June 2006 Issue of Wines & Vines

How Yeast Affects Wine Flavors

by Jane Firstenfeld
How yeast affects wine flavors
In two decades of research, Henick-Kling has made many different varietal wines with many different yeast strains, like this batch of Rieslings, each with a different culture.

  • ASEV presenter Dr. Thomas Henick-Kling has researched the impact of yeast cultures on wine flavor at Cornell University for two decades.
  • Yeast's influence is strongest on wines that display the most distinct varietal characters.
  • When experimenting with yeast strains, many winemakers test their wines the wrong way, at the wrong time. Yeast's influence cannot truly be evaluated until at least six months after fermentation is complete.
Articles you've read in these pages, suppliers' brochures or conversations with fellow winemakers may have aroused your curiosity: Can using a different strain of fermentation yeast actually affect the ultimate flavor profile of your wine? Cornell University professor Thomas Henick-Kling, Ph.D., has researched that issue for two decades, and assures us that the answer is a resounding "Yes."

Henick-Kling, whose presentation, "Wine Flavor Modification by Yeast and Bacteria," will be part of ASEV's Aroma & Flavor Symposium on Tuesday, June 27, spoke with Wines & Vines in mid-April, when he was beginning to prepare for the symposium.

"I've been working on wine flavor for the 20 years that I've been here," he said. "Our research into the effect of yeast strains on wine flavor came from our work here in experimental batches of wine from different varietals and viticultural practices."

The researchers originally changed yeast strains for practical reasons (cost, stuck fermentations, etc.). "We noticed a big flavor difference," according to Henick-Kling. "We did more experiments with several different varieties (of grape), using different yeasts, and found a very distinct influence on the final flavor profile." This is not just speculation, he emphasized. "We've shown there is."

Over the course of his research, Henick-Kling tested many strains of yeast cultures, with many varietals of grapes grown in varied locations and under different viticultural practices. Yeast's influence on flavor is strongest in wines with the most distinct varietal characteristics. "If you use a flavor-neutral type grape, or, say a Chardonnay with huge yields (and only weak varietal character), the only difference you'll find is how many fermentation esters a yeast makes," he said.

How yeast affects wine flavors
In Cornell's experimental labs, red wine fermenters are individually temperature-controlled to maintain consistent conditions.
His team measured grape varietal flavors using traditional gas chromatographic mass spectrometry methods, but relied most on trained tasting panels to evaluate the wines, a time-consuming process not easily available to most producing wineries. Henick-Kling explained that winemakers who are still dubious about yeast's effect on wine flavors have not been sampling correctly. "People compare yeasts, measuring fermenting esters with gas chromatography, and they'll still be all the same. In the past, people would say, 'They don't make a difference. Some are just faster, some are slower.' They've been analyzing at the wrong time, and the wrong way," he said. While yeast of whatever strain, whether free-swimming or encapsulated, begins to affect fermentation as soon as it's added, its long-term effects are not detectable until at least six months to one year after fermentation is complete. Until that point, what are being measured and detected, according to Henick-Kling, are merely the fermentation esters, which produce that "new wine" aroma.

"All yeasts produce fermentation esters," he said. "The ones that ferment properly vary a little in how much they produce. Rarely can you smell a difference. During active fermentation and a few months after, you won't find much of a difference among strains. Then the fermentation aromas decay; when they fall away, then you see the grape varietal, the regional differences. These don't show up until one-half year to a year.

"It's tempting, always, to taste and smell, but there really won't be much difference," among young wines fermented with different yeasts, he observed. Although he has worked with a leading yeast supplier during years of research, Henick-Kling does not recommend specific yeast strains for specific winegrapes. "I tell winemakers, different yeast cultures are tools to get more diverse flavors into their wines," and leaves them to perform their own trials and draw their own conclusions.

If it takes a while for yeast strains to reveal their influence on wine flavors, how long does this influence persist in the barrel, tank or bottle? According to Henick-Kling, if you start with a flavorful grape, the correct yeast's flavor enhancement should become evident in six months or so, then, barring oxidation, last indefinitely. "Some winemakers say it gets even stronger."

(Learn more about at ASEV's Aroma & Flavor Symposium, 8 a.m.-5:30 p.m., Tuesday, June 27. Professor Henick-Kling's presentation, Wine Flavor Modification by Yeast and Bacteria, is scheduled for 3:30 p.m. Other featured topics include: Wine Flavor Chemistry, Measurement of Taste, Making Sense of Scents, Perception of Odor Objects: Neurobiology and Behavior, The Chemistry of Varietal Aroma, Understanding Wine Flavor from Vine to Wine, Technique to Study the Impact of Grape Maturity and Irrigation on Pinot Noir Wine Aroma, The Flavors of Modern Cooking and Wine Taste Beyond Sugars, Acids and Tannins. Visit asev.org for details.)
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