A cork stopper is a venting closure and will allow the escape of fluid when the pressure inside the bottle is high enough to force fluid between the cork and the glass.  Not all cork sealed wine bottles leak, but if they do, the amount of leakage does not usually entail a significant commercial loss of wine. In some cases, there is only apparent leakage, i.e., a very slight lowering of the wine level or a staining of the cylindrical surface of the cork, but no escape of wine from the bottle.

There is a very real problem if the leakage is widespread and the average contents of the bottles is less than the stated contents; and there are additional problems if leakage occurs after the bottles have been packaged, because of any wine residues, staining, vinagry odours or microbial growth on the packaging materials will make the product unsaleable in that condition.

The quality of the wine is not affected by minor leakage in the years following bottling, because the sealing pressure of the cork is usually still sufficient to exclude atmospheric air, even though it has not been able to resist the pressure of the wine. When slight leakage and oxidation occur together in a recently bottled wine, it is not "cause and effect", but a result of a common cause - the presence of air.  

In the long term, i.e., years or decades, the seal pressure of a cork will decline to the point where it can no longer resist the small pressure differences.  With minor fluctuations in temperature, there will be alternately, loss of some liquid and ingress of atmospheric air.  Unless old bottles can be topped up and recorked, it is important that they be protected from changes in temperature.

The simple answer to the question, "Why do wine bottles leak?" is, "Because there is pressure inside these bottles which is greater than the sealing pressure of their closures." This answer is certainly correct, but unfortunately, it is not a very helpful answer.  Intellectually, it is equivalent to the Aristotelian notion that objects fall because there is nothing to hold them up.  Worthwhile explanations of the causes of leakage can only be obtained if the questions are directed at the factors which determine the sealing ability of the cork and at those which determine the pressure inside a sealed bottle of still wine.

Because these factors are many, varied and interelated there can be no simple answers.  Therefore, in dealing with the leakage problem, it is important to give careful consideration to the following questions and to focus attention on the relevance an plausibility of each of the possible explanations. There may be explanations other than the ones provided, and there may be other questions which need to be asked.

The bottles are stored lying down

The corks are damaged or creased

The corks are no good

The pressure inside the bottle is too high

The bottle bores are too big  

The corks are too small

Particulate matter trapped between the cork and the glass prevents the cork from making complete contact with the glass.

Gravity

Pressure inside the bottle  

Capillary action  

Compression of headspace gases when the cork is driven into the bottle

The weight of other bottles in the stack

Expansion of the wine with increases in its temperature after bottling

Air diffusing out of the cork into the bottle

The vapour pressure of gases dissolved in the wine

Hetero-fermentative microbial activity

Fluctuations in atmospheric pressure

Air picked up during bottling

Unsublimated dry ice

Not enough time to look  

Condensate on the outside of the bottles obscures any leakage

The bottles are not laid down immediately after corking  

It takes time for the bottled wine to warm up  

It takes time for the dissolved gases to exert their full vapour pressure

The rate of leaking is too slow to observe  

Me sealing pressure of the cork declines very slowly  

Intermittent elevated storage temperature

The bottles are laid on their sides after corking  

The wine was extremely cold  

There is little or no headspace

High concentration of dissolved gas(es)

The corks are no good

The corks are badly damaged or creased

The corking pressure is very high

Larger bottle bores  

Higher post-bottling storage temperatures  

Lower wine temperature at bottling  

Higher levels of carbon dioxide

Higher levels of dissolved nitrogen  

Poor maintenance and performance of corking machine  

Inferior corks  

Smaller headspace volumes  

The wine has a higher coefficient of thermal expansion

Irregular and/or insufficient vacuum or CO2 flush

Maladjustment or wear causes incomplete compression of the cork

Scraping and tearing of the cork surface by damaged or misaligned machine components  

