Hydrogen sulfide, H₂S, is an undesirable compound produced during wine fermentation. Though colorless, hydrogen sulfide has a strong smell of rotten eggs. Even at concentrations lower than detectable by a human nose, hydrogen sulfide can contribute to a closed smell and taste, as well as contributing to formation of mercaptans and other undesirable compounds in the final product. Formation of hydrogen sulfide typically occurs during the primary fermentation when yeast activity is high. Copper sulfate is commonly added to remove H₂S when high concentrations are detected.
Rather than react to high H₂S concentrations (typically detected by smell), many winemakers are taking a proactive approach to minimize the production of hydrogen sulfide. These methods include selecting yeast strains that produce less H₂S, adding certain nutrients during fermentation, or aerating the wine.
Oxygen addition helps because hydrogen sulfide is largely formed from yeast products due to the reductive level reached by the wine environment. “Reductive” is a chemistry term used to describe the electron sharing tendency of a solution. Sulfates and elemental sulfur present in wine are reduced (gain electrons from the environment) to form hydrogen sulfide.
In order to combat a reductive environment, an oxidative species must be incorporated, the most well-known of which is oxygen. Sufficient oxygen counteracts hydrogen sulfide production and the associated negative effects. The balance between a reductive and oxidative environment is known as Oxidation-Reduction Potential, also called ORP or redox.
Adding oxygen to a wine fermentation is commonly done via pumpover, punchdown, or devatting (délestage). Pumpover provides the best control of aeration and also provides a compelling possibility for automated process control. Instead of scheduling or manually triggering a pumpover, automation could activate a pumpover only when the environment approaches a dangerously reductive state and end it before excess oxygen is added. Lately, winemakers have been turning to online sensors to provide a continuous measurement of ORP during fermentation to provide the critical control signal for pump over.
With a quantitative analysis of ORP, pumpovers (or oxygen additions by other means) can be initiated well before hydrogen sulfide can be detected by the human nose, preventing excess H₂S accumulation and the efforts needed to repair it. This quantitative approach also ensures that wine quality is consistent between batches. Through use of ORP sensors, wineries can prevent the formation of excess hydrogen sulfide, and ensure batch-to-batch consistency in wine fermentation.
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