Reduction
A wine fault caused by insufficient oxygen during winemaking, producing sulphur-based off-aromas such as struck match, rotten egg, rubber, garlic, or onion.
In depth
Reduction is the opposite of oxidation. It occurs when wine develops under anaerobic (oxygen-deficient) conditions, allowing sulphur compounds to accumulate. Yeast and bacteria naturally produce small amounts of sulphur compounds during fermentation — in normal winemaking these dissipate. Under anaerobic stress, production is amplified.
The specific compounds and their aromas: hydrogen sulphide (H₂S) — rotten egg, drains; methanethiol — rotten cabbage, garlic; ethanethiol — onion, rubber; dimethyl sulphide (DMS) — canned corn, asparagus, cooked vegetables.
Unlike oxidation, reduction is often correctable: exposing the wine to oxygen (through splashing, decanting, or swirling in the glass) can dissipate the volatile sulphur compounds. This is why a "reduced" wine can sometimes "blow off" with aeration — the fault disappears with air contact. However, if the reduction is severe or the wine has been in bottle for a long time, it may not fully resolve.
At Level 3, candidates must identify reduction by its sulphur-based aroma character and know that it can be corrected by aeration — unlike TCA or severe oxidation which are permanent faults.
Related exam topics
Frequently asked questions
- Can reduced wine be fixed?
- Often yes — reduction is frequently correctable by exposing the wine to air. Swirling the glass, decanting, or splashing the wine into the glass can allow the volatile sulphur compounds to dissipate. A reduced wine that "blows off" with 20 minutes in the glass is quite different from a wine that permanently smells of rotten egg regardless of aeration. TCA (cork taint) cannot be fixed by aeration — this distinguishes the two faults.
Practise questions on this topic
Use Vinlecta to practise exam-style questions that test your knowledge of reduction and related topics.