Tannin varies by region

Samples will be made into wine to compare differences between Merlot grown on rootstocks and own-rooted vines.

Dr. James Harbertson’s wine phenolic research could help Washington wine grape growers as much as winemakers by providing baseline information about tannin levels of wines –produced in their region.

The subject of wine phenolics is not just for winemakers and enologists. Wine phenolics are responsible for important wine attributes and include compounds like tannins, anthocyanins, polymer pigments, and antioxidants. Grape tannins, which are developed in the vineyard, primarily in the seeds and skins, help stabilize the color in wine and cause the "mouth astringency," explained Harbertson, Washington State University Extension enologist.

Harbertson has a particular affinity for measuring tannins in wine. As a graduate student at the University of California, Davis, he worked with UCD’s Dr. Doug Adams to develop a simple and inexpensive tannin assay that measures wine phenolics.

Harbertson recently completed research that measured the range, distribution, and average values for total phenolics, tannin, anthyocyanin, and polymer pigments of more than 1,300 finished red wines from Washington, California, Oregon, and Australia. The wines represented samples of Pinot Noir, Cabernet Sauvignon, Syrah, Merlot, and Zinfandel. His research has been submitted to the American Journal of Viticulture and Enology for publication, but the procedure is simple enough to do in a winery laboratory, he added.

Different regions

The last time a wine composition study was done on Washington wine was more than 20 years ago, according to Harbertson. Since then, the laboratory methods have changed, and grapes are being grown in new areas.

"I’d always been told that different places in Washington had different phenolic levels in the wines," he said. To see if that was really true, he gathered commercially available wines from the major wine regions (Yakima Valley, Columbia Valley, Walla Walla Valley, and Red Mountain) and analyzed tannins and other phenolics. Of the 1,300 wines sampled, more than 350 came from Washington.

From the survey, Harbertson found differences between the tannin levels of Cabernet Sauvignon and Merlot and between different regions. Cabernet tannin concentrations were higher than Merlot.

Red Mountain

The wine region that really stood out in the Washington survey was Red Mountain, located near Benton City. The Red Mountain appellation had the highest tannin levels for Merlot and Cabernet, while the Walla Walla Valley appellation, or American Viticultural Area, had lower tannin concentrations in Cabernet Sauvignon than other Washington AVAs, he reports.

"Now we have proof that Red Mountain is different," he said. "And we have evidence that there are differences in tannin levels and phenolics between regions.

"We now know the differences are there, but we haven’t explained the ‘why.’ The next step is to bring in viticulture expertise to look at things like soil and irrigation. I’m just the chemistry guy."

He speculated that Walla Walla could be lower in tannins because it receives more rainfall than Red Mountain, and it may be more difficult to control soil moisture than in more arid regions.

Harbertson said that he is part of an important three-way team at WSU that includes viticulture, chemistry (enology), and sensory science. Dr. Carolyn Ross, who teaches sensory analysis of food and wine at Pullman, is analyzing consumer preferences of wine tannin levels

Grape and wine tannins

Harbertson noted that when reviewing tannin concentrations, one must remember that there is not a straightforward relationship between tannin levels in the grapes (seeds, skin, and pulp) and in the wine. Adams, his mentor at UCD, found that only 50 percent of the tannins are released from the skin and seeds, and that the tannin level in grapes cannot be used to predict the tannin concentration of finished wines.

"Much of the tannin release depends on how the winemaker deals with the skins," Harbertson said. "It’s like the differences between soaking a tea bag for 30 seconds versus 3 hours. It’s really about knowing the potential of the grapes and then it’s how the winemaker goes after it."

Tannin importance

So, what do tannin levels mean to the grower? Harbertson likens them to growth charts used by pediatricians to monitor the growth progress of infants and children. "If you see that your wine is in the 13th percentile of what is the average tannin range for your region, then you know there could be a problem with your grapes."

He believes his tannin survey provides baseline numbers for growers that can be useful in gauging where the grapes (and wine) fit in terms of the regional average. "It gives you a bouncing-off point," he said. When a winemaker comes back to a grower and says ‘your grapes aren’t tannic enough,’ the grower will know if his or her wine numbers are average for the area, Harbertson explained. If the grower is half of the baseline average, then he or she knows there is a problem and can consider if there are things in the vineyard that could be done to manipulate the grape tannin levels.

By knowing the regional average, the grower can better understand where his or her grapes fit in the overall picture and be more knowledgeable when discussing wine styles and tannin levels.

"If your wines are not tannic enough, can you manipulate tannins in the vineyard, or are the levels beyond what a winemaker can do to improve the wine?" he asked. Growers may be able to influence grape tannin levels in the vineyard, by implementing regulated deficit irrigation practices or changing the vigor of the vine. Research has shown that changing vine vigor can affect the tannin –concentration, Harbertson said.

Winemakers can also gain from the tannin study by understanding where their wines fit in the general wine population. He is also hoping that the data will encourage wineries that are not doing lab analyses on finished wines to use the assay as a winemaking tool.