Good performance in Cava corks, both on the bottling line and during uncorking, is closely linked to a series of physical parameters inherent in the cork, the rigor of the manufacturing process, the final treatment and its hygiene conditions.

These parameters must respond to the demands which, prior to being established, must be set forth and evaluated in accordance with the final destination of the product.

This study compares the physical performance of the corks made by the 14 most representative manufacturers on the market, amongst which we find corks manufactured both by the individual mould process and by the continuous extrusion process. It aims at contributing to improved, optimised cork performance from the time it enters the corking machine until the moment uncorking takes place.

This research work was been made possible thanks to the assistance and collaboration of numerous Spanish and French wine cellars.

Our intention is neither to establish nor to set a qualitative order amongst the different manufacturers but rather to present previously unknown parameters and thus determine the place our product occupies amongst the corks of the most important manufacturers.

MATERIAL AND METHOD

The corks were selected from different sources, classified in lots of 20 units each, with similar characteristics and conditioned to an identical degree of moisture. These lots were then enumerated, at random and anonymously, between 1 and 14.

Testing was carried out under identical laboratory conditions for all corks.

The following equipment was used:

A single-headed bottling machine with 4 sets of clamps to compress the cork into a cone shape, reducing it to a diameter of 15.5 mm on the bottom and 17.9 on the top, and adapting it by means of a series of electronic devices. This procedure enables us to obtain the values of two such important parameters as compressive stress and penetration stress for each cork. For each parameter, we obtain curves upon which more than 30 points are recorded. Thus, for example, Graph 1 shows us both readings taken for each cork. Curve A shows us the compressive stress (DcN) from its start to its end diameter. Curve B shows us the penetration stress (DcN) for a corking of 23.0 mm deep.
A special torque wrench to measure the effort involved in extracting champagne corks.
A test bench for creating an inner pressure in the bottles which enables us to discover the pressure withstood by the corks and the evaluation of their tightness.
A precision balance of 0.01 g.
A moisture potentiometer.

The methods employed are described in the following pages.

Graph 1.
Curve A: Compressive stress. Curve B: Penetration stress.

1. COMPRESSIVE STRESS

This parameter has to do with the source and quality of the raw material and also with monitoring the processes for obtaining the agglomerate.

The exertion of the clamps in compressing the cork is an important parameter, one that in theory should be checked but in practice generally is not. An overly hard cork may make it impossible to close the clamps completely. This means that the cork is then left with a greater diameter than that of the inner neck of the bottle, the result being that it touches the border of the neck, producing folds in the last rings and with insufficient, off-centre penetration. The consequences that may derive from this problem are loss of inner pressure, loss of liquid and problems when uncorking.

This compression value marks the exertion of the clamps of the bottling machine and shows us the degree of difficulty involved in this operation, thus reflecting the wear and tear to which the machine is subjected.

For each cork, we obtain a value in DcN (*) for this exertion or stress, by recording the integral obtained from the moment compression begins to its final diameter at the phase previous to penetrating the bottle. As for this integral, more than 30 points are recorded during the interval of time needed for compression. See Curve A of Graph 1.

Graph 2, Compressive stress (DcN)

The values shown in this graph are the averages of each of the series being monitored. All elements are for corks measuring 48.0 X 31.0 mm on identical bases weighing 9 g and with 5.0% moisture in the agglomerate.

Bac corks by E. Trachsler S.A. require the least compressive stress.

Compared with the ìmodus operandiî of a bottling chain (industrial process), we have slowed down the process of cork compression, thus obtaining wider, more detailed curves. Although not appreciable in an industrial dynamic owing to the very quick pace of the machines, significant differences are seen.

2. PENETRATION STRESS

This factor is linked in part to the quality of the agglomerate and, more importantly, to the surface treatment the corks receive. From the values obtained, we can deduce the quality of a treatment insofar as it concerns the regularity of its distribution. Moreover, comparing different treatments used in the aspects of even distribution and gliding power will help us to reach genuine conclusions.

This parameter is of enormous importance for achieving correct, uniform corking depth, which will later ensure controlled, reliable uncorking.

Differences in corking depth cause problems such as uncontrolled uncorking, loss of pressure and even cases of insufficient depth of the liquid. It can also bring about significant stress in uncorking with excessive depths, an aspect which, in extreme cases, may lead to cork breakage.

The graph showing the exertion involved in uncorking each bottle is the result of more than 30 readings throughout the process.

Graph 3, Penetration stress (DcN)

Bac corks by E. Trachsler S.A. show a significant difference from the rest, due to the lower amount of exertion required.

3. PRESSURE WITHSTOOD

On a test bench, corked bottles without agrafe are subjected to a gradual increase in inner pressure in order to provide us with the values needed for expelling the cork. The margin of 6.5-8.5 bars is the one that we consider best for attaining an optimum "stick-slip" factor. Lower values signify uncontrolled cork release and higher values mean excessive exertion required for uncorking. The results are shown in graph 4, where it should be pointed out that 80% of the monitored series is within a correct margin and the remainder is below the lower limit.

