Wine Faults and Preservation: Cork Taint (TCA), Reduction, Brett and SO2

What counts as a wine fault

In this article, a wine fault is a smell or taste caused by an unwanted compound or by spoilage microbes, rather than by the grape, the place or the winemaking style. The compounds involved are often present in tiny amounts, so a fault is usually noticed by the nose before any analysis is done.

The faults covered here have different origins: cork taint comes from a chlorinated compound, reductive notes from volatile sulfur compounds that yeast can form, Brett character from a spoilage yeast, and ethyl acetate from acetic acid bacteria. Sulfur dioxide (SO2), discussed in the later sections, helps protect wine against oxidation and microbial spoilage, and how well it works depends on the wine's pH.

Tasters do not always agree on where character ends and a fault begins. This article gives published thresholds where they exist and treats everything else as a matter of judgement.

Cork taint: how TCA forms and why it mutes a wine

TCA (2,4,6-trichloroanisole) is the main compound behind cork taint, and it has a distinct musty, mouldy aroma. In consumer tests, the detection threshold was 2.1 ng/L and the rejection threshold 3.1 ng/L; some tasters could detect TCA below 1 ng/L.

TCA forms mainly by two routes: microbial O-methylation of chlorophenols, and chlorination of anisole. A study of cork bark and finished corks concluded that TCA already present in the bark of living cork oaks can account for most cases of cork taint in bottled wine, so the problem does not start only in the winery.

TCA does more than add a musty note. A 2017 sensory study found that concentrations below the detection threshold can mask several other olfactory notes, and laboratory work published in PNAS in 2013 showed that TCA suppresses the cyclic nucleotide-gated channels of olfactory signal transduction without producing an odour response of its own, even at attomolar concentrations. This helps explain why a lightly tainted wine can seem flat and muted rather than obviously mouldy.

Closure makers have responded in different ways. Diam, for example, cleans its cork with supercritical carbon dioxide (its DIAMANT process) and guarantees a releasable TCA level of 0.3 ng/L or less for each closure.

Reduction: hydrogen sulfide, thiols and disulfides

Reductive faults are caused by volatile sulfur compounds. Yeast metabolism can produce hydrogen sulfide (H2S, with its rotten-egg smell) and thiols such as methanethiol (MeSH) and ethanethiol (EtSH), which are grouped together as sulfhydryls.

The usual treatment is copper sulfate fining: copper reacts with H2S, methanethiol and ethanethiol and removes them from red and white wines. The copper does not necessarily drop out as a solid, as was once thought; a large part of it may stay dissolved in the wine after filtration.

The order of steps matters. If a wine containing thiols is aerated to get rid of a suspected reductive smell, the thiols can be oxidised to disulfides, which do not react with copper. To remove them, the winemaker first has to create reducing conditions with ascorbic acid and SO2, which turn the disulfides back into methanethiol and ethanethiol, and only then fine with copper.

Brettanomyces and volatile phenols

Brett character comes from the yeast Dekkera/Brettanomyces. Its main spoilage compound is 4-ethylphenol (4-EP), which gives a Band-Aid, medicinal or pharmaceutical character. Together with 4-ethylguaiacol (4-EG) and 4-ethylcatechol (4-EC), it is associated with notes described as Band-Aid, medicinal, horsey and barnyard, known collectively as Brett character.

The aroma threshold of 4-ethylphenol in a neutral red wine has been put at 368 µg/L. The unit is micrograms, not nanograms: it starts to be noticed at far higher concentrations than TCA.

Oxygen, acetic acid bacteria and ethyl acetate

Oxygen is not a fault in itself, but uncontrolled exposure during storage opens the door to oxidation and to spoilage microbes. Ethyl acetate is one example: it is produced by acetic acid bacteria, and its formation is related to the dissolved oxygen in the wine.

Free SO2 is the part of a wine's sulfur dioxide that is available to protect it against oxidation and microbial spoilage. How much protection that free SO2 gives is not fixed: it depends on the wine's pH, which is the subject of the next section.

Sulfur dioxide: why pH decides how well it protects

Free SO2 in wine exists in three forms: molecular SO2, bisulfite (HSO3-) and sulfite (SO3 2-). At wine pH the bisulfite form is the most common, and the lower the pH, the larger the molecular share. Between pH 3.0 and 4.0, less than 10 per cent of free SO2 is in the molecular form.

Targets against spoilage are set in molecular SO2: molecular SO2 levels of 0.5 to 0.8 mg/L have been suggested as effective for inhibiting wine spoilage bacteria and yeasts. The free SO2 needed then follows from the pH. The molecular share is calculated with a pKa of 1.81: molecular SO2 equals the molecular percentage multiplied by the free SO2, so the free SO2 needed equals the target divided by that percentage.

For a target of 0.8 mg/L of molecular SO2, this gives about 13 mg/L of free SO2 at pH 3.0, where about 6 per cent is molecular. The figures are 40 mg/L at pH 3.5 and about 99 mg/L at pH 3.9. A small rise in pH therefore multiplies the SO2 a wine needs.

Practice question 1: cork taint

The two practice questions in this article were written by Maison for self-study; they are not taken from any official examination.

Question:
Explain where the TCA in a corked wine can come from, why a wine can seem muted before it smells clearly musty, and what one closure maker guarantees about TCA.

Model answer:
TCA (2,4,6-trichloroanisole) forms mainly by microbial O-methylation of chlorophenols or by chlorination of anisole, and one study concluded that TCA already present in the bark of living cork oaks can account for most cases of cork taint in bottled wine. In consumer tests, the detection threshold was 2.1 ng/L and the rejection threshold 3.1 ng/L. Below the detection threshold TCA can still mask other aromas, and it has been shown to suppress the cyclic nucleotide-gated channels of olfactory signal transduction, so the fruit can seem muted. Diam treats its cork with supercritical CO2 and guarantees a releasable TCA level of 0.3 ng/L or less for each closure.

Mark scheme (10 marks):
- The two formation routes (microbial O-methylation of chlorophenols, chlorination of anisole), with cork bark as a source (3 marks)
- Detection at 2.1 ng/L and rejection at 3.1 ng/L, with the musty, mouldy descriptor (3 marks)
- Masking below the threshold and suppression of olfactory signalling (2 marks)
- The supercritical CO2 treatment and the 0.3 ng/L guarantee (2 marks)

Practice question 2: a rotten-egg smell

Question:
A young white wine smells of rotten eggs. Explain which compounds are likely to be involved, how copper fining acts on them, and why aerating the wine before fining can make the problem harder to fix.

Model answer:
The smell points to volatile sulfur compounds: hydrogen sulfide and thiols such as methanethiol and ethanethiol, which yeast metabolism can produce. Copper sulfate fining reacts with these sulfhydryls and removes them, although part of the copper may stay dissolved in the wine after filtration. If the wine is aerated first, the thiols can be oxidised to disulfides, which do not react with copper. The winemaker then has to create reducing conditions with ascorbic acid and SO2 to turn the disulfides back into thiols before copper can remove them.

Mark scheme (10 marks):
- Naming H2S and thiols (methanethiol, ethanethiol) as the likely compounds (3 marks)
- Copper sulfate reacting with sulfhydryls, and the note on copper that stays dissolved (3 marks)
- Aeration oxidising thiols to disulfides, and why copper then fails (2 marks)
- Ascorbic acid and SO2 to restore the thiols before copper fining (2 marks)