Reduction is a chemical reaction that occur when something like e.g. wine are held in absence of air (oxygen). When a wine is reduced, unfortunate aromas can arise which can be stinky and smelly like rotten egg, burned rubber, burnt matches or garlic.
The smell comes from volatile sulfur compounds and can arise from the use of too much sulfur in the winery or in the field too close to harvest, or simple just bad wine making.
Reduction in wine though can also be controlled by the wine maker and create pleasant and complex aromas.
