Faults in wines are not always easy to determine. Rob MacCulloch MW explain Decanter a little more about faulty wines.

Ask Decanter: Mystery wine fault

Xavier Auerbach, Netherlands, asks: Occasionally I encounter bottles of red wine that have a very specific flaw – and not brett, oxidation, reduction, volatile acidity or the main recognisable faults. The nose is raw, bitter and stalky and the palate leaves an unpleasant impression of cocoa powder and pure alcohol. Recently a bottle of Sandeman 1994 Vintage Port was thus affected (a second bottle from the same case was fine). Can you explain this?

Rob MacCulloch MW, for Decanter, replies: This is unusual, but is probably due to the wine’s phenolic content degrading. Over time, changes in a wine’s phenolic matter can produce volatile aromas. Some of these aroma compounds, such as methylpyrazines, are shared with cocoa, and produce the aromas described. These aromas overpower the remaining fruit character in the wine and will make alcohol more apparent, too.

Why would only some bottles in a case be affected? Individual bottles have very slight cork inconsistencies which can create unstable conditions, causing phenolic content to change more rapidly. As this is a combination of factors, which may include just enough oxidation to kickstart a more rapid phenolic degradation, well-known wine faults like TCA or overt oxidation are not detectable.

Rob MacCulloch MW is training for a career in viticulture and oenology.


  • Hylton McLean

    Not Much of a mystery here if other bottles from the same case were OK and the perceptions/memories of the taster are reliable.
    Cork taint, and/or oxidation caused by a bad fitting cork in a bad formed bottle neck are a constant cause of bottle variation, especially in wines ,such as VP, that are stored for long periods.

    Hylton McLean Honey Moon Vineyard

  • Damian Espinase

    This is an interesting fault description and resulting prognosis. The description of the wine of a ‘bitter nose’ is a challenge to understand. Bitterness is one of the 5-6 tastes (if you include umamai) and is not an aroma. However, ‘raw and stalky’ I would interpret as underripe or what winemakers call ‘green’ aromas. Although colours as aromas is just as confusing as tastes equating smells. I think the class of molecules Rob is trying to allude to are methoxypyrazines not methylpyrazines. Methoxypyrazines are associated with under-ripe fruit or the ‘green’ aroma of sauvignon blanc. These compounds can also be caused by lady beetle taint. Although I have never come across methylpyrazine in wine chemistry there seems to be some evidence that this different class molecule can be synthesized during Maillard reactions. This could explain the bottle variation if one bottle was stored at the shop by the heating duct or something. Dimethylpyrazine seems to smell of roasted sesame seeds, is that the aroma? Nevertheless, the ‘phenolic degradation’ hypothesis seems a stretch as methoxypyrazines and methylpyrazines are not phenols. Phenols are 6 carbon rings structures and the common phenols referred to in wine are larger multiple ringed molecules called generally as flavonols, anthocyanins etc… these make up the astringency, bitterness and colour of wine depending on their polymerization and structural conformation. Pyrazines are also ring structures however with 2 carbons replaced with nitrogen atoms and subsequent functional groups and to my knowledge are not formed in the Shikimate pathway of the grape as flavonols are. My guess would be that pyrazines are alkaloids or formed through rearrangements of amino acids such as histidine. Anyway that’s enough bio-organic chemistry for one day. Chalk it up to bottle variation and possibly poor storage. Cheers D.