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Grape seed color not helpful in assessing wine tannins

By Scott Weybright, College of Agricultural, Human & Natural Resource Sciences

HarbertsonPROSSER, Wash. – For decades, if not centuries, the changing color of a grape’s seed has played a role in determining when winemakers harvest grapes.

After some complex experiments, though, researchers at Washington State University have determined that seed colors don’t have the long-held impact, contrary to wine mythology.

“Grape growers and winemakers have looked at the coloration of grape seeds, believing it can tell the amount of tannins the grape will impart into wine,” said Jim Harbertson, a professor in WSU’s viticulture and enology program.

“It just wasn’t true,” he said. “Winemakers should probably pay attention to the color and taste of grapes, but not worry about seed color for tannin extraction.”

Extracted tannins not affected

The old theory held that immature grapes, with green seeds, would impart more tannins into wine. Harbertson said people generally don’t like too many tannins in wine because they add bitterness and astringency.

More mature grapes, whose seeds become more brown than green, have fewer tannins. But the WSU researchers found that didn’t really impact the amount of tannin extracted into the wine.

“My grad student, Federico Casassa, and I thought it would be a good opportunity to evaluate a long-standing myth,” Harbertson said. “We thought we would get way more tannins from green seeds and prove the myth.”

To test the myth, the team made wines from both immature and overripe grapes, then tested for the number of tannins in the finished product. They waited about 30 days between picking the fruit, then double-checked the results.

“We repeated the experiment twice because we found a result that a lot of people wouldn’t necessarily believe,” Harbertson said. “We found the same thing every time, so decided the results were valid.”

Winemakers encouraged to re-evaluate

Because tannin extraction in red winemaking is influenced by alcohol concentration and different skin and seed contact times, the researchers couldn’t make the same wine from both maturities because the end result wouldn’t be comparable. During each harvest, they took a portion of the fruit and added sugar, or removed juice and added water, in an attempt to mimic the amount of sugar present at the different grape maturities.

This allowed the team to account for the difference in alcohol content. The researchers also changed the maceration period, or length of time the grapes were softened by soaking.

The only difference, Harbertson said, was that in both the immature and ripe grapes, extended maceration led to increased tannins – but at essentially the same amounts, independent of when the fruit was picked or how much alcohol was present. The extended maceration allowed more tannins to be extracted into the wine.

The researchers aren’t telling winemakers to make changes to how they’ve operated for years, but they do hope the study leads to re-evaluation of when winemakers harvest their grape crop.

“We’d like them to just re-think how they make their decisions to harvest,” Harbertson said.
“We know a lot of older winemakers have spent a lot of time looking at grape seeds. But we’ve been pleasantly surprised that they’re taking our study seriously and have been interested in learning more about what we’ve found.”


James F. Harbertson, WSU professor of enology, 509-372-7506,



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