Natural Cork Quality Council presents
CTCOR Research Library
Article #2
Sensory Assessment of Cork stoppers using “Dry Heat”
SHORT SUMMARY:
Description:
This article reports a series of experiments with Champagne Corks where a sample of several thousand untreated corks were exposed to dry heat to stimulate volatile compounds. The corks immediately underwent sensory evaluation. The procedure allowed the sensory evaluation to occur without the use of any destructive practices or immersion procedures. Corks were categorized as (1) low risk, (2) medium risk or (3) high risk. Samples of each risk category were immersed in a neutral white wine for 5 to 7 days. Follow-up analysis showed a strong correlation between flaws detected using dry heat and flaws exposed after standard immersion tests.
Results:
Overall the samples exhibited cork taint in 1.6% of the immersion studies. The incidence of taint was highly skewed to those corks previously identified as “medium” or “high” risk. These corks which consisted of 4.9% of the original sample accounted for 70% of the cork taint that appeared after immersion treatment. The results by category were:
- Level 1 - low risk - 95.1% of the sample. It exhibited cork taint at a rate of 0.5%.
- Level 2 - medium risk - 4.2% of the sample. It exhibited cork taint at a rate of 14.3%.
- Level 3 - high risk - .07% of the sample. It exhibited cork taint at a rate of 71.4%
Importance:
This study describes a methodology that provides a non destructive means to evaluate cork stoppers for potential taint. A high correlation was reported between those corks identified as medium to high risk and those corks which developed taint symptoms in immersion studies. The methodology may someday provide a useful means to improve QC practices by greatly increasing potential sample sizes beyond the current levels specified by international standards.
A WHOLE TRANSLATION OF THE STUDY APPEARS BELOW:
AN OLFACTIVE TEST TO ASSESS CORK STOPPERS
Frederic Panaiotis
Isabelle Tribaut-Sohier
Michel Valade
CIVC Technical Deparment
The main problem with the use of cork stoppers in Champagne is connected with the so-called "cork taints". After an enquiry set forth among several professionals, oenologists and cup-bearers, the percentage of bottles affected by this flaw is estimated between 0.5% and 2% (see "Le Vigneron Champenois" no. 3, March 1993). Considering an average annual production of 200 million bottles, these off-flavours would then fall upon 1 to 4 million champagne bottles per year. Even nowadays, problems with taste are often seen as a major fatality, since there are few methods of preventing them. In order to meet the pressing demands as far as control procedures, cork-producer Barange, S.A., has decided to recover and improve an old traditional type of practice known as the olfactive assessment of cork stoppers. In former times, when corks were marked by fire in ancient champagne cellars, the person appointed for the job used to scent the aromas coming from the cork stoppers. Those corks showing an unusuall odour were thence eliminated. The test performed by Barange, S.A. is based on the same principle. Its methodology is actually registered in the National Institute of Industrial Property.
TEST METHODOLOGY
Despite having different sizes, the lots of cork stoppers are equally important: they may vary from thousands to dozens of thousands of units. Therefore, control procedures should fall upon a specific sample whose size is set by the AFNOR Standards (NFX 06-022, NFX 06-024). According to the type of control procedure, the sample would range from 300 to 800 cork stoppers.
The test is actually performed by slightly warming the mirror where the raw cork stoppers lay (without any marking nor surface treatment) and by immediately smelling the resulting aromas. According to the nature and intensity of these odours, cork stoppers will be classified into three categories, as follows:
- low-risk cork stopper;
- medium-risk cork stopper;
- high-risk cork stopper.
These three levels of riskiness have been settled through the experience of Barange, S.A. controllers while performing several tests in an attempt to establish a correlation between the intensity of some odours and the appearance of the so-called "cork taints".
These risks show if each cork stopper is likely to develop or not such defect.
After being smelled, each cork stopper belonging to the medium and high-risk
categories is tested on wine, in an individual container.
- Here, only the first disc (side mirror) will be put in contact with 50 ml of neutral white wine.
- The wine is then tasted after 5 to 7 days.
- The absence or presence of a special "taste" deriving from the cork stopper will be
- stressed, thus specifying the defect's type and intensity.
VALIDATION OF THE TEST'S EFFECTIVENESS
Since the CIVC aimed to check the actual effectiveness of this olfactive assessment, an experiment was prepared in order to:
- check how effective was the range of selected cork stoppers in champagne;
- characterize the several types of "bad taste" that were found in champagne and compare them to the aromas described during selection of the cork stoppers;
- assess the organoleptic changes that were due to the capsulated control bottles.
