Reduction

Insufficient oxygen during fermentation or aging, leading to formation of hydrogen sulfide (H2S) and mercaptans (thiols). Common in wines fermented in sealed stainless steel tanks or on heavy lees. Nutrient-deficient musts (low nitrogen) are more susceptible. Certain yeast strains produce more H2S.

Struck match, rotten eggs, burnt rubber, garlic, or onion aromas depending on the specific sulfur compound. Mild reduction (struck match) often blows off with aeration or decanting. Severe reduction (mercaptans, disulfides) is harder to remedy and may be permanent.

Adequate yeast nutrition (diammonium phosphate), gentle aeration during fermentation, copper fining for H2S, and racking for moderate reduction. Adding copper sulfate at micro-doses can treat mild reduction. Preventing is easier than curing.