork) all contain oxygen trapped in the closure material itself that is released in the first few months after bottling. Compression of the closure during insertion creates pressure inside the closure that eventually equilibrates into the headspace. The same open cellular structures that give natural cork its compressibility and elasticity also lead to higher OTR, as these empty spaces contain a lot of oxygen. Other closures that have to be compressed for insertion (microaglomerate and synthetic corks) also show some OIR while screw caps do not introduce much oxygen from this phenomenon2. However, recent bottling audits using new sensory technologies have shown a large variation in TPO introduced at bottling with screwcap closures due to oxygen trapped in the cap itself as it is placed on the bottle, which introduces 2-3 times more oxygen than cork OIR1
  2. Oxygen Transmission Rate (OTR) becomes more important after 6-12 months, when the OIR has finished equilibration2,3. Many compounds consume oxygen quickly upon its entry into the wine, so overall DO is not always altered, even when oxygen is added, but new technologies have been developed to better measure this rate2. Once OIR has been equilibrated, natural cork and microaglomerate cork have low rates of transmission that remain steady for several years. The mechanisms that govern oxygen diffusion through the structure of natural cork (leading to OTR) are not well understood2. OTR for natural corks varies based on length and grade. Longer corks have lower OTR. Lower grade corks having higher OTR due to more lenticels1. Synthetic closures have a high transmission of oxygen, which moves relatively easily through the polyethylene plastic, while most screwcaps have very little oxygen transmission, though screwcap liners are now available with higher OTR rates. 

After 25-30 years, corks will lose elasticity and begin to leak oxygen between the side of the cork and the bottle, even under good storage conditions1,3. Long term aging under other closures is not very well understood at present.


(1) Wong, D. P. Taking Control of Total Package Oxygen. Wine Business Monthly 2020, No. March 2020.

(2) Furtado, I.; Lopes, P.; Oliveira, A. S.; Amaro, F.; Bastos, M. de L.; Cabral, M.; Guedes de Pinho, P.; Pinto, J. The Impact of Different Closures on the Flavor Composition of Wines during Bottle Aging. Foods2021, 10 (9). 

(3) A cork closure at the service of your wine. Diam.

How does closure type affect wine aromatics?

When considering which closure type will best preserve the quality of your wine, keep in mind that the “complex aromatic pool is affected in complex ways”1. Wine is made up of many odor-active compounds, each of which will be impacted by oxidative or reductive conditions in different ways at different rates. Some compounds become less perceptible in their reduced form while others become less perceptible in their oxidized form. Also, some will be more affected than others. Figure 1 gives a broad generalization of the evolution of odor active compounds with the presence or absence of oxygen, but the impact on any single wine depends on the wine itself. There is no single study that will answer the question of which closure is best for YOUR wine. However, we can learn from the experience of others. Following is a short summary of published experiments testing the effects of different closures on wine sensory characteristics and wine quality over time.

Figure 1: Generalized evolution of wine in different states of oxidation.

Experimental setup: Sauvignon Blanc bottled with synthetic cork, Saran-tin screwcaps, Saran-ex screw cap, and natural cork the aged in the bottle for 24 months.2

Results: The synthetic cork had the highest OTR. Wine bottled with synthetic cork was oxidized, low in SO2, with some browning and loss in thiols, but was low in H2S. Saran-tin had the lowest OTR. Wine aged in bottles with this closure had no browning, retained SO2, and retained thiols, but was also reduced with pronounced H2S. Cork and Saran-ex showed no reduction but also no browning.

Take home/considerations: Oxygen is not the only enemy. Wine aged under very reductive conditions may show reductive flaws.

Experimental setup: A thiol-rich Bandol Mourvèdre Rosé was bottled with three types of synthetic corks with different OTR1

Results: Wine bottled with a low OTR closure had a reductive mask, muting some fruit aromas. Wine bottled with an intermediate OTR closure showed thiols (grapefruit) without reduction. Wine bottled with a higher OTR cork had “mature” fruits (pears/white fruits) but no grapefruit. 

Take home/recommendations: To preserve thiol expression and maximize fruit expression, split the bottling run into two closure types; one lot bottled with an intermediate OTR closure for early release, one set bottled with a low OTR closure designated for later release.

Experimental Setup

This study included two varieties, Riesling and Gwertztraminer, with two different winemaking approaches to SO2, “optimized” and “reduced” (Table 1), each bottled in 3 types of synthetic corks3

Results: SO2 decreased rapidly in first year. For both wines, wine made with low SOtreatments showed rapid oxidation, regardless of the OTR of the closure. For wines with “optimized” SO2 bottled with high OTR closure, quality deteriorated quickly (in the first year). For Riesling, the quality increased in low OTR closure for the first year, then remained stable for 4 years before it was considered oxidized. For Gwertztraminer, wine bottled with the intermediate permeability closure was preferred initially (for first 2 years) with wines described as fruitier, spicier, with aromatic notes. The lower OTR had a reductive mask and presented as less “open”.

