Available at: https://digitalcommons.calpoly.edu/theses/3373
Date of Award
6-2026
Degree Name
MS in Agriculture - Crop Science
College
College of Agriculture, Food, and Environmental Sciences
Advisor
L. Federico Casassa
Advisor Department
Horticulture and Crop Science
Advisor College
College of Agriculture, Food, and Environmental Sciences
Abstract
In the 2024 and 2025 vintage four experiments were performed each with a different varietal, namely Albariño, Grenache blanc, Loureiro, and Vermentino. These experiments investigated the effects of hyperoxygenation, reductive winemaking techniques, while monitoring the oxidation-reduction potential.
Albariño grapes were made following two juice protocols, control/reductive (Reductive Control) and oxidative (HyOx). Reductive Control received antioxidants (50 ppm SO2, 160 mg/L ascorbic acid) and CO2 blanketing, and HyOx underwent hyperoxygenation at approximately 700 mL/L/h of air for two hours through a stainless-steel sparging stone with 10-µm pores. Juice was cold-settled and racked. Reductive Control juice was split into four treatments: WhiteCon, SkinCon, WhiteReduct, and SkinReduct. HyOx juice was split into WhiteHO and SkinHO. Reduct treatments were handled reductively, and HO treatments were handled oxidatively during alcoholic fermentation. WhiteReduct and SkinReduct received 100 mg/L reduced glutathione on Day 2 of alcoholic fermentation. Oxidation-reduction potential was measured throughout juice handling and alcoholic fermentation. Results showed a decrease in ORP for Reductive Control to 100 mV while HyOx reached a peak ORP of 376 mV highlighting the contrasting redox statuses at the juice stage. Expectedly, increased total phenolics and tannins were observed in skin contact wines, and decreases were observed for hyperoxygenated wines not treated with skin contact. Skin contact wines displayed greater golden hue and color saturation, while traditionally fermented wines had greater green hues. Increases of 50 to 110% were observed in odor activity values (OAVs) of esters in wines that were fermented traditionally (i.e. no skin contact) relative to their skin contact counterparts. Aroma characteristics for wines made reductively were dominated by spice and matchstick, while WhiteHO displayed fruity and floral notes. Together the use of oxidative and reductive handling of juice, and alcoholic fermentation in conjunction with skin contact, led to specific sensory profiles of the wines.
Grenache blanc grapes were similarly made following two juice handling protocols, reductive (Red) and oxidative (HyOx), which mimicked exactly those of the Albariño juice treatments. Juice was cold-settled and clarified. Prior to alcoholic fermentation both Red and HyOx juice were split into low and high turbidity treatments with turbidity measured as nephelometer turbidity units (NTUs). High turbidity treatments received an addition of yeast cell walls of inactivated yeasts to adjust turbidity. Treatments were thus Red_LowNTU, Red_HiNTU, HyOx_LowNTU, and HyOx_HiNTU. Reductive treatments received 100 mg/L of glutathione on Day 2 of alcoholic fermentation. All treatments received sparging with air at 450 mL/L/h for 30 min at one-third °Brix drop. ORP was monitored throughout juice handling and alcoholic fermentation. Reductive and oxidative sensory profiles of the wines, and their evolution during aging were investigated. Sensory analysis was conducted two times, firstly on freshly bottled wines, and then again after a period of accelerated aging. ORP on the juice averaged 66 mV for Red. Contrastingly, HyOx started at 175 mV, and peaked at 356 mV. However, an upper was likely not reached for HyOx. This observation in conjunction with a decrease of 66 to 69% of total phenolics from the juice to the HyOx wines after alcoholic fermentation, indicates that only partial oxidation of phenolics occurred during hyperoxygenation. Red treatments, however, had increased levels of GSH, 9 mg/L for Red_LowNTU and 8 mg/L for Red_HiNTU after the completion of alcoholic fermentation, while HyOx treatments had less than 1 mg/L. After six months of bottle aging, Red_LowNTU and Red_HiNTU had 4 mg/L GSH suggesting greater protection from oxidative degradation during aging. Sensory analysis after accelerated aging supported this assumption as HyOx wines evolved more into oxidative aromas (i.e. caramel), while Red treatment wines evolved towards reductive aromas while maintaining some fruit aromas as well. Notably, oxidative and reductive handling of the juice, had much greater impact on chemical and sensory parameters than adjustments to turbidity did.
