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Cross section of grape and seeds. <b>(istockphoto.com)</b>

Cross section of grape and seeds. (istockphoto.com)

Winemakers have long held that the changing color of a grape’s seed serves as a harvest signal, that the greener the seed at harvest, the more tannin characteristics and bitter taste imparted to the wine.

However, researchers at Washington State University have found that changes in seed color have less to do with wine tannins than previously thought.

In fact, their results after one year’s study were so different — and opposite — from what was previously thought, they completed a second year of study to confirm the results.

“It’s an interesting one, mostly because we don’t think winemakers should be paying attention to seed browning,” said James Harbertson, associate professor of enology at the Chateau Ste. Michelle WSU Wine Science Center in Richland, Washington. “It just doesn’t really change how much tannin gets extracted in the wine.”

The project was born, Harbertson said, by an argument. He and former WSU doctoral candidate Federico Casassa of Argentina, now an assistant professor of enology at California Polytechnic State University’s wine program, both believed that if winemakers have more green seeds at harvest, they’ll end up with horrible, bitter-tasting wine that no one will want to drink.

The argument centered on whether the theory could be tested.

“He didn’t think we could actually do the study as we designed it. It was a debate about experimental design,” he said. “We came up with three-fourths of the parameters and had discussions about controlling for different things. It was fun, because we really got into it and eventually finished the experimental design.”

Embarking on the research

A previous study in Australia established a color schematic showing the development of wine grape seeds, with the color progressing from an olive green to an almost coffee-colored brown.

The study, however, lacked information about tannin concentration and what happens if wine is made at each of these different color points, Harbertson said.

The researchers selected a typical cultivar in Washington, Merlot, which is one of the earlier-ripening red wine grape varieties in the region, and the Paterson Ranch of Ste. Michelle Wine Estates in Paterson, Washington.

The vineyard, which was planted from 2000 to 2003 with clone 3 and row spacing of 7 feet in a north-south orientation, is drip irrigated.

The grapes first were picked relatively early, Sept. 22, 2011, at a soluble solids of 20°Brix.

Few growers and winemakers would consider picking 20°Brix fruit today, Harbertson said, and the researchers figured that “if you don’t see any difference in tannin extraction and sensory profile at this level, you won’t see it at all.”

They divided the must — the freshly pressed juice that includes the stems, skins and seeds — into two lots. One served as a control lot, while sugar was added to the second lot to push it to 25°Brix and to increase alcohol levels, a process known as chaptalization.

The researchers made wine from both lots twice: after a standard maceration time of 10 days and after an extended maceration time of 30 days.

Maceration is the cold-soaking process by which the grape skins, seeds and stems leach the phenolic materials of the grape — tannins, anthocyanins or pigments, flavor compounds — into the must and, ultimately, the juice. It’s where red wine receives its color.

The researchers harvested fruit again 33 days later on Oct. 25, 2011, when the fruit finally ripened to 25°Brix. They repeated the process, except this time, instead of adding sugar to one lot, they bled the juice and watered it down to reduce it to 20°Brix. This enabled them to work with more mature fruit with lower alcohol levels.

They followed the same process again in 2012, harvesting on Sept. 13 and Oct. 17.

The results

Wines without extended maceration had significantly higher anthocyanin content, saturation and red color component, whereas the extended maceration wines had enhanced tannin extraction from seeds, lower anthocyanin content and lower saturation.

The sugar level and alcohol content showed no significant effect on tannin and anthocyanin extraction, the study found.

In terms of sensory profile of the wine, those made under extended maceration showed higher astringency, lighter and yellower color components and cooked vegetal aromas. Chaptalization of early-harvest fruit to 25°Brix shifted the sensory profile to a sweeter taste, alcoholic, floral, with chocolate/caramel attributes and higher astringency.

The later harvest date, meanwhile, had an even more positive effect on the sensory profile of the wines than maceration length and sugar and alcohol levels: Wines from the late-harvest fruit were defined by viscous mouthfeel (the wine feels heavier, thicker in the mouth), sweet taste and fruit-derived aromas.

Overall, unripe fruit and the application of extended maceration had a negative impact on the sensory profile of the wines, whereas chaptalization of unripe fruit yielded wines with an improved sensory profile.

“Seed tannin extraction didn’t really matter if you had really unripe fruit at 20°Brix versus really ripe fruit at 25°Brix, where the grapes are more likely to be picked nowadays,” he said. “We saw there was a little bit of difference between vintages, but for the most part, it was constant depending on when you picked. When we did the sensory test, we really couldn’t differentiate the astringency.”

Harbertson said some growers might be more concerned about a red variety that is picked even earlier — Pinot Noir. “Those growers say their seeds are really, really green, and the longer they ripen, the less they worry about it,” he said. “My guess is that it takes shorter time to get from 20° to 25°Brix, so you have even less to worry about.”

The research tells winemakers and growers that seed color and tannins aren’t as important a factor at harvest, Harbertson said. Instead, they should place the emphasis on those factors that weigh more heavily and are easier to measure or taste, such as fruit color, flavor and acidity.

“Most of those things you can measure quite easily, and tannins are actually quite hard to measure,” he said. “The message is that you don’t have to spend all this money on analysis of tannins in the vineyard. It’s not as mission critical, especially if the vineyards are trying to do some of that themselves.”

The study was published in the American Journal of Enology and Viticulture in 2013. •

– by Shannon Dininny