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.