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The Sweet Smell of...Sugar?

August 2009
 
by Tim Patterson
 
 
    HIGHLIGHTS
     

     
  • Many winemakers report a connection between the level of residual sugar and the nature and intensity of aromas in white wines.
     
  • Some research indicates that while the presence of sugar, not itself volatile, may cause subtle differences in volatility of other compounds, the differences are likely to be sub-threshold in the nose.
     
  • Sugar-derived sweetness can trigger an association with fruitiness. Intriguing evidence also exists to indicate that the type of sugar and method of sweetening can also make a difference in aromatics.
     

Aromatic wines often arrive with some amount of residual sugar--from off-dry to sticky. But does the sugar have anything to do with the aromatics?

More difficult than it appears, this question popped up for some discussion at the technical conference portion of this past February's International Alsatian Varieties Festival in the Anderson Valley. The winemakers on the panel were split on the effects of sugar: Some thought there is an impact in the mouth; some even suggested a difference can be picked up simply in naked smell; some were not convinced, period. The discussion moved on, but this possible relationship stuck in the back of my mind and finally came up on the to-do list.

The short answer seems to be "maybe." This is another of those areas in which researchers in other parts of the food and beverage world have invested more time and energy ferreting out the interrelationships--the wine literature is spottier. But the drift points to "yes:" When tasting wine, heightened sweetness leads to an increased perception of fruitiness, and as for the nose alone, there may be something gong on, but it may not be detectable.

Volatile aroma thresholