Corking

The weak point in the system used to seal the bottle is the cork; a natural product which adds to the final appearance of the bottle and its efficiency.

Corking depends on:

• The fit in the bottle neck
• The corking device clamp
• The quality and dimensions of the cork

The basic factor is the quality of the cork. It is assumed that the other factors are of minimum importance due to the fact that:

Today, bottling plants pay attention to the compression of the cork so as not to leave folds on the surface as was the case with the clamps of old corking devices. The moulds used by bottle manufacturers now tend to control the diameter of the neck of the bottle where the cork is housed so that it is perfectly compressed in all bottles.It should be said that many years ago there may have been an appreciable variation in the internal diameter of the necks of bottles, causing the unsatisfactory fit of some corks.

However the most important question is the quality of the cork.

Corks

Biological aspects. Duration and evolution of corks

When we wish to extend the duration of the agreeable taste of a wine we tend to consider that the only variable factor is the wine, and that the glass and cork are inert. However we observe that glass is the only inert element; not the cork, which is subject to attack by moths and moulds. Nevertheless, from the enological point of view, we are interested in determining whether cork, apart from its use in bottling, has permanent characteristics or whether it evolves in any way from the moment it is taken from the cork oak.

"Corks come from a living element which evolves in the original tangential direction. The quality of the cork over a long period of time depends, especially in the case of the first layer of bark, on the conditions which bring forward or delay this contraction".

The dimensions of corks evolve differently with respect to wine and the atmosphere; their oscillation being greater in the annular direction of the sheet. This creates a discrepancy between cork diameters from the moment they are cut, depending on:

a. The age, from the moment when the cork is cut.

b. The dryness which has been endured.

c. The damaging effects of dampness.

For this reason, we make reference in this study to the concept of "different diameters" in corks as a difference of measurements in the direction of its particles and perpendicular.

To conclude, the dimensions of corks evolve naturally and in the form of an annular contraction of the sheet. It can be said that:

"Corks come from a living element which evolves in the original tangential direction. The quality of the cork over a long period of time depends, especially in the case of the first layer of bark, on the conditions which bring forward or delay this contraction".

Physical aspects

Corks have variable densities.

The densities recorded cover a wide range from 0.1 to 0.23.

From a practical point of view, the lowest densities usually correspond to bigger cells and these corks tend:

• To fall into the bottle when the corkscrew is introduced.
• To push out during hot spells or when bottles are filled without an air space.
• To stay fast even when the corkscrew passes right through.

The higher density of some corks is due to a more compact structure or due to a heterogeneous softer part, compensated by cracks with woody walls. The defect associated with high-density corks is, in principle, that they crack on being extracted.

The measurements we have made of elasticity demonstrate that this is the result of variations in density.

Therefore, in practice, there is a wide variety of commercially-available corks, from the density point of view, and this includes, in addition to the basic group of medium density corks, very light or very heavy corks.

It is considered that a quality methodology for corking can be established, based on this study of cork density; it consists of:

• Visual observation (of 50 corks).
• Weighing of 50 corks (those submitted to observation beforehand).

Visually, corks can be eliminated in accordance with quality requirements by observing abnormal instances of pores, cracked veins, deformations, the greenish ring of moulds or the obstruction of pores.

Based on weighing the evaluation process will afterwards narrow the range of selection for the more demanding qualities, trimming the "wings of the Gauss curve".ccbsWe recommend, for three different quality levels and for corks of 49 mm, the following characteristics for long-term conservation:

	Min. Density	Max. Density	Weight range
The most demanding	0,16	0,19	3,3-4,2
Intermediate	0,14	0,20	3,1-4,4
Low(*)	0,13	0,21	2,9-4,7

 

(*) In this case a visually rejectable cork can be admitted.

Microbian aspects

Studies we have carried out on Rioja Reserva wines with bottling periods ranging from six months to thirty-two years, have shown that:

Moulds are the natural inhabitants of corks.

During ageing in bottles placed horizontally, wine penetrates the entire length of the cork, neutralising a large part of the moulds in the interior.

Paradoxically, corks of very old wines reveal large growths of moulds but these are situated on the outside of the cork.

The dominant types of moulds in long-life corks are Aspergillus and Penicillium.

From the microbian point of view, moulds are already present in corks available for bottling.

Moulds persist and are maintained in all corks. However, the capacity of these moulds to reproduce themselves in an appropriate environment does not imply that the cork can transfer taste to the mould; this would occur in the case of exaggerated levels of dampness in corks.

The presence of yeasts and bacteria in commercial corks is not frequent, but they can be seen in the veins of low-quality corks filled with cork shavings and mastics.

From the point of view of sterile bottling, we have determined that moulds do not constitute a risk but, considering the low numbers of yeasts and the assumption that corks are important transmitters of yeasts, the sampling of sacks of corks is recommended.

In addition to these microscopic agents, corks can be attacked by moths if the bottle is not capped and if humidity is very low.

Chemical aspects. The harmful effect of wine on the cork.

Although wine is the substance to be conserved, the behaviour of the cork as a seal does not only depend on its evolution in time, in accordance with the atmosphere; but also on the characteristics of the wine in the bottle which can affect the condition of the cork during long-term ageing.

For this study we have investigated several corks of the highest quality available on the market in order to consider their behaviour in the presence of several components of wine.

Behaviour in the presence of tannin

Corks made by three manufacturers were submitted to a hydroalcohol bath at 10° at a pH of 3.2; in some cases with tannin at 1 g/l, with the following results:

Tapón	Medio	Peso escurrido	Peso en seco	Diferencia Diámetro
I	testigo	+45,0%	-5,6%	-0,15%
I	con tanino	+51,0%	-4,1%	-0,15%
II	testigo	+37,5%	-6,3%	-0,25%
II	con tanino	+42,9%	-7,45%	-0,34%
III	testigo	+29,6%	-8,4%	-0,33%
III	con tanino	+45,7%	-8,75%	-0,37%

Tannin appears to accelerate the contraction of the diameter of corks and the impregnation of dampness.

Behaviour in the presence of alcohol

Tests carried out were made with vacuum-evaporated white wine and with alcohol made up at varying degrees. The corks remained immersed in the wine for one month.

Degree of the liquid	Diameter difference	Drained weight
6°	0.06	+7.0%
12.5°	0.10	+9.8%
22°	0.14	+20.8%

It is shown that the degree of alcohol is important in the diffusion of liquid inside the cork although it does not appear to have a considerable effect on changes in diameter.

Influence of the position of bottles

We have studied a group of bottles of red wine from the same bottling operation. Some having remained in a horizontal position and others vertical, with an extremely variable atmosphere ranging from -10° C to +40° C and humidity from 30% to 98%.

The horizontal group: weight from 4.03 to 4.44 g and diameters ranging within 10 days from 20.5 mm outside to 20.2 mm on the inner surface, dampened by the wine.

The vertical group (dry): weight from 3.5 to 3.7 g, the average dimensions within 10 days being 20.8 mm on the outside and 21.3 mm inside (next to the wine).

Influence of the grape variety

We have studied the quality and evolution of the wine/cork relationship for the Cabernet and Tempranillo varieties with at least four years of storage in bottle and with four different types of cork:

Red Cabernet wine:

Type of cork	External diameter	Internal diameter
a	20.2 mm	20.5 mm
b	20.2 mm	19.8 mm
c	20.6 mm	20.6 mm
d	19.5 mm	20.2 mm

All these corks were heavily stained.

Tempranillo red:

Type of cork	External diameter	Internal diameter
a	21.8 mm	23.0 mm
b	22.2 mm	22.2 mm
c	21.2 mm	22.3 mm
d	21.2 mm	22.2 mm

Lightly stained corks.

These results are not conclusive but seem to show that wine from the different grape varieties has a widely ranging effect on the cork during long periods in bottle. In the case of some types of wine, corks contract readily inside when they are in contact with wine, as is shown by the diameter of corks extracted from bottles of old wines.

The harmful effect of corks on wine.

The taste of the cork.

Some corks give a disagreeable taste to wine. This subject is controversial and is extremely important for old wine or wine which must remain a long time in bottle.

Rigault, Issanchou, Sarris and Langlois determined that the intrinsic "cork taste" which some wines have is due to 2-4-6 trichloroanisole (2-4-6 TCA). Nevertheless, some authors such as Riboullet attribute this taste specifically to cork moulds; Wurdig to moulds in the bodega and the use of pentachlorophenol as a disinfectant for wood. Maujean, Millery and Lemarseguier also attribute this to 2-4-6 TCA and to the use of chlorated compounds as a disinfectant for corks.

We attribute certain responsibility to the wine for this strange taste. In tests using wines of different origin and the same corks, some acquire a disagreeable taste and others do not. This occurs with wines from a grape with oxidative polyphenols, such as Garnacha, which readily acquire this disagreeable taste. Therefore, although no precise evidence is available, we understand that the cork taste comes from the cork but is especially noticeable in some wines.

We have also observed that the cork taste is more noticeable in wines with a low pH.

After studying a series of corks, the taste of which could be noticed in the wine and comparing these with others which did not, we could not find any difference, either in quality or in quantity, of the population of microbes.