Available at: https://digitalcommons.calpoly.edu/theses/3240
Date of Award
3-2026
Degree Name
MS in Agriculture - Crop Science
Department/Program
Horticulture and Crop Science
College
College of Agriculture, Food, and Environmental Sciences
Advisor
Federico Casassa
Advisor Department
Horticulture and Crop Science
Advisor College
College of Agriculture, Food, and Environmental Sciences
Abstract
The oxidation-reduction potential (ORP) is a measurement thought to reflect the overall ability of a solution to oxidize or reduce substances within that solution. In wine, there is no absolute agreement as to which substances control or dominate the ORP; however, oxygen, ethanol, iron-tartrate and copper-tartrate complexes, and glutathione are suspected to play key roles, while sulfur dioxide and phenolics may act as buffers. Anecdotally, some varieties of Vitis vinifera are prone to oxidation while others are prone to reduction, though the chemical basis of this remains unclear. The following works aim to further elucidate the influence of specific chemical species on the ORP in model wine solutions, as well as the effects of macro-oxygenation and redox-controlled air sparging during alcoholic fermentation of Syrah wines, and redox-controlled air sparging during alcoholic fermentation of Grenache, Syrah, and Mourvèdre wines, including the resulting GSM blends.
The first study investigated the individual and combined effects of caffeic acid (CAF), reduced glutathione (GSH), and ferric ion (Fe3+) on the oxidation-reduction potential (ORP) of an oxygenated model wine system to better understand the roles of phenolic- and sulfhydryl-containing compounds, and transition metals, in governing the ORP. Additions of CAF (150 mg/L), GSH (150 mg/L), and CAF+GSH were followed by the addition of Fe3+ (10 mg/L Fe3+ chloride hexahydrate). Addition of CAF alone had little effect on the ORP, whereas GSH and CAF+GSH rapidly decreased ORP by approximately 50 mV, converging from initial values of 243 mV (Ag/AgCl reference electrode) and 237 mV to a resting value of 189 mV, indicating that glutathione dominated the ORP regardless of the presence of caffeic acid. Caffeic acid concentrations remained stable, and absorbance at 420 nm showed no evidence of browning over the two-hour experimental period, confirming phenolics did not oxidize despite superfluous dissolved oxygen. Following Fe3+ addition, ORP increased to similar values across treatments (266 to 284 mV), demonstrating the dominant redox activity of iron-tartrate at the electrode surface. The CAF+GSH treatment exhibited the steepest post-iron addition ORP decline, suggesting synergistic transition metal reduction by caffeic acid and glutathione. Collectively, these results showed that glutathione strongly lowered the ORP, the Fe3+ addition overrode the effects of glutathione and caffeic acid and increased the ORP, and caffeic acid had only a marginal impact on the ORP under wine-like conditions, highlighting the role of transition metal and sulfhydryl redox couples as contributors to the ORP.
The second study evaluated the redox, chemical, and sensory effects of macro-oxygenation and redox-controlled air sparging during alcoholic fermentation of Syrah wines compared to those made with traditional punch down cap management. Syrah must was subjected to four treatments: punch downs (PD), redox-controlled air sparging triggered at −40 mV (Ag/AgCl reference electrode) (RedoxCon), standard-rate macro-oxygenation (MOX), and double the standard rate macro-oxygenation (2MOX). RedoxCon received 147.6 mL O2/L must; that is, 1125% and 512% more O2 than MOX (12.1 mL O2/L) and 2MOX (24.1 mL O2/L), respectively, and maintained higher average ORP values (−25 mV) than MOX (−67 mV) and 2MOX (−74 mV). PD and RedoxCon completed fermentation two days prior to MOX and 2MOX, consuming 89.4% of sugars by day 2 compared to 61.3% in the MOX treatments. At pressing, MOX and 2MOX wines exhibited 30 and 31% higher total color, 41 and 27% higher tannins, and 31 and 32% higher total phenolics than PD and RedoxCon wines, respectively. MOX and 2MOX retained significantly more tannin during aging, losing approximately 30% relative to 43 to 51% losses in RedoxCon and PD wines. Ester concentrations at pressing were 61% (MOX) and 53% (2MOX) higher than in PD, driven by ethyl n-octanoate and isoamyl acetate (odor activity values (OAVs) >150), while terpenoids and norisoprenoids were higher in PD and RedoxCon wines. Sensory analysis by Pivot© Profile indicated that 2MOX wines were associated with the perception of fruit aroma, RedoxCon wines with the perception of spicy and floral aromas, and both MOX treatments were perceived as astringent and reductive. Collectively, these results demonstrated that macro-oxygenation during alcoholic fermentation preserved phenolics and esters by maintaining lower ORP environments and minimized volatile stripping, overall highlighting ORP as a controllable variable in alcoholic fermentation of red wines.
The final study examined the effects of ORP control during alcoholic fermentation on fermentation kinetics, chemical composition, and blending consequences of Grenache (GR), Syrah (SY), and Mourvèdre (MO) wines. Alcoholic fermentation was conducted in quadruplicate using three ORP protocols: traditional pump overs (CON), ORP controlled air sparging activated upon the ORP reaching 50 mV (Ag/AgCl reference electrode) (OXI), and ORP controlled air sparging activated upon the ORP reaching −80 mV (Ag/AgCl reference electrode) (RED). The ORP differed by variety, with GR exhibiting higher mean ORP values, reaching averages of 35 mV in CON and 58 mV in OXI, compared to lower values in SY and MO. OXI treated musts accelerated alcoholic fermentation, with sugar consumption rates up to 4.82 Brix per day, while RED treated musts experienced delays by up to 3 days across all varieties. OXI wines contained up to 15% less flavan-3-ols, less monomeric anthocyanins after bottle aging, increased polymeric pigments, and increased yellowness as indicated by higher CIELab b* values. Ester concentrations and OAVs were generally lower in OXI wines; RED wines contained higher levels of acetate and ethyl esters. Grenache was most responsive to the control of the ORP, while Syrah showed comparatively fewer chemical changes. GSM blend (33:33:33 GR, SY, and MO) wines exhibited sub-additive behavior for most phenolics and volatile compounds, while ethanol, pH, and anthocyanins followed additive trends. These results demonstrated that control of the ORP during alcoholic fermentation exerted a strong, variety-dependent effect on wine composition and that blending outcomes were not entirely additive.
In summary, these studies investigated the effects of transition metals, phenolics, and glutathione on the ORP of model wines, as well as the impact of minimal intervention air and oxygen injection cap management techniques on the ORP of Grenache, Syrah, and Mourvèdre musts during alcoholic fermentation. The results showed that ORP was affected by transition metals and glutathione, whereas phenolics showed no significant effect in model wines. During alcoholic fermentation of red wines under minimal cap management and ORP setpoint protocols, phenolic composition was only marginally influenced by ORP control. Altogether, these findings suggest that minimal cap management combined with macro-oxygenation or air sparging to maintain selected ORP values may be a viable strategy during alcoholic fermentation of red wines without increasing the risk of severe reductive aromas or substantial phenolic loss. However, these studies should be considered preliminary, as the experiments were conducted at a relatively small scale, where vessel size and geometry may have favored phenolic extraction and gas mixing.