Creasing of the cork by irregular or excessive compression

Slow insertion

Non-uniform bottles

Variations in bottle bores

Some of the corks have inferior sealing pressures  

Some of the corks have superior sealing pressure  

Variations in initial headspace volumes  

Variable performance by the corking machine  

Only some of the bottles are exposed to radiant or convective heat  

Different orientations of the corks in the compression chamber

Variable or intermittent air pick-up during filling

The bottle bore is too large

The cork is too small  

The pressure in the headspace is very high

The stained areas are the softest regions of the cork

The stained areas are adjacent to enlarged regions of the bore

The stained areas have insufficient surface coating  

Wine residues on the bore at the time of corking

That while the wine has been in contact with the cork the pressure in the bottle has (perhaps briefly) been sufficient to force the softer regions of the cork away from the glass surface

The cork has been creased or damaged by the corking machine  

The pressure in the bottle is or has been too high  

The cork is too soft  

The bottle had been overfilled  

The bottle bore is too large

The bottled wine has been stored or transported at elevated temperatures

More prolonged contact with the wine

Repeated contact with the wine as it ebbs and flows with changes in pressure

Less surface coating  

A stronger affinity for sorption of wine pigments

They are closer to the wine  

Absorption of liquid vapour by the cork  

Absorption of 1-2 ml of wine by the cork

Shrinkage of the wine with decrease in temperature after bottling

Some of the wine is trapped between the cork and the glas

The overall physical properties of the cork, e.g., its density, diameter relative to the bottle bore, flexibility, growth rings, lenticels, moisture content, resilience, stiffness, surface integrity

A localised lack of resistance to pressure, e.g., a large, soft growth ring  

The hydrophobic nature of the surface coating

Loss of stiffness of cell walls as their moisture content increases

Buckling and/or collapse of cell walls by prolonged pressure induced stress

Permeation of gas out of the cells  

Condensation of liquid vapour in the cells

The cork improves with age

The bottle is empty

After some leakage, internal pressure in the bottle falls below the sealing pressure of the cork

After some leakage, the increased headspace volume reduces the  magnitude of pressure changes caused by fluctuations in temperature

Some of the gases dissolve in the wine

Any oxygen reacts with constituents of the wine

Some of the gases diffuse into the cork

Excess pressure will be reduced to the sealing pressure of the cork if the cork vents liquid or gas

The pressure due to gravity would be 0. 2 - 0.3 kPa for a bottle stored horizontally, and about 3 kPa for a bottle stored upside down.  Cling-film stretched over the neck of the bottle will provide an adequate seal for liquids at pressures of these magnitudes.

The author has measured pressures of 0 - 500 kPa in bottles of still wine shortly after corking.  Comprehensive sampling of bottled wines in the retail market for pressure measurements is restricted by financial and metabolic limits, and the fifty or so measurements made over the last six months may not be truly representative.  The majority of bottles (which showed no signs of leakage), had headspace pressures in the range -10 to +10 kPa, and the remainder (with signs of past leakage) had headspace pressures in the range 5, to 80 kPa at ambient temperatures.  The latter pressures are lower than the normal seal pressure of cork stoppers (100 - 200+ kPa), but they would have been much higher before the. loss of fluid and the dissolution of headspace gases.

Liquid tension can exert tremendous pressures at submicroscopic dimensions, but it cannot cause flow out of the bottle.  However, when internal pressure forces the cork away from the glass, liquid penetration into the minute gap will be impeded or enhanced by capillary forces which depend on the micro-contour and the degree of wettability of the exposed surfaces.

The figure above shows the way that headspace pressure can be produced at corking; with vacuum corkers, the compression stroke begins when the vacuum line is blocked by the cork at the start of insertion.  Although the use of CO, flushing and/or vacuum corkers is designed to circumvent this problem, inadequate vacuum or CO, flush, malfunction, long compression stroke and small headspace volume will cause or contribute to excessive headspace pressure.

Increases in the temperature of the wine will increase the vapour pressure of dissolved gases and will also reduce the volume of the headspace as the wine expands.  Estimating the combined effects is difficult, and it is much easier to insert a hollow needle attached to a pressure gauge through the cork and record the pressure as the wine is warmed. A correction must be made for the volume of the hollow needle and the Bourdon tube of the gauge.

