Introduction

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 years^1,2. 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 20^th 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 wine^3,4. 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.

References

(1) Waterhouse, A. L. A chemist explains why corks matter when storing wine. Wine Folly. https://winefolly.com/deep-dive/chemist-explains-corks-matter-storing-wine/.

(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 2021, 26 (1). https://doi.org/10.3390/molecules26010232.

(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. Foods2021, 10 (9). https://doi.org/10.3390/foods10092070.

(5) How to Modulate the Wine Aromatic Evolution by Closure Oxygen Ingress; InfoWine; 2021. infowine.com (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. https://www.wineanorak.com/new_zealand_screwcap_initiative.htm.

(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.

TPO = OIR + HSO + DO

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 headspace^2,3

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

References

(1) Oxygen pick-up during packaging - understanding total package oxygen. AWRI. https://www.awri.com.au/industry_support/winemaking_resources/storage-and-packaging/packaging-operations/oxygen-pick-up-during-packaging-understanding-total-package-oxygen/.

(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. https://www.awri.com.au/wp-content/uploads/tpo_fact_sheet.pdf.

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 equilibration^2,3. 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 conditions^1,3. Long term aging under other closures is not very well understood at present.

References

(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. https://www.diam-closures.com.

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 SO₂ treatments 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.

References

(1) How to Modulate the Wine Aromatic Evolution by Closure Oxygen Ingress; InfoWine; 2021. infowine.com (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

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.