Here we see the "stick-slip" factor, which is to say the halfway point between the power to glide and the power to be retained, thus ensuring that the cork remains in the bottle when the end consumer removes the agrafe. Normal uncorking requires only a _ turn.

On another test bench and using bottles where the agrafe remains fastened and which are immersed in water, a test for tightness is carried out. These are subjected to a gradual increase in inner pressure of up to 15 bars, thus exceeding the most exacting conditions that a bottle of champagne will have to withstand (graph 5).

Bac corks by E.Trachsler S.A. have once more obtained the finest overall results in these two tests (pressure withstood and loss of tightness).

4. CORK EXTRACTION STRESS

This parameter, measured with a special wrench, shows us the proper adherence of the cork to the bottleneck and the exertion required in extracting it. Values are generally considered to be acceptable if they are between 2 and 3 Nm. This value depends largely on the gliding power of the final treatment which, in the case of bac corks, can be adapted to the needs of each customer.

If the customer does not provide specific instructions, E. Trachsler sets the standard value of this parameter between 2.5 and 3.0 Nm.

85% of the cork series tested have yielded values of between 2 and 3 Nm; the rest have shown higher values.

Values of lower than 2 Nm mean that the cork runs the risk of uncontrolled release, whereas values of higher than 3 Nm show the need for excessive exertion in uncorking.

5. CORKING DEPTH

To carry out this test, we have followed the theory that, in order to attain ideal cork performance, the inner part of the bottleneck should have the same volume of agglomerated cork as it does of natural cork. Consequently, a depth of 23 mm ( 1 mm is the most recommended. The correctness and regularity of this parameter influence, as do other factors, in the performance expected from the cork during uncorking.

Insufficient or excessive depth results in a series of problems already described in Section 2 (Penetration Stress).

Graph 6, Corking depth (mm.)

Graph 7, Deviation of corking depth (mm.)

Of the 14 series tested, only the bac corks by E. Trachsler S.A. reach the theoretical corking depth of 23.0 mm with no deviation (0.0 mm) in 100% of the cases.

The corking machine has been adjusted to a corking depth of 23.0 mm.

CONCLUSIONS

The conclusion drawn from these results is that a series of key factors directly influence cork performance. These are the following:

THE QUALITY OF RAW MATERIAL (Granulated substance and discs):
Although lacking in visibility to the user, this factor influences cork performance directly in all of the parameters we have mentioned. Proof of its importance is the fact that approximately 50% of the cork mass entering the bottle is natural cork and that it is in direct contact with the wine.

The user should take this into account when choosing the most adequate cork for his needs.

Selecting the raw material from its source to its inclusion in the cork, either as granulated substance or discs, is thus absolutely essential.

Aware of the importance of all of the foregoing, E. Trachsler also subjects its discs to a complete and specific washing, with absolutely proven efficacy in reducing organoleptic alterations of the wines.

CHECKING THE AGGLOMERATION PROCESS AND THE DEGREE OF POLYMERISATION:
Our mastery of the process gives us a product that is highly homogeneous, a facet that involves reliable performance, hence eliminating the need for continuous adjustments to the corking machine. Given the complexity of this matter, a study will be carried out in the near future.

CORK WEIGHT:
This must be in accordance at all times with the product for which the cork is intended. One must always ensure that the weight used is that which is specifically required in the inside of the bottleneck for the cork mass to be introduced (*). Far from an advantage, excess weight can cause numerous problems.

CORK MOISTURE:
Experience at both the practical and laboratory levels has taught us that the moisture level needed for optimum cork performance is between 4.5 and 5.5% for the agglomerate and 6-8% for the discs. Moisture values of lower than 4% mean that the cork has lost part of its combined water, a problem that will lower its physical performance and, more especially, its power of elastic recovery.

FINAL TREATMENT:
The best finishing is a homogeneous application that perfectly combines the gliding power with correct adhesion to the bottleneck (ìstick-slip factorî). The numerous problems caused by the classic paraffin band must be avoided at all costs. These include adhesion-welding of corks to the bottleneck at low temperatures, resoftening this band at high temperatures, uncontrolled uncorking due to oxidisation, or absorption of the very band. Tests have shown that, far from bringing about a favourable effect, placing a paraffin band causes numerous uncorking problems.

Having studied all of these aspects, we can sum up by remarking that it is not the system of manufacturing the agglomerate itself ñ be this by continuous extrusion or individual mould ñ that influences the final cork performance but rather the use made of each system. It has been proven beyond a doubt that a cork whose agglomerate has been obtained by continuous extrusion can amply surpass the quality of one that is manufactured according to the individual mould technique.

Another factor that directly affects all subsequent cork response is the final treatment applied. E. Trachslerís exclusive treatment provides bac corks with features that make it stand out from the rest.

Today as yesterday, there is no better alternative to the bac cork.

(*) Specific weight inside the bottle = the specific weight of the cork mass (agglomerate + discs) to be inserted in the bottle multiplied by a correction factor.

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