Tracking Redox Potential during Fermentation as an Enological Parameter and Indicator of Yeast Metabolism
Gordon Walker, Arlene Mains, Roger Boulton, and
Michael Silacci*
*Opus One Winery, 7900 St. Helena Hwy, Oakville, CA 94562
(Michael.Silacci@opusonewinery.com)
Redox potential is an electrochemical measure of the reactive chemical environment within a solution that determines the rates and types of electron transfer reactions that can occur in a must/wine matrix. The redox potential is buffered by many reduction-oxidation couples and organic/inorganic complexes with metal species that are highly pH-dependent. Previous research has shown that the redox state of fermentation has important implications for the rate, yeast metabolism, and hydrogen sulfide (H2S) formation. When the redox potential of a fermentation drops below a certain point, elemental sulfur is reduced to H2S, providing a spontaneous chemical pathway for H2S formation independent of the yeast methionine pathway. Platinum electrodes can be used to measure the oxidation reduction potential (ORP), a quantifiable measurement of the tendency of a molecule or ion to gain or lose an electron. Tracking ORP provides valuable insight into the dynamics of fermentation beyond just Brix and temperature. From our observations, the ORP value of must starts high (300 to 400 ORP), but will begin dropping rapidly with the onset of yeast activity. Regular spikes are seen every 12 hrs, corresponding to pump-overs. As yeast enter into exponential growth, the ORP drops dramatically. We hypothesize that when the redox drops below ~200 to 150 ORP, the fermentation enters a reductive danger zone, where the elemental sulfur can be spontaneously reduced to H2S. From our data, nutrient and oxygen additions had a significant impact on both the fermentation rate and ORP values. Ultimately, it is the yeast metabolism driving the drop in ORP, with the lowest values corresponding to the yeast reaching a biomass maximum and ceasing growth. We have demonstrated that tracking redox potential as ORP is a valuable enological parameter for analyzing fermentation dynamics in real time.
Funding Support: Opus One Winery
