How can a winemaker craft a white wine that does not have the unexciting vinous smell of white wine? According to Dr. Anna Katherine Mansfield of Cornell University, the winemaker’s art is accentuating a quality already in the grape that will rise above the baseline aroma giving the wine sensory impact.
Two well established characteristics of wine conspire to reduce aroma, Mansfield explained at the 2012 Cold Climate Conference, St. Paul, Minnesota on February 25th.
First, ethanol and the 22 other compounds in all wine produce an aroma that is blandly perceived as “generally vinous.” In order to get a pronounced wine aroma, the active compound must either be powerful or in large quantities, such as the methoxypyrazines in Cab Sauv that produce its distinctive bell pepper smell.
The second factor working against wine aromas is “Sack’s Theory” which essentially says that unripe and ripe aromas mask each other. In other words, “If you have unripe grapes, it’s going to suppress any nice, fruity notes you might have, ” Mansfield said.
The secret to overcoming the tendency of white wine to not smell is to produce more favorable aromatic compounds like monoterpenes, thiols and esters. “We’re not talking about a bunch of grandmothers in Florida,” Mansfield said. “Esters are compounds that produce floral and fruit flavors. They’re the volatiles we’re trying to increase.”
There are many techniques that increase production of “good volatiles.” The most controllable factors are yeast selection and nutrition, Mansfield said.
In a cool, anaerobic fermentation environment, yeast becomes stressed which causes the microbes to produce mid chain fatty acids. Mid chain fatty acids normally smell like limberger cheese, but ethanol reacts with theses lipids to produce fruity, floral notes.
Mansfield said the basic equation for how yeast creates favorable odors is: Limberger cheese + ethanol= strawberries. “If you already do cool fermentations for whites, you’re pushing things in the right direction,” she said. To enhance aromatics, fermentation not only needs to be cool, it also needs to be cool early in the process. “We know ester production peaks early in fermentation, so get it going strong early and don’t let fermentation creep,” she said.
Mansfield said that clarifying the must can also help increase pleasant aromatics. That’s because must contains lots of long chain fatty acids. And if yeast can access these long chain fatty acids from must, then they don’t produce their own desirable mid chain fatty acids. “Clarifying your must to 100-200 NTU’s is what you want,” she said. “Don’t filter your must, but you do want it settled.”
Harvest timing enhances wine aromas but is not a factor that can easily controlled, Mansfield said. “Studies of Sauv Blanc ripening show that there is approximately a two-week window where flavor and aroma compounds peaked, but not all at once, then dropped and got reabsorbed into the grape,” she said. “We also know that thiols will peak at some point and then diminish, so this is not an instance where it’s good to just let grapes hang.”
Thiols, which are sulfur derived aroma precursors, are good volatiles that have been discovered more recently. While sulfur has bad connotations with winemakers, in small concentrations thiols produce tropical fruit aromas. (In large concentrations thiols smell like cat urine.)
Mansfield said all that’s needed to detect thiols in grapes is the saliva in your mouth. Thirty seconds after chewing on a grape, an experienced taster can perceive the presence of thiols in the form of gooseberry and passionfruit flavors and aromas. Sauvignon Blanc, and particularly Sauv Blanc from New Zealand, contains signature volatile thiols.
Mansfield said suspicions are growing about the presence of thiols in Frontenac Gris. Frontenac Gris grown in southern Midwest areas, like Southern Missouri, has aroma characteristics similar to Sauv Blanc so there may be a thiol precursor in this cold climate grape, she said.
Mansfield said preliminary research by Dr. Murli Dharmadhikari at Iowa State has uncovered monterpenes in La Crescent. (Monoterpenes form much of the aromatic fingerprint of Riesling, Gewürztraminer and Muscat.) Mansfield said that she would guess that monoterpenes are also present in Frontenac Gris, although more study is required.
Skin contact can be helpful for expressing monoterpenes and other aromatics in whites because 20% to 60% of aroma precursors are found in the skin of the grape. While cold soaking can extract more favorable compounds, Mansfield advised caution with this technique.
“Don’t cold soak with rotten or unripe grapes or you’re just pulling out more of the bad stuff,” she said. “Cold soak grapes only in the good years when you have the ripest fruit.”