The Control of Redox Potential during Wine Fermentations
David Killeen, Roger Boulton,* and Andre Knoesen
*Department of Viticulture and Enology, University of California,
Davis, CA 95616 (firstname.lastname@example.org)
Redox potential is a common electrochemical measurement of the status of competing electron transfer reactions in biological or chemical media, including fermentations. The redox potential of juice is usually between +300 to +350 mV before the onset of yeast growth. The anaerobic fermentation of glucose to ethanol within yeast requires that cells maintain an internal redox potential of
-290 mV for both NAD and NADH to exist. To maintain this internal value, yeast transport a number of components from and into the fermentation medium, causing the redox potential patterns to change significantly during fermentation. The redox condition of the juice during fermentation can determine whether or not certain reactions take place. One example is the electrochemical reduction of suspended elemental sulfur to hydrogen sulfide, which is sometimes formed during fermentation. Being able to control the redox potential during wine fermentation might mitigate this and have other effects on yeast metabolism and other redox reactions. This study developed a controller for the redox potential during fermentations using air and demonstrated its efficacy at the experimental scale
(100 L fermentations) to control the potential at a set value. The control system consisted of a microcontroller that communicated with a commercial redox meter and probe. The controller would sample the meter every 15 min and turn on the air if the probe reading was below the redox potential setpoint. The controller and its related air spargers could raise the redox potential by as much as 100 mV during mid-fermentation. Uncontrolled, inoculated fermentations were allowed to proceed normally without added air. Experiments have been performed successfully in triplicate white fermentations (Chardonnay) and red fermentations (Grenache and Mataro). Differences in the Brix curves were noted. These appear to be the first examples of wine fermentations with a controlled redox potential.
Funding Support: T.J. Rodgers Fellowship in Electrical and Computer Engineering, Stephen Sinclair Scott Endowment in Viticulture and Enology