Available at: https://digitalcommons.calpoly.edu/theses/3120
Date of Award
6-2025
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
Cap management during alcoholic fermentation of red wines plays a critical role in modulating the phenolic and volatile extraction from grape solids, influencing the homogenization of chemical and temperature gradients, while imparting oxygen to the must. Different cap management alternatives are also thought to influence the evolution of the oxidation-reduction potential (ORP). It is critical to understand how different cap management protocols affect these variables to design a cap management protocol tailored to wine style and varietal. This thesis investigated the effects common commercially applied cap management protocols, as well as the efficacy of novel, automated gas mixing protocols as potential alternatives to labor-intensive cap management. Additionally, this research sought to investigate how some specific and contrasting phenolic and chemical makeup between grape varietals may respond to varying cap management protocols.
In the 2023 and 2024 vintages, contrasting cap management protocols were applied to Pinot noir and Petite Sirah wines from the Central Coast of California. These include the commercially traditional protocols of punch-downs and pump-overs, as well as no cap management with and without various automated air or nitrogen gas mixing regimes. The ORP was monitored during alcoholic fermentation of the gas mixing wines of the 2023 vintage, and all wines of the 2024 vintage. The results detailed how cap management regimes can be designed to produce targeted ORP evolutions. Additionally, the results detailed the effects of these protocols on the phenolic and volatile composition of the wines.
In the 2023 vintage, Pinot noir and Petite Sirah wines were produced with the following cap management protocols: punch-downs (PD), pump-overs (PO), no cap management (NoCapMgmt), and two gas mixing protocols, whereby air or nitrogen (N2) gas was injected into the bottom of the fermentation vessel for 1 h, twice daily, (AirMix; N2Mix). These varietals were chosen as representations of how wines of typically low (Pinot noir) and high (Petite Sirah) phenolic potential may respond differently to contrasting cap management protocols. ORP of AirMix wines reached peaks of 340 mV and 240 mV in Pinot noir and Petite Sirah, respectively, while N2Mix wines were consistently below −50 mV during alcoholic fermentation. At pressing, PD wines contained more total phenolics than PO wines in Pinot noir, but not in Petite Sirah. However, PD wines of both varietals in the 2023 vintage increased flavan-3-ol concentrations when compared to all other treatments. NoCapMgmt wines contained >50% increases in esters, when compared to PD wines, revealing the effects of physical mixing on CO2 stripping of volatiles. Of all volatiles, the largest differences were seen in isoamyl acetate. AirMix wines showed decreased astringency and increased red fruit character, while N2Mix wines had higher color saturation as perceived by sensory analysis. Neither AirMix nor N2Mix wines showed detectable reductive aromas at 3 months of bottle aging despite receiving no cap management other than gas injections.
In the 2024 vintage, Pinot noir and Petite Sirah wines were produced using five and six cap management protocols, respectively: PD; PO; AirMix; and N₂Mix; as well as ORP-control through air injections triggered by ORP measurements ≤-40 mV (RedoxConAir), and, in Pinot noir only, N2 additions paired to inject gas simultaneously with respective RedoxConAir replicates (RedoxConN2). ORP, reduced glutathione (GSH), and oxidized glutathione (GSSG) were monitored throughout a 10-day alcoholic fermentation in all wines. This study revealed the temporary effects of oxidative pump-overs on the ORP of wine. PO wines exhibited peaks in ORP of ≥100 mV during pump-overs, whereas PD wines increased 20-40 mV during punch-downs during peak fermentation. At pressing, the ratio of GSH:GSSG was correlated with the prevailing ORP of the wines before pressing (p< 0.04), suggesting the glutathione redox couple is an effective marker of the oxidative or reductive history of a wine. AirMix wines showed similar phenolic losses to the 2023 wines. In further continuity with the 2023 vintage, PD wines extracted more flavan-3-ols, and wines with the least physical mixing and degassing (2023, no cap management; 2024, RedoxConAir) retained significantly more esters, namely, isoamyl acetate.
Collectively, these findings confirm that different cap management protocols distinctly affect the ORP of wine fermentations, and in turn affect their chemical makeups. Gas injection protocols can serve as viable, automated alternative or supplementation to manual cap management in cases of favorable fermentation size and geometry. Differences in phenolic response between Pinot noir and Petite Sirah underscore the importance of varietal context in designing a cap management protocol. However, the ORP does not appear directly related to the oxidation or extraction of phenolic compounds and alternatively is better suited as a tool to regulate volatile sulfur compound production and yeast performance. Lastly, automated gas mixing systems present an opportunity to design fermentation strategies that align with stylistic goals while improving efficiency and consistency in wine production.