Oregon geologists dish up new dirt on wine

April 29, 2016 - 6:00am

Wine enthusiasts say the strangest things. When describing a particular glass of grape, they seem to draw from a colorful and endless array of terms that baffle the average consumer.

A wine might be minerally. It might be chalky. Perhaps it’s slaty or earthy or evocative of wet stones.  

“Taste experiences like these are often attributed to the soil in which the wine grapes were grown,” said Greg Retallack, a UO geologist and expert on fossil soils.

But does soil affect the taste of wine so directly? Should the French notion of "gout de terroir" — taste of the soil — be taken literally to mean that tastes are simply transferred from soil to vine to wine?

Retallack, director of the Condon Fossil Collection at the UO Museum of Natural and Cultural History, sheds some light on these questions in a new paper co-authored with Portland State University geologist Scott Burns.  

The study — published in the May issue of GSA Today, a journal of the Geological Society of America — reveals that we can indeed taste some aspects of soil in wine, especially acidity. In talks about the research, Retallack often ends with a short song he put together about the soil-wine connections.

Using data from hundreds of Willamette Valley pinot noir wines of different vintages, the authors found that there is an inverse relationship between the acidity of wine and the acidity of soil: Grapes grown in fertile, low-acid soils result in more acidic wines while infertile soils produce wines with less acidity.  

“In the Willamette Valley, grapes grown on the fertile, 10,000-year-old Missoula flood deposits lead to greater astringency in the final wine product, while those grown on the leaner, 100,000-year-old terraces of sedimentary rock and basalt support more rounded flavors,” Retallack said.  

“Some acid is desirable in wine because it provides a fresh tingle on the tongue, but too much can be unpleasantly astringent, like vinegar,” he said. “Less acidic wines have a rounder flavor and finish that bring out the subtleties in the organic flavor compounds.”

The study confirms the traditional wisdom of vintners, which says that lean, acidic soils with fewer nutrients produce grapes better suited to winemaking.

“A struggling vine produces more grapes than leaves, and more enticing flavors in those grapes, in order to elicit ingestion and dispersal by animals,” Retallack said, “while overfed grapes form mainly simple acids and sugars, resulting in less desirable flavors in both the grapes and the finished wine.”

Making the cover of the GSA Today is exciting both for the research and the region, said PSU's Burns.

"One of the best places in the world to taste differences in wines based on soil differences is the Willamette Valley, where we live," he said. "There are three main groups of soils that produce three different flavors in the pinot noir wines. Wine tasters love tasting these differences."

—By Kristin Strommer, Museum of Natural and Cultural History