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 (rbboulton@ucdavis.edu)
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
