Fresno State Winery winemaker Matt Brain with stainless steel drums used for research on the role of glutathione in preventing oxidation and preserving aromas in white and rosé wines.

Fresno, Calif.—Fresno State Winery winemaker Matt Brain discussed a new research project and current knowledge about glutathione in winemaking at Fresno State Grape Day, held biennially on the California State University, Fresno, campus by the Department of Viticulture & Enology, the Viticulture and Enology Research Center (VERC), and the Jordan College of Agricultural Sciences and Technology.

Brain joined Fresno State in 2015 as the winemaker at the 10,000-case production Fresno State Winery and teaches wine production courses for enology students. He has worked extensively with grapes from Central Coast vineyards, currently as co-owner and co-winemaker for the 1,600-case production Baker & Brain brand that produces Pinot Noir and Rhône varietals. Past winemaking experience includes stints at Tolosa Winery and Edna Valley Vineyards in San Luis Obispo County.

While working at Edna Valley he became interested in the role of glutathione in preserving freshness and aromas in aromatic white varieties and rosés. “I liked these wines for their aromatics after fermentation, but after they were in bottle, they could prematurely lose their freshness and aromas,” he said.

Varieties such as Riesling, Sauvignon Blanc, Chardonnay and others can have desirable but fragile aromas associated with volatile thiols that are commonly described as box tree, guava, passionfruit, grapefruit, pineapple, green apple, banana, floral, honey, and rose.

Glutathione in grapes and wine
Glutathione (GSH) is a tripeptide antioxidant (glutamate, cysteine, glycine) found in plant and animal cells, and it can play an important role in protecting white and rosé wines against oxidation. Significant levels of GSH correlate with higher levels of desired fruit and floral aromatics in finished wine, and slower oxidation rates. GSH is more important as an antioxidant in white wines, as compared with red wines that have tannins with antioxidant properties for protection during aging.

GSH can potentially be managed in the vineyard. GSH levels in grapes correspond with nitrogen (N) status, and healthy, properly fertilized soils can improve N and GSH content in grapes. Recent research has investigated methods to maximize GSH levels in grapes and wine. Brain is focused on understanding the protective power of GSH in production scale winemaking, and how GSH and sulfur dioxide work synergistically to preserve fresh wine aromas.

Brain said it is currently not legal to add glutathione to wine, however, certain yeasts produce more GSH as part of the fermentation process. Brain said one such yeast is Laffort Zymaflore X16. In addition, yeast nutrition aid products are available that have GSH precursors (cysteine, N-acetyl cysteine) that can be consumed by yeast to produce GSH and increase levels in the wine during fermentation. One product is “Fresharom” from Laffort USA, designed to increase aromatic and aging potential and inhibit browning in white wines.

Brain’s research at Fresno State is part of a project started in 2015 through the Laffort Applied Research Cooperative (ARC) in which multiple wineries work on a single trial using the same defined protocols. Wines being tested at Fresno State include Semillon, Barbera Rosé and Cabernet Sauvignon Rosé. Other California wineries are doing in-house trials with Sauvignon Blanc, Riesling and Chardonnay. Additional wineries and more varieties will be included in the ARC project this year and in future vintages.

Under the ARC trial protocol, fermentation begins in standard commercial stainless steel tanks. After the wines go through about one-third Brix depletion, they are then transferred into stainless steel drums. Fresharom is added as a fermentation nutrient to create lots with higher GSH levels. Lower GSH level lots continue fermentation in drums without Fresharom additions. The wines are bottled with high and low GSH levels, and at a mid-level with a blend of the two. Each of these three GSH level lots are bottled with three different levels of sulfur dioxide—0, 10 parts per million (ppm) and 20 ppm. Bottle closures used are Diam 10 technical corks designed for low oxygen ingress.

During bottle aging, the wines will be analyzed for sulfur dioxide and GSH degradation rates over time, and for levels of dissolved oxygen and oxygen appetite. The wines will be analyzed at three month intervals after bottling for 18 months, and will also be evaluated by a sensory panel for preference and fruit expression. “A goal of this project is to create a chart, similar to the chart created years ago on the relationship of wine pH to sulfur dioxide additions,” he said. “My idea is to propose new SO2 addition targets based on the GSH levels in wines.”

As an example, it is possible that with wines having GSH levels of 10 ppm or less, winemakers should maintain their current levels of sulfur dioxide, but with GSH levels above 10 ppm, the sulfur dioxide addition rate could potentially start being reduced. It is generally believed that GSH levels of 20 ppm and above in wine are needed to offer significant protection. “Understanding how GSH levels impact aging curves will allow winemakers to safely reduce SO2 levels through aging and at bottling, allowing the production of more expressive and desirable aromatics in relation to the expected wine sales and wine consumption time frame,” he said.

Grape Day attendees tour new research facility
In addition to lectures and research presentations, the Grape Day event, which took place in early August, included a tour of the campus vineyards that produce wine, raisin and table grapes managed and processed by students as part of their course work and educational training. In addition, attendees received a tour of the new Jordan Agricultural Research Center (JARC) that held a grand opening in May and is receiving the finishing touches on laboratory and research space to begin use during the fall 2016 semester.

The JARC, located about a half mile from the V&E Department and VERC facilities, is a 30,000 square foot, three-story building designed as an interdisciplinary agricultural research center that will allow researchers across all ag related sciences to perform individual projects, as well as collaborate on projects of mutual interest. Each floor is roughly divided in half, with laboratory facilities on one side of the hallway, and space for interaction and meeting among researchers, students and faculty on the other side of the hallway.

The JARC includes a Sensory Evaluation Laboratory that will be used by the V&E Department for wine sensory research and product evaluations, and by other food science researchers. Other lab space will be used by the California Water Institute to coordinate campus water-related programs and research. A robotics laboratory will be used by ag scientists and engineers to develop and test robotic instruments and sensors for ag applications. Instrument analysis laboratories will enable students and faculty to test, calibrate and experiment with new instrument technologies for research and field applications.

Individual laboratory spaces have been dedicated for plant physiology, plant pathology, microbiology, entomology, plant genomics, and bioenergy systems. Although V&E Department research will continue at the VERC facility, the JARC is available for additional projects and to collaborate with other ag science and engineering departments on multi-disciplinary projects.