A scientist from Fresno State’s Viticulture and Enology Research Center (VERC) will soon join the “hang time” debate with San Joaquin Valley wine grape growers.

Viticulture research specialist Dr. Sanliang Gu is examining several “regulated deficit irrigation” (RDI) strategies using extended hang time in a study to determine how the methods affect quality and color of grapes grown for wine in the valley.

Hang time refers to that point at which the grapes are ready for harvest but are allowed to “hang” a few days longer, resulting in increased sugar content and further accumulation of acids and color, Gu said in explaining his research. The right combination of these characteristics can make for a successful fermentation and outstanding wine. A hang time that’s too long can result in grapes with a sugar content that’s too high, prompting sluggish or “stuck” fermentations and a significant loss of wine quality.

Grower opinions on hang time vary from being “required” for better wine quality regardless of variety, location, and vintage, to it being absolutely unnecessary, with only negative impacts on fruit quality and vine health, Gu noted.

The debate continues because relatively little research has been done in the San Joaquin Valley to determine the relationship between hang time, fruit quality and vine health, Gu said. One reason is because premium wine grape growing is relatively new to the Valley, and summer weather conditions here can make hang time management especially challenging.

“Under the stressful temperatures and humidity conditions during and beyond the ripening period, hang time may negatively impact the grapevine’s capacity and functionality the most in this region, especially under regulated deficit irrigation, which has become a common management practice throughout the wine grape industry in California,” Gu said. “We need to find a management strategy to synchronize the sugar accumulation and color-flavor development to improve fruit quality and minimize or eliminate the need of hang time.”

Gu’s goal is to maximize fruit sugar development with color and flavor using regulated deficit irrigation, using a minimum amount of hang time, this avoiding damage to the vines.

The experiment is being conducted in two locations: a 10-acre vineyard of Cabernet Sauvignon on Fresno State’s university farm, and a 75-acre block of Syrah farmed by a cooperating grower in the Fresno area. Treatments will include conventional drip and regulated deficit irrigation.

The deficit irrigation treatment will be conducted using information based on weather conditions, soil moisture and vine vigor. Trials will include the use of a pioneering computer-based technology that will govern the irrigation system. The goal will be to reduce water use by 30 percent to 60 percent of conventional irrigation practices, Gu said.

In combination with the deficit irrigation treatments, three hang time treatments will be used: grapes will be harvested at 24 degrees on the Brix scale, which is a conventional harvest date; at two weeks after conventional harvest date, or at 28 Brix; and fruit not harvested.

Regulated deficit irrigation offers other benefits in addition to potentially better wine quality, Gu noted.

“A reduction in water use means less electricity needed to pump water and pressurize the irrigation system, and therefore represents an economic incentive for the grower. In addition, when managed properly, RDI results in less shoot growth and generally more open canopies that increase the exposure in the fruit zone to sunlight and air movement,” Gu said. “This exposure results in better fruit color and flavors, while decreasing the potential for diseases and pests.”

Results of the study will help to answer questions and concerns about hang time with sound scientific research and justification for the practice of hang time. Gu anticipates release of results and analysis next year. Overseeing the study, along with Gu, is VERC director Robert Wample.

For more information on the project, Gu may be contacted at sanliang@csufresno.edu.

(Copy by Steve Olson of the California Agricultural Technology Institute
at Fresno State.)