Take home/considerations: Riesling is more sensitive to oxidation and can tolerate slight reduction, so it is better to use a less permeable closure. Gwertztraminer is more susceptible to a reductive mask in a low permeability closure, but tolerates oxidation better (opens up), so an intermediate OTR is better. 

Experimental setup: A barrel-fermented Hungarian Chardonnay was aged on lees for 12 months, bottled with three types of synthetic closures of varying OTR, then aged for an additional 42 months.1

Results: Wine bottled with a lower OTR closure had an initial reductive mask that eventually evolved. There was no mask with any of the closures after 18 months.

Take home/recommendations: Closure type is less important in wines that are relatively resistant to oxygen.

Experimental Setup: Pinot Noir and Chardonnay aged for 36 months in bottle with natural cork, synthetic, and 3 types of screwcaps2

Results: H2S, methanethiol, and thioacetates (reduced compounds with negative sensory impacts) all decreased with age. Wines bottled with high OTR screwcaps showed the fastest decreases, but also accumulated acetaldehyde.

Take home/considerations: Oxidation and reduction are a continuum. The best closure type depends on the type of wine and amount of aging time.


(1) How to Modulate the Wine Aromatic Evolution by Closure Oxygen Ingress; InfoWine; 2021. (accessed 2021-11-02)

(2) Furtado, I.; Lopes, P.; Oliveira, A. S.; Amaro, F.; Bastos, M. de L.; Cabral, M.; Guedes de Pinho, P.; Pinto, J. The Impact of Different Closures on the Flavor Composition of Wines during Bottle Aging. Foods2021, 10 (9)

(3) Meistermann, E.; Diéval, J. B. Impact of Closure Oxygen Permeability on the Conservation and Ageing of White Wines in Bottles. InfoWine 2022, 7

Experimental Results


Assessing chemical and sensory impacts of different types of corks during bottle aging of reserve Chardonnay (2021)

Theo Smith & Dani Romero, Rappahannock Cellars

Though natural cork still dominates the market for wine closures, several alternatives are also available to winemakers. In this study, the same wine was bottled with natural cork (Amorim medium) and three different grades of Diam microagglomerated corks (3, 5, and 10), then aged for 18 months before analysis. Wine chemistry after aging was nearly identical among the wines. Chardonnay aged with natural cork had slightly lower free and total SO2 post aging. Dissolved oxygen measures of the wine post aging were more affected by fill height than by cork type, exposing a need for diligent quality control of bottle fill when using manual fillers. Concentrations of odor active compounds were similar for wines aged with natural cork and Diam 3, and differed some for wines aged with Diam 5 and Diam 10. However, there were no significant differences in scores for Chardonnay varietal character, fruit intensity, reduction or oxidation when evaluated by winemakers in a sensory session.

Click here for the full report


How does bottling with different oxygen transmission rate screwcaps affect aging in Chardonnay and Viognier? (2015, 2022)


There are many aspects of screwcaps that make them an excellent closure for wine bottles. Early concerns about screwcaps included the idea that wine would not evolve in the bottle during aging due to reductive conditions, however in recent years screwcap manufacturers have introduced cap types with different oxygen transmission rates, allowing the winemaker to choose a cap type with appropriate aging potential for each wine. The purpose of this trial was to test the chemical and sensory impacts of screwcaps with different OTR on Chardonnay and Viognier wines after 7 years of bottle aging. Chardonnay and Viognier from the 2013 vintage were bottled in February 2014. For each variety, wine from a single tank was bottled with Stelvin screwcaps of different OTR. Wines were first evaluated in 2015, then again in 2023. For both Chardonnay and Viognier, wines bottled with screwcaps with the highest OTR (7) showed many symptoms of oxidation: low free and total SO2, high dissolved oxygen, and measurable browning. These wines also received significantly lower sensory scores for varietal character and fruit intensity. Wines aged with OTR 1 and 3 (Chardonnay) and OTR 1, 3, and 5 showed no significant differences in sensory scores, though OTR1 wines were described as reductive by some panelists. When asked to estimate the age of these wines, those bottled with lower OTR were estimated to be less than 3 years on average, while those under OTR 7 caps were estimated to be over 6 years. 

Click here for the full report




Virtual Sensory Session: Pre-bottling decisions impacting post-bottling wine quality

Lee Hartman and Kirsty Harmon

January 11, 2024

On January 11, 2024 winemakers from around the state of Virginia gathered virtually to discuss and taste the results of two experiments focused on bottle closures, sulfur dioxide, and oxygen management. We were joined by winemakers Lee Hartman (Bluestone Vineyards) and Kirsty Harmon (Blenheim Vineyards) to discuss the results of their experiments. 

Watch Video Here

For more information, check out the Closures Learn module, here


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