Winemakers call it "hang time" -- letting grapes ripen on the vine as long as possible.
Lately, grape growers say they feel as if they're the ones being hung out to dry.
A dispute between California's grape growers and buyers over the right time to pick fruit has become a hot topic in the state's $15-billion wine industry. The timing of the harvest isn't just a matter of taste: There are millions of dollars at stake.
The problem, as growers see it, is that the longer grapes hang on the vine in the late summer heat, the more water evaporates from the fruit.
That can reduce the weight by as much as 25%. Napa Valley farmers are paid by weight -- an average of $4,000 a ton for Cabernet Sauvignon grapes -- and of course don't like to see potential profits disappearing into thin air.
Especially irksome to growers is that some wineries add water back into a vintage. Growers say they should have been paid for that water content upfront.
For their part, winemakers say that leaving fruit until it begins to wrinkle is the best way to get the high sugar content and concentrated flavors needed to make the high-alcohol, fruity red wines demanded by consumers.
Winemakers see growers' complaints as just a ploy to get more money.
"We want the fruit to hang as long as it possibly can," said Bill Brinton, owner of Charles Creek Vineyard in Sonoma. "You get different flavors when you let the grapes hang, and that seems to be what the public likes."
The practice of hang time, also called physiological ripening, has been around at least 15 years, although it has become prevalent only in the last four or five years.
During California's recent grape glut, growers weren't about to raise a stink out of fear they would endanger badly needed sales.
But as the market for wine grapes has tightened over the last year, the issue has taken center stage.
The two sides are set to air their differences today at an industry meeting on the issue in Napa Valley that is expected to draw as many as 500 people, prompting organizer Andy Beckstoffer to worry that there won't be enough room for everyone at the Native Sons Hall in St. Helena.
"The real issue is whether there is any science behind this concept of hang time," said Beckstoffer, one of the largest independent growers in Napa Valley.
Besides losing money, growers worry that letting the grapes hang an extra two to three weeks is causing long-term damage to their vineyards by decreasing production and shortening the life of vines.
With the expense of replanting a vineyard running $15,000 to $40,000 an acre, depending on the location, growers want to keep their vines as productive as possible.
"We are finding our bunch counts in the spring declining," Beckstoffer said.
Although there is no argument that leaving the grapes on the vine reduces their weight, it is less clear whether there are long-term physiological effects on the vine.
Andrew Walker, a viticulture professor at UC Davis, said that "surprisingly little research" has been done on whether the practice improves wines -- as vintners claim -- or ultimately does damage.
Nick Dokoozlian, vice president of viticulture for E. & J. Gallo Winery, believes there could be damage when the practice is combined with other techniques used in California -- including hillside planting, shallow-soil farming and deficit irrigation -- to reduce the vegetative growth of the vine and improve berry quality.
"Hang time in and of itself is not the culprit," Dokoozlian said.
He acknowledged that grape yields have declined in the last two harvests, but he said statewide weather conditions were more to blame than farming practices.
The debate has leaped the Pacific Ocean to Australia, where a similar style of wines is popular.
Australian viticulturist Richard Smart, who runs an international vineyard consulting business, agrees there is no evidence that delayed harvest alone damages the vines. But he also said there is no evidence that leaving the grapes on the vine longer improves wine quality.
"There are at least a dozen factors that affect wine quality, and once the grapes reach a certain degree of ripeness, the time of harvest is one of the least important," Smart said.
What he calls the "mythology" of hang time has caught on quickly on both sides of the Pacific, which surprises Smart because the wine industry prides itself on being based in science and technology.
"We are seeing winemakers come into the vineyard, taste the fruit, examine the color and taste of the seeds, feel the skin and then pronounce the date of harvest," Smart said. Much more import for the quality of the wine, he said, are the earlier steps such as pruning and irrigation.
Linda Bisson, a UC Davis winemaking professor, said the taste of a grape on a specific day can't predict how the wine eventually will turn out; taste is subjective compared with empirical measures such as sugar and acid levels.
Vintners aren't convinced.