Although the partial pressures of nitrogen and CO, in the bottle may be individually less than atmospheric pressure, their combined pressure may be significantly greater.

If there is hetero-fermentative activity in the bottle, it is desirable that leakage occurs before high pressures are produced.

When a particular bottling has a higher rate of leakage than to other bottlings, it is tempting Attribute the problem to an intrinsic lack of sealing ability of the corks.  The likelihood of this being the case can be assessed by checking the bottles for residual pressure, SO2 loss, gas content, headspace volume, weight and dry weight of the corks and current seal pressure, and calculating the initial headspace pressure.

Irregular compression will not crease every cork, only those which enter the compression chamber with their growth rings at right angles to the major tangential stress, (Figure above).

Excessive pressure after corking prevents the cork from making complete contact with the neck of the bottle.  It is not true to say that the lack of contact is because the cork requires time to recover its original diameter.  The oft cited data on recovery times refer to corks which have been compressed for 24 hours. Depending on the moisture content of the cork, momentary compression to 15 - 16 mm has little or no lasting effect on its diameter, and in the absence of excessive headspace pressure, the cork makes effective contact with the glass surface, (Figure above).

Wine staining on the cylindrical surface of the cork shows the extent of wine contact at some stage with that part of the cork.  It does not necessarily mean that the bottle has leaked or that it is likely to leak.  Figure 4 shows some common staining patterns.

Corks commonly increase in weight by a gram, or more after insertion as they absorb liquid vapour.  This is enough to lower the wine level by several millimetres; in conjunction with wine staining of the cylindrical surface of the cork, it can create the impression that the bottle has leaked.

The sealing ability of many corks is only as good as their least effective region, (Figure above).

After a certain amount of leakage, the headspace volume is increased and the maximum pressure in the bottle is reduced to less than the seal pressure of the cork.  There will be no further leakage unless there are unusually high temperatures during storage or transport, or until the terminal stages of the cork's useful life

The decline in pressure produced at corking is due mainly to the dissolution of headspace gases in the wine, and the venting of gas or liquid between the cork and the glass.  Reaction of oxygen with constituents of the wine, and the permeation of CO, and liquid vapour into the cork can also contribute to the reduction in pressure, (Figure above).

Wine bottles leak when their internal pressure exceeds the sealing pressure of the closure, i.e. because of excessive internal pressure, inadequate sealing pressure or a combination of both.

Leakage of wine usually stops after a certain amount of wine has been expelled from the bottle, because of the consequent decrease in internal pressure. Although leakage may recommence if the temperature of the wine increases, the extent of any subsequent leakage is diminished by the cushioning effect of the now enlarged headspace volume.

Pressure in wine bottles can be generated by compression of the gas in the headspace when the cork is driven into the neck of the bottle, and also when the wine expands with increases in temperature.  Although excessive pressure produced at corking decreases gradually as some of the compressed gas dissolves in the wine, it has a deleterious effect on the seal pressure of the cork, particularly if the wine remains in contact with the cork during the period immediately after bottling, i.e., before the pressure has decreased.  In addition, any residual corking pressure also increases the amount of pressure generated when the temperature of the bottled wine rises during storage and transport.

Excessive pressure produced at corking is directly and indirectly a major cause of leakage, although wine bottlers will always claim that other batches of corks inserted under similar conditions have not leaked.  These claims may be genuine in some cases, but they still do not exonerate deficiencies in the corking operation.  Failure to observe best practice in the corking and storage of wine bottles increases the incidence and magnitude of any subsequent leakage.

After insertion, the sealing pressure of all corks declines over a period of time.  The sealing life of a cork will depend on the extent of its exposure to prolonged pressure and liquid, and also on its intrinsic ability to endure strain and impede the entry of liquid vapour.  Nevertheless, even when the sealing pressure of a cork is very low, for leakage to occur, the internal pressure in the bottle must be greater than atmospheric pressure.