SELECTION AND DESCRIPTION OF THE CORK STOPPERS
As an introductory note to the following experiment, several thousands of corks were "smelled" so as to obtain:
- 126 medium-risk cork stoppers;
- 21 high-risk cork stoppers.
All the other cork stoppers were classified as low-risk. Among them, only 200 were chosen at random and preserved for future reference.
The low-risk cork stoppers showed a high content of "boisee" and vanilla odours as well as slight sugar nuances (candies, confectionery). Their density is generally reported as weak.
A great diversity of odours was reported among the 147 medium and high-risk cork stoppers. However, even the highest skilled panellists cannot fully agree as to the describers that are used to classify the odours. Nevertheless, most medium and high-risk ranges feature vegetable odours (green wood, hay, vegetables) or mould (mouldy wood, cellar). Some cork stoppers release an amazing set of odours, namely: hydrocarbons, plastic, coffee, eucalyptus, mint....
CHAMPAGNE CORKING
The experiment was voluntarily performed in half bottles of champagne where the content had been previously racked. This technique aimed to neutralize early organoleptic variations in the selected bottles. Setting of cork stoppers was made through dry bottling. Simultaneously, 200 half bottles were capsulated so as to form a control lot. In order to prevent contact between the cork and the wine, the capsules had incorporated plugs (withouth the cork-gasket). All the other half bottles, that is 547, were kept laid down in cellars for 3 months before tasting.
TASTING
The half bottles were tasted by a trained panel during 3 consecutive sessions, according to a CIVC-developped technique.
The bottles are uncorked and decapsulated (thereby preventing their differentiation) before being distributed randomly among the several tasting places. Each panel receives a reference bottle (the capsulated control bottle) so that each member is familiar with the type of wine.
If these quality standards are observed, the panellists will be able to find all the slightest alterations in the wine which sometimes are perfectly undetectable to the ordinary customer.
A first panel, working in binomials, evaluates and rejects the half bottles that display defects or "organoleptic deviations" according to the reference. Each binomial tastes at least fifty samples.
After being isolated, each bottle is then reassessed by three experts. If the wine is to be classified as having a "cork tainted taste" or another organoleptic deviation, it is necessary that at least two out of three tasters agree as to the specific description of the defect.
Through this technique it is possible then to control a significant amount of bottles with a restricted number of panellists. Provided each bottle is tasted by at least two experienced technicians error risk will be therefore reduced.
TASTE RESULTS
The overall results from the three sessions were as follows:
- As expected, no cork taint was detected in the capsulated control half bottles. However, two of them exhibited a slight variation in taste. It is amazing to detect such alterations in racked wines that would normally exhibit identical aromatic profiles.
- As to the 200 half bottles that had been corked with low risk-cork stoppers, only one tainted taste was detected (that is 0.5%). 9 organoleptic deviations (that is 4.4%) were registered. Six of them derived from chemical defects (hydrocarbons or pharmaceuticals odours).The other three bottles displayed a slight vegetable or"boisee" character which isn't at all unpleasant.
- These are the results from the 126 medium-risk half bottles:
- cork taints, that is 14%, half of which was very strong;
- organoleptic deviations (approximatelly 12%) with a focus on "boisee"/vegetable or chemical/hydrocarbons.
- As to the 21 high-risk half bottles, 15 cork taints were found (more than 70%). Among these 15 cork taints, 9 were reported as being intense or very intense.
Table 1. Taste results (percentage) 3 months after corking
Lots
| WINE DESCRIPTION |
Crown Cap Control |
Low-Risk Stoppers |
Medium Risk Stoppers |
High Risk Stoppers |
| Faultless (%) |
99% |
95% |
74% |
28% |
| Cork Taints (%) |
0% |
0.5% |
14% |
72% |
| Other Deviations (%) |
1% |
4.5% |
12% |
0% |
The results prove that the olfactive assessment of cork stoppers after warming permits finding those that will transmit bad taste after corking. However, this technique is not fully effective for the detection of defective cork stoppers, as confirmed by the cork taint found in a low risk-cork stopper. Similarly, the present technique should not be seen as a perfect model to suppress all kinds of bad taste from cork stoppers.
In fact, this test corresponds effectivelly to its main objective: to detect and deflect the lots which might lead to an unusually high rate of bad taste in cork stoppers.
Besides, the quality of selection depends entirely on the person in charge of the olfactive assessment of cork stoppers: someone experienced and highly-skilled in the art of identifying the resulting aromas.
