Growing & Winemaking


Seed Removal Remains Challenging

January 2010
by Dan Berger

About 18 years ago I had a rare and candid conversation with Julio Gallo. During a lunch break, when Gallo was mixing the ingredients for a tossed salad, I asked him if he had ever thought about the effects of seeds on the tannin structure of red wines.

Yes, he said, he had thought of it, and in fact back in 1947 and 1948 he tried a couple of different experiments aimed at removing seeds from the fermentation tank. One such method, he said, was a sweeping device that removed seeds from tanks. It was inefficient, however, and didn’t remove very many seeds, so he abandoned the idea.

That day in the early 1990s, Gallo mused about that test and asked if there was any news on the subject. Just the usual, I said, mainly scientific research to see if seed removal was a good idea.

By now, in 2010, you’d think winemakers would have a definitive answer about the benefits or drawbacks of seed tannins—and whether removing them before completion of fermentation allows for better tannin structure, which some (inaccurately) allege. (See Tim Patterson’s column, “Everything You Know About Tannin Is Wrong,” Wines & Vines, October 2009.)

Indeed, in the four years since I last wrote an article about seed tannins for Wines & Vines, the process of removing seeds from large-tank fermentations has remained an arduous one, especially for bigger wineries. Yet researchers, equipment suppliers and winemakers continue to sift for answers—and seeds.

The Australian Wine Research Institute (AWRI) has conducted some serious research into this subject, and some of the technical papers published by AWRI include such arcane material as “characterization of proanthocyanidins in grape seeds using electrospray mass spectrometry,” by five researchers in 2003.

Dr. James Harbertson, an assistant professor and extension enologist at Washington State University, Prosser, also has done important work in this field.

To date there is no definitive answer as to whether removing seeds from red wine fermentations makes for better tannin structure, although anecdotal evidence abounds. One such story comes from a winemaker in Colorado who firmly believes in seed tannin removal, contending it works for her because her winery is small.

The assumption driving this issue is that seeds have more bitter and thus more astringent tannins than skin tannins, and that, in theory at least, removing them early is the best course of action to produce a wine that doesn’t have such astringency from post-fermentation maceration—standard treatment for most red wines in California, especially Cabernet Sauvignon.

Among other crucial factors leading to quality red wines, tannin management is one of the most important for quality, and the method used in dealing with tannins—both in the vineyard and in the winery—determines whether the amount of tannin will be appropriate for a particular wine.

Harbertson is a chemist and biochemist, and thus he does not make value judgments as to whether a wine has too little, too much or just the right amount of tannin. That, he said, is for the winemaker to determine. He is mainly interested in how tannins get into red wine

But his research has shown that skin tannins are actually just as astringent as seed tannins (a report on tannins is soon to be published in the American Journal of Enology and Viticulture).

I asked if seed removal wouldn’t be one way to lower total tannins, especially for wines that undergo extended post-fermentation maceration. Harbertson said that was “a fine idea, (but) no one has ever carried the demonstration out.”

He added, “I’m not yet convinced that seed tannins are all that bad. We did extended macerations, and what we saw was that the proportion of skin and seed tannins extracted into the wine are fairly similar to what is present in the fruit. Typically, short macerations favor proportions that are 50-50 and not the 80% seed, 20% skin typically found in grapes.”

Moreover, in higher-alcohol wines, he said, “even though there was proportionally more seed tannin, people couldn’t tell” in sensory evaluations.

He looked at tannins in various wines, one of which was lower in alcohol and another higher in alcohol. “There was more seed tannin in the high-alcohol wines, but the tannin concentrations of the wines were fairly similar.”

Still, anecdotally at least, seed tannin removal is a huge area of investigation. Dr. Christian Butzke, associate professor of enology in the Department of Food Science at Purdue University, said seed tannin removal seems to be on the minds of many wineries around the world.

“You see it in Priorato, in Montepulciano and other areas—newer wineries and some larger, older wineries are buying fermenters that allow for seed tannin removal,” Butzke said.

