Welcome to the fifth part of Tasting analysis like a professional. So far we have built up the systematic method from sight through smell to the structure and balance of taste. Now we turn our attention to what can go wrong: the wine faults. Being able to recognise them in the glass is one of the most practical skills you can acquire as a taster, because it sets the trained palate apart from the enthusiastic one.
It requires two things at once. You need to know what you are sensing, and why it occurs. And you need to be able to determine whether what you encounter is a genuine fault or simply a stylistic choice. That distinction is harder than it sounds, and it is the whole point of this part.
What you will learn
- To recognise the most frequent wine faults purely sensorially and connect them with the microbiological and chemical mechanisms behind them.
- To distinguish between a fault and a deliberate stylistic choice.
- To assess when a bottle should be discarded, and when it merely divides opinion.
- A little about how the wine's own microbiology and analysis help explain what you are sensing.
Cork and TCA
The classic cork fault is most often caused by the compound 2,4,6-trichloroanisole, abbreviated TCA. Sensorially you recognise it as a musty, earthy, damp cardboard or cellar-like note, which in mild cases does not smell directly off but simply dampens and veils the fruit. In pronounced cases it dominates everything.
What is interesting about TCA is how low the detection level lies. Analytical methods such as GC/MS operate with a detection threshold for TCA around two parts per billion, that is to say a vanishingly small content. That tells you something important about sensing: even trace amounts that you cannot necessarily put into words can take the life and aroma out of a glass. When a wine seems mute and dull without an obvious reason, a slight TCA influence is a frequent suspicion.
In practice, cork taint is screened in the laboratory with techniques such as solid-phase microextraction coupled to gas chromatography, where headspace-SPME has proven well suited precisely to this type of aroma compound. You should of course not be doing that at the table, but it underlines that your nose here is measuring something very real.
Oxidation and reduction
Oxidation and reduction are each other's opposites and both concern the wine's relationship with oxygen.
Oxidation
When wine comes into too close contact with air, oxidative yeast species from the genera Candida, Pichia and Metschnikowia can develop on the surface, form a biofilm and produce acetaldehyde. Sensorially this gives notes in the direction of overripe apple, nuts, sherry-like depth and a dull, brownish colour that does not match the wine's age. The fruit gives way to something tired and flat.
The producer counteracts this by minimising oxygen, among other things by storage under inert gas such as argon, CO2 or nitrogen, which inhibits the oxygen-dependent microorganisms. When the protection fails, the result is what you encounter in the glass.
Reduction
Reduction is the opposite: the wine has lacked oxygen, typically sulphur-related notes in the direction of struck match, boiled cabbage, garlic or rubber. The important thing here is that reduction is often volatile. A little air in the glass or a decanting can drive off the lightest reductive notes and reveal a healthy wine underneath. This is one of the reasons you should never judge a bottle definitively on the first sniff.
Brettanomyces and volatile acidity
Two of the most debated influences deserve their own section, because they both balance on the border between fault and character.
Brettanomyces
The yeast species Brettanomyces bruxellensis produces volatile phenols, among them 4-ethylphenol and 4-ethylguaiacol, through decarboxylation and reduction of hydroxycinnamic acids such as ferulic acid and p-coumaric acid. Sensorially you recognise them as stable, leather, plaster, sweat or smoked meat. The compounds become noticeable when the concentration exceeds a threshold in the range of a few hundred micrograms per litre, and the risk rises as the yeast population grows, particularly during barrel ageing when the cellar temperature rises.
The central tool against Brett is sulphur. A level around 0.5 mg/L molecular SO2, which at pH 3.75 corresponds to about 45 mg/L free SO2, effectively inhibits most strains. But not all: the species comprises three phylogenetic groups with differing SO2 sensitivity, from very sensitive through tolerant to resistant. That explains why Brett shows up even in well-run cellars.
Related is the mousy note, mousiness, which stems from N-heterocyclic bases formed by both Brettanomyces and certain heterofermentative lactic acid bacteria. It is not perceived in the nose, but as an aftertaste of wet dog or old popcorn that grows on the palate.
Volatile acidity
Volatile acidity, dominated by acetic acid and the associated ethyl acetate, gives notes of vinegar and nail polish remover. A little perspective: wine is microbiologically more stable than must, because during fermentation the yeast produces ethanol and fatty acids that increase toxicity towards other microorganisms. When that balance is shifted, acetic acid can take over. In small amounts a hint of volatile acidity can seem lifting and upward. In large amounts it is unequivocally a fault.
Fault or style?
This is where it gets interesting, because the line is not always sharp. A small amount of reduction, a discreet Brett note or a touch of volatile acidity can be perceived as complexity by one taster and as a fault by the next. Three principles help you navigate.
First: intensity and balance. The question is rarely whether a compound is present, but whether it dominates. A hint of stable behind dark fruit is something other than a wine that smells solely of stable.
Second: consistency. A stylistic note appears consistently and seems integrated. A fault stands out and drags down the whole.
Third: take the bottle seriously over time. Reduction can disappear with air. As a rule, therefore, you should discard a bottle when an influence is unequivocal, dominant and does not disappear with time in the glass, particularly in the case of clear TCA, strong oxidation or pronounced acetic sting. These are objective faults that cannot be tasted away.
Sensory analysis is precisely defined as a scientific discipline for eliciting, measuring, analysing and interpreting reactions to what the senses perceive. That gives you the freedom to separate the measurable, the fault, from the subjective, the preference.
In short
- TCA gives a musty, cellar-like note and can extinguish a wine even in trace amounts, down around the detection level of two parts per billion.
- Oxidation gives nutty, tired notes from acetaldehyde, while reduction gives sulphur notes that often disappear with air.
- Brettanomyces forms volatile phenols such as 4-ethylphenol with notes of stable and leather, and is controlled primarily with sulphur.
- Volatile acidity in the form of vinegar and nail polish remover is lifting in small amounts and a clear fault in large ones.
- Distinguish between fault and style based on intensity, balance and whether the influence disappears over time.
Frequently asked questions
Is Brettanomyces always a fault?
Not necessarily. In small, integrated amounts some tasters perceive Brett as complexity, while others reject it entirely. If the stable note becomes dominant and pulls fruit and freshness out of the wine, it is unequivocally a fault. It is about intensity and balance, not about presence alone.
Can I rescue a bottle that seems reductive?
Often yes. Reductive sulphur notes are typically volatile and can be driven off with air, either by letting the wine stand in the glass or by decanting. Therefore never judge a reductive bottle definitively on the first sniff.
Ready for the next step?
Now you can put a name to what goes wrong in the glass, and just as importantly distinguish faults from deliberate style. In Blind tasting: method and pitfalls we take the next step and remove the label entirely, because this is where your ability to read a wine objectively is truly put to the test.
Do bring along your new attentiveness when you explore our selection, and taste at your leisure. Remember at the same time that the best bottle is often the one you yourself enjoy, with the food you love. The technique is a tool, not an answer key.