Loureiro grapes followed two juice protocols: reductive (Red) and oxidative (HyOx). Red received 50 ppm SO2, 160 mg/L ascorbic acid, and N2 mixing at 500 mL/L/h for 2 h. HyOx received no additives and underwent hyperoxygenation with air at 250 mL/L/h for 2 h. After cold-settling and racking, each juice was split into low and high turbidities. Low turbidity treatments had a target of 45 NTUs and high turbidity treatments were adjusted to a target of 650 NTUs. Adjustments to turbidity were made with the remaining lees from each treatments’ respective juice tank. Fermentation was completed in four days for all treatments with one pumpover occurring on Day 2 and temperatures peaking at 25.6°C. The day after inoculation one full volume pumpover was performed. Very few chemical differences were observed in these wines. However, differences were observed in total phenolics and in linalool contents. Total phenolics at the end of alcoholic fermentation were 66 mg/L for Red treatments and 27 mg/L for HyOx treatments. Specific phenolic compounds measured included tannins, flavonols, and flavan-3-olsn which were at similar levels for all treatments and below sensory thresholds. Concentrations of esters and terpenes were similar for all treatments with the exception of linalool which ranged from 277 μg/L in Red_HiNTU to 400 μg/L in Red_LowNTU after alcoholic fermentation. With a low starting °Brix of 18.5, high initial YAN (224 mg/L), and a depletion of most sensorially active phenolic compounds in both Red and HyOx treatments, reductive versus oxidative handling of the juice and turbidity adjustments had only minor effects on the resultant wines.
Vermentino grapes were made following three different juice protocols: Con (50 ppm SO2 and CO2 blanketing), N2 (50 ppm SO2, 160 mg/L ascorbic acid, and mixing with N2 gas at 950 mL/L/h for 2 h), and HyOx (mixing with air at 950 mL/L/h for 2 h). Juice was cold-settled and racked. Air was further split into AirVL3 and AirMon. Con, N2, and AirVL3 were inoculated with VL3 yeast. AirMon was inoculated with Montrachet yeast (UCD 522). Glutathione was added to N2 and AirMon treatments on Day 2 at 100 mg/L. ORP was monitored throughout the juice stage and alcoholic fermentation. Fermentations with VL3 yeast were completed in nine days while AirMon was sluggish, but eventually completed alcoholic fermentation. Average ORPs at the juice stage were 127 mV for Con, 58 mV for N2, and 310 for Air. Air reached a peak of 420 mV which it maintained during the course of hyperoxygenation. These differences in ORP are evidence of the effectiveness of antioxidants and inert gases in excluding oxygen and deterring the formation of oxidation products in the medium. Additionally, the peak ORP in Air suggested the cycling of redox active species to their oxidized forms, reaching some form of oxidative limit. However, the contribution of species derived from enzymatic oxidation remains elusive. This has implications for chemical oxidation. Indeed, Con and N2 juice had significantly higher total phenolics, 140 mg/L and 120 mg/L respectively, than AirVL3 and AirMon which had 21 mg/L. All treatments showed similar absorbance at 420 nm. Notably, OAVs of volatile sulfur compounds (VSCs) on finished wines were 12 for Con, 67 for N2, 4 for AirVL3 and 1 for AirMon. Total ester OAVs were similar across all treatments. As such, the aromatic profiles of the wines differed dramatically. Levels of GSH and GRP in finished wines varied by treatment, with AirMon retaining 3 mg/L of GSH after three months of bottle aging, suggesting added protection against oxidative degradation.
The studies herein investigate the effects of reductive and oxidative handling at the juice stage and during alcoholic fermentation on wines, and their effects on ORP. Reductive versus oxidative handling led to stylistic differences with HyOx wines being generally more fruit forward, while Red wines tended to have greater aging potential, but also the potential to display excessive notes of reduction (i.e. due to VSC generation). Turbidity adjustments showed relatively minor differences. Furthermore, antioxidant additions, hyperoxygenation, skin contact, turbidity adjustments prior to alcoholic fermentation, and yeast strain for inoculation were employed. Chemical and sensory effects of these white winemaking practices are herein described and discussed primarily in light of potential practical implications.