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By Steve Gorman and Ian Simpson (Reuters) - Wine connoisseurs may find some of northern California's 2017 vintage to be a bit smokier than usual on the palate. Wildfires have damaged or demolished at least 13 Napa Valley wineries, a trade group for vintners there said on Tuesday, but experts say smoke rather than flames may pose a wider risk to the delicate grapes still waiting to be picked. A spate of deadly wind-driven fires hit the world famous wine-producing regions in Napa, Sonoma and adjacent counties on Sunday night, near the end of the grape harvest, though 10 percent to a third of the crop remains on the vines, according to various estimates. Those fully ripened grapes, if exposed to sustained heavy smoke, would be especially vulnerable to contamination by the pollutants, said Anita Oberholster, a professor of viticulture and enology at the University of California at Davis. Smoke taint, in the parlance of the industry, occurs when smoke is absorbed into the plant and concentrates in the fruit, altering a grape's chemistry, and ultimately its taste and aroma - but not in a way likely to win wine-tasting ribbons. "It can have a severe smoke aftertaste. In really bad cases, it can really have the taste of an ashtray," Oberholster said. However, in some slightly tainted wine she has sampled, the effect "was so subtle that I thought it added complexity." Red grapes are more susceptible than white because white wine is produced with less contact from the grapes' skin, where the smoke-induced compounds accumulate, she said. Experts said those compounds, which bind to sugars in the grape, persist after bottling and intensify as the wine ages. Following 2008 wildfires in northern California, producers bottled smoke-tainted wine under a special label and sold it to buyers with the caveat that it was a fire-damaged vintage, Washington State University enologist Jim Harberston said. He said the trend toward higher "hang time," the practise of leaving some grapes on the vine longer to achieve a higher alcohol content for a richer, bolder wine, may have backfired on some growers this year. Barry Waitte, owner and vintner of Tamber Bey Vineyards in the town of Calistoga, said on Monday that smoke and ash around his property was light. "I'm not seeing much right now, just a little tint of yellow because of it in the air," he said by telephone. "If this happens and continues for four or five days, then maybe we’ll see some effect." Oberholster, however, said grapes could be affected by exposure to extremely heavy smoke for as little as 30 minutes. She said roughly a third of the grapes in the overall fire region remained to be harvested. But the Napa Valley Vintners association said 90 percent of the fruit raised by its 550 members was already picked. The group also said that most grapes still on the vine were Cabernet Sauvignon, a thick-skinned variety it hopes would be at relatively little risk. Neighbouring Sonoma Valley, which produces even more grapes than Napa, was also hard hit by fires, but information about the scope of damage to wineries there was still spotty. Napa, Sonoma and adjacent counties collectively account for 12 percent of the wine produced throughout California, but they are home to the state's most highly prized grapes, experts said. Retail sales of California wines total nearly $32 billion a year and the state ranks as the world's fourth leading wine producer after France, Italy and Spain, according to the Wine Institute's 2015 economic survey. (Reporting by Ian Simpson in Washington and Steve Gorman in Los Angeles; Writing by Steve Gorman)