“(Dr. Roger) Boulton (at the University of California, Davis) has been talking about this subject for 30 years, and with wineries that do extended macerations, this seems to be a substantial problem.

“And yet skin tannins are just as astringent, so you have to be careful about making conclusions. I think it’s good to have more control of the seeds in the tank, but you also have to look at many other issues, such as seed ripeness, seed numbers and so forth.

“For now, I still think this is a work in progress. For one thing, the phenolic aspects are extraordinarily complicated, and there are only a few people in the world who really understand this.”

Jackie Thompson is not one of them, but the winemaker for her own Boulder Creek winery in Boulder, Colo., believes that seed tannin removal is a great tactic for her red wines.

“I noted that the seeds, when you bite into them, have an astringency, and breaking seeds would release that” into the wine. She said that articles from UC Davis indicated that at higher alcohol levels, “when wine is sitting on those seeds, it seems to extract the higher tannins.”

In 2003, the first vintage she crushed fruit, “I did 30 days of extended maceration, and Colorado wines are a little high in alcohol, over 14%, and the juice was sitting on seeds, extracting like crazy.

“And it definitely showed in the wine,” which, she said, wasn’t really approachable until years later. By 2008, she re alized that making red wines that took four or five years to become approachable “didn’t work with our business model; we couldn’t produce wines that tannic.”

Already using délestage twice daily for her earlier wines, Thompson said she decided to try seed removal, as well as making wine with a bit lower alcohol.

She began to remove seeds as early in the process as she could, since it seemed to work with délestage. “I start to remove seeds as soon as the cap forms solid enough to actually get juice out of that bottom drain hole, usually about day three.”

Early on, she cools the fermentation tank to slow the fermentation a bit. At that stage the fermentation is slightly anaerobic, and whole berries gather at the bottom of the tank.

She uses a sump tank with a big screen on the top, and juice from the bottom of the fermentation tank runs into the screen, enabling her to capture seeds in the screen. It’s messy, and difficult, but it works. “It’s also a great time to mix things in, such as nutrients,” she said.

Thompson notices definite differences between grape varieties. “Merlot tends to separate out more quickly: The berries float to the top more quickly. Syrah is the messiest one; the screen gets gummed up with a lot of particulates. With Syrah, I can only sort it to about 10 gallons before you have to clean the screen. With Merlot you get about 30 gallons before the screen clogs up.”

Laurie Hook, winemaker at Beringer Vineyards in Napa Valley, acknowledged that for many of her vineyard-designated Cabernet Sauvignons, she works to remove some seeds, but in a large winery, the process is difficult and varies from tank to tank.

Thompson said Boulder Creek’s small production makes her seed removal easier and gives her a better, grainier (as opposed to grittier) tannin for her Cabernet Sauvignons, Merlots and Syrahs. (Her 2006 Merlot just won a Double Gold medal at Colorado Mountain Winefest’s Commercial Wine Competition.)

I asked Thompson if her seed removal method could work for larger commercial wineries. “They’d have to come up with something that’s automated: a screen and a cleaner.”

Despite the délestage and seed removal, she said that whole berries get mixed up in the screening, and that causes complications. A small percentage of the berries stay whole, “so I suspect we’re getting a little sugar that goes into the barrel—I always allow for it to happen.” The wine completes fermentation in the barrel.

When she has done a number of seed removals, “I put it all back in the tank and let it settle for at least a week. I rack again and let it rest for another week. This is more necessary for Syrah especially, since it likes to hold onto its solids, so we want them to settle out.”

Harbertson was intrigued by Thompson’s work and said that ideally he would be able to do a more scientific test to see just what one would get with seed tannin removal.

For the time being, seed removal is still a great tactic, but how it really works—and whether it’s worth the effort—remain questions to be answered.

Dan Berger has been a wine columnist since 1976. Currently he issues weekly wine commentary, Dan Berger’s Vintage Experiences and a nationally syndicated wine column. He also coordinates the Riverside International Wine Competition. To comment on this article, contact him through

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