Sophisticated micro-agglomerate corks have become increasingly popular among wine producers looking for a reliable TCA-proof and oxidation-resistant alternative to natural corks and screwcaps. They are now being widely used by super-premium producers in regions such as Burgundy and Champagne. German scientists wonder if they create a 'potentially flavour-active contaminant' called suberic acid. This paper by Rolf Cordes, Volker Schneider, Wolfgang Schwack and Dr. Paul Haase-Aschoff has not been supported by other research.
Some Australians and New Zealanders who prefer screwcaps claim to have detected TCA taint or a 'gluey' note in micro-agglomerate closures, but comments like these are rare exceptions to the satisfied rule. Now, however, research in Germany by Rolf Cordes, Volker Schneider, Wolfgang Schwack and Dr. Paul Haase-Aschoff suggests that what they describe as a 'potentially flavour-active contaminant' called suberic acid may be creating an astringent effect in wines sealed with micro-agglomerate corks.
- Cork quality control has always been about the perception of odours. Taste aspects have traditionally been neglected.
- New research is looking into closure-derived astringency with sensory tests and an electronic tongue.
- Suberic acid is proposed to be the compound in micro-agglomerate corks that is associated with astringency
- Astringency is said to build up over time, affecting the appreciation of subsequent wines sealed with different closures
Natural and agglomerated corks vary in many aspects such as their oxygen permeability and their susceptibility of contributing taints of varying character to the wine. Among the latter, the cork taint caused by TCA is the most important. Its perception takes place primarily at the olfactory level. Hence, the focus on cork quality control has always been on the perception of odours in combination with olfactory perceptions, in conjunction with the analytical quantification of TCA and similar compounds by gas chromatography. However, this analytical approach does not detect odourless non-volatile compounds that only become active on the palate. Therefore, taste aspects of the use of corks have traditionally been neglected.
Anecdotal tasting results from originally identical wines sealed with different closures gave rise to the assumption that the lots treated with certain types of agglomerated stoppers exhibited a lingering astringency on the mouth and throat mucous membranes, which caused an alteration of the wines’ gustative profile. Astringency is known as a perception that increases in intensity and duration with repeated ingestion. The reason for this is that residual astringency of the previous wine is carried over to the next sample, to whose astringency it adds to if the residual astringency is not allowed to decay to zero before the next ingestion (1). This sensory bias is called carry-over effect. It shows that past astringency becomes the limiting factor in its own discrimination during subsequent evaluations. Additionally, it demonstrates the need for careful experimental design of trials involving astringency scorings. In particular, the triangle test has been shown to not be the correct choice when there are significant carry-over effects between samples (2, 3, 4).
Sensory and instrumental discrimination of cork-derived astringency
Due to the long-lasting persistence of the astringency elicited by the corks used in this study and the carry-over effect resulting therefrom, it became evident that samples could not be tasted in short intervals required in triangle tests. Therefore, paired-comparison tests were used as ‘same/different’ tests performed by 11 panelists tasting 8 pairs (A and B) of wine by mouth. In each pair, A was a commercial reference wine and B the cork treatment of that wine. Treatments were (i) two corks soaked in 1,500 mL wine for 32-72 hours, and (ii) bottles sealed with the cork under investigation instead of screw caps and stored horizontally for 37-94 days. Tasters were asked to evaluate the A and B samples served in the order ‘reference’ – ‘treatment’ without re-tasting, and to indicate whether samples were the same or different, and whether they varied in astringency. A waiting period of at least four hours was observed between each pairwise comparison. Under these conditions, 7 out of 8 agglomerated corks changed the wine with statistical significance.
|
Table 1: Paired-comparison tests by mouth on the impact of three agglomerated corks from the same manufacturer on various wines and water. |
||||
|
Type of cork |
Variety and origin of wine |
Treatment |
Panelists, total |
Panelists distinguishing difference |
|
1 |
Müller-Thurgau, Germany |
2 corks soaked in 1500 mL wine for 48 hours |
11 |
10** |
|
1 |
Muscat blanc, Germany |
Bottles sealed with cork and stored laid down for 37 days |
11 |
11*** |
|
2 |
Syrah Rosé, Italy |
2 corks soaked in 1500 mL wine for 72 hours |
11 |
9* |
|
2 |
Pinot blanc, Germany |
Bottles sealed with cork and stored laid down for 41 days. |
11 |
9** |
|
3 |
Silvaner, Germany |
2 cork soaked in 1500 mL wine for 32 hours |
11 |
10** |
|
3 |
Riesling, half-dry, Germany |
2 corks soaked in 1500 mL wine for 72 hours |
11 |
11*** |
|
3 |
Riesling, dry, Germany |
2 corks soaked in 1500 mL wine for 72 hours |
11 |
8 |
|
3 |
Pinot noir, Switzerland |
Bottles sealed with cork and stored laid down for 94 days. |
11 |
9* |
|
Mix of corks 1 + 2 + 2 x 3 |
Water |
4 corks soaked in 1500 mL water for 48 hours. |
11 |
8 |
|
Significance levels: * p=0.05, ** p=0.01, *** p=0.001 |
||||
As the order of samples could not be randomized due to the carry-over effect of the sample matrices, the statistical power of the paired comparison tests was obviously reduced. Therefore, they were complemented by an Astree electronic tongue. The electronic tongue confirmed the results obtained by the sensory tests; the samples were not identical.