Establishing connections between the cork odours and the wine odours after corking
This type of relationship is quite difficult to establish since there are many different comments on taste (as to wines and above all corks). Nevertheless, one can easily detect several trends:
- half of the 26 "mouldy" cork stoppers led to cork taints.
- more than 50% of the 25 cork stoppers exhibiting vegetable odours led to cork taints and vegetable or "boisee" organoleptic deviations.
- as to the other cork stoppers, the low number of individuals available by describer does not allow to draw accurate conclusions.
These first results should however be carefully assessed, considering the restricted amount of cork stoppers which have been tested. However, after this first assessment we managed to confirm most of the results through other experiments.
We've specifically detected transmission of vegetable taste (hay, green wood) or balsamic (eucaliptus) in champagnes, through cork stoppers that had already exhibited such odours before being corked.
POSSIBLE APPLICATIONS
The selection of risk cork stoppers through olfactive assessment after warming is a perfect technique specially designed for the organoleptic control of cork lots.
The test, as described in the first section, can start directly in the manufacturer or user as a method for quality control assessment. However, the technique should not be used in cork stoppers that hadn't yet underwent marking and treatment.
In fact, the silicons which are usually used for the surface treatment normally affect the olfactive assessment of the cork stopper. Similarly, the process of marking with fire confers an intense smoke odour to the cork.
The possibility of obtaining risk cork stoppers opens new research areas for the study of cork taints, thus allowing:
- to verify the effectiveness of some treatments seeking to suppress cork taints.
- to assess and identify the compounds responsible for the cork's bad taste.
- to answer some of the most practical questions, such as "Is it possible for a defective cork stopper to contaminate the pure cork stoppers by sheer contact?" or "At what rate does the cork taint appear?".
The list is not exhaustive. These applications are only restricted by the adequate time schedules necessary to select enough quantities of risk cork stoppers.
THE CORK - a non-neutral material
In the three months after corking, the cork stopper may eventually lead to organoleptic changes in the wine.
Comparing the half bottles that were corked with low risk stoppers, one can detect a decrease in the caracter of the wines that were simply corked with cork, together with the appearance of"boisee" and vanilla odours.
In these wines, this feature is considered to be a positive quality. However, the more "boisee" is the odour, the more dangerous it is for the quality of the wine.
These results confirm that however pure it may be, the cork isn't neutral before the wine. It can both lead to positive or negative changes in the cork's organoleptic features.
These remarks derive from other conclusions set forth by different authors: Riboulet (1982), Boidron (1984) and Mazzoleni (1993) showed that the cork has many volatile aromatic compounds that can easily migrate to the wine after corking.
Hence, the comparison between the cork as a material that is extracted from the cork-oak and the timber that is used in the ageing of wines in casks.
CONCLUSION
The cork stopper is closely associated with the concept and image of champagne: a noble product as well as the traditional symbolism of uncorking the bottle.
Cork is a natural product whose production cannot be fully mastered. In order to avoid certain accidents after corking, such as high cork taint rates, it is of the utmost importance to perform a sort of organoleptic control in all lots of cork stoppers.
The experience shows that the selection of cork stoppers through olfactive assessment after warming meets the main control requirements: to reject defective cork stopper lots.
This test is quite simple to implement and offers quick, effective answers.
The technique of selecting cork stoppers will inevitably improve the basic knowledge around cork taints. It is an example of how the combined efforts of the professionals operating in the cork and wine industries will result in a significant progress as to the quality and reliability of the cork stopper.
There's no doubt as to the future perspectives of this corking method which, in our opinion, is vital for the assessment of quality wines.
BIBLIOGRAPHY
- BOIDRON J.-N, LEFEBVRE A., RIBOULET J.-M., RIBEREAU-GAYON P. Les substances volatiles susceptibles d'etre cedees au vin par le bouchon liege. Sci. Alim, 4, 1984, 609-616.
- RIBOULET J.-M. Contribution a l'etude chimique et microbiologique des "gouts de bouchon" dans les vins. These Universite de Bordeaux II, 1982.
- MAZZOLENI V., CALDENTEY DEL POZO P., CARERI M., MONGUZZI F., CHIALVA F. Studio della composizione chimica del sughero: i composi volatili 5e Symposium International Sur le Vin "Le liege en Oenologie". Pavie, 13 - 14 mai 1993.
- VALADE M., PANAIOTIS F., TRIBAUT-SOHIER I. Les problemes organoleptiques lies au bouchon liege. Le Vigneron Champenois, no. 3, mars 1993, 35-40.
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