Why we dislike H2S H2S (Hydrogen Sulfide) is a rotten egg gas.
I suppose I could stop here and everyone would know why we dislike H2S, however its personality is a little more sinister. A little Chemistry: In bottled wine H2S is a byproduct formed when there is a lack of oxygen.
Young wines (especially reds) continue evolving in bottle and are hungry for oxygen so wine benefits if there is some dissolved oxygen available. Dissolved oxygen can be beneficial whereas leaving a wine open to the air oxidizes wine because the oxygen concentration on the surface is too high and the number of wine molecules in contact with it are too few.
When the concentration of H2S is high enough one quick sniff tells us ‘this wine pongs’! If the concentration is quite low it will simply mute the fruit and the wine will be drinkable but not very exciting.
Now for the sinister personality of H2S: A wine that has unacceptable levels of H2S can score quite high in a wine show, this is because H2S is very pungent at first but quickly deadens the sense of smell and we become less sensitive to it.
So in a lineup the first glass with elevated levels of H2S will be detected but the following wines with only slightly lower levels of H2S appear acceptable.
Later when a show winner is evaluated on its own it can smell very ordinary. Decanting a smelly bottle or vigorously swirling the glass to introduce oxygen will usually greatly improve wines showing mild levels of H2S.