Measurement of suberic acid
Extracts of cork slices soaked in 10% ethanol overnight were analyzed by LC-HRMS. Differential analyses of the chromatograms showed more than 40 compounds that occurred in agglomerated corks. There were specific compounds that were typical for agglomerated corks and which did not occur in natural corks, or only in substantially lower amounts. Among these were floionic acid and suberic acid. These both belong to the group of so-called cork acids making up the cork suberin polymer. Since floionic acid is not commercially available in its pure form, further research focused upon the odourless suberic acid to check whether it is sensorially active and able to migrate from the normal cork surface into the wine.
When the end of seven agglomerated corks was soaked in 15 ml model wine under conditions reflecting a one-month storage at ambient temperature, subsequent measurements of suberic acid yielded concentrations corresponding to 1,240 to 2,160 ng/l in a standard bottle of 750 ml. In order to approximately record the time course of its release, the same procedure applied to an agglomerated cork showed an increase of 40% from day 7 to day 14.
In a subsequent step, four wines were spiked with 1,200 and 2,200 ng/l suberic acid (Sigma Aldrich 60930) and submitted to analysis by the electronic tongue in comparison with the unspiked reference. For each wine, the electronic tongue distinguished the two samples as not identical.
Taste threshold of suberic acid
Additional tastings were performed after addition of increasing amounts of suberic acid to 40 white and red wine samples. Paired-comparison tests with the untreated reference tasted first showed that additions of 1,400 ng/l suberic acid caused a significant difference. Trained tasters are able to respond to less than 200 ng/Ll. Threshold concentrations were more dependent on tasters than on wines.
Astringency elicited by acids
The generally accepted mechanism evoking astringency is that proteins in saliva combine with hydroxyl groups of tannins and precipitate. Many organic acids such as tartaric or suberic acid also contain hydroxyl groups and thus fit in this theory. Thus, variable astringency intensities and sub-qualities have been shown for the acids contained in wine, as well as their dependence on pH suggesting that the acidic properties of these acids are an additional cause of their concurrent astringency. Suberic acid has similar properties, as can easily be understood when ones tastes aqueous solutions containing as little as 1,000 ng/l of it. The same amount added to wine causes additional changes in the mouthfeel that go far beyond the well-defined tactile sensation of astringency, as can be seen from the sensory fingerprints of the electronic tongue shown in the figure.
In contrast to cork taint, these changes are identical in all bottles when the bottles are sealed with agglomerated corks, since the production process of such corks delivers product homogeneity. Hence, they do not attract sensory attention when all bottles are sealed with the same cork. In the case of natural corks, which are known to display differences between individual specimens, the measured suberic acid covers a wide range with values below or above those of agglomerated corks. Thus, its sensory effect also occurs to a varying degree after bottling with natural cork.
Literature
- Lee C.B. and Lawless H.T., 1991. Time-course of astringent sensations. Chem. Senses 16 (3): 225-238.
- Williams A.A. and Arnold G.A., 1991. The influence of presentation factors on the sensory assessment of beverages. Food Qual. Pref. 3 (2): 101-107.
- Lau S., O'Mahony M., Rousseau B., 2004. Are three-sample tasks less sensitive than two-sample tasks? Memory effects in the testing of taste discrimination. Percept. Psychophys. 66 (3): 464-474.
- Yang Q. and Ng M.L., 2017. Paired Comparison/Directional Difference Test/2-Alternative Forced Choice (2-AFC) test, simple difference test/same-different test. In: Discrimination Testing in Sensory Science, A Practical Handbook. Woodhead Publishing Series in Food Science, Technology and Nutrition, 2017, p. 109-134.
A longer version of this report, including a full set of references appeared in Infowine and Researchgate in 2021, while suberic acid is mentioned in a 1906 paper written by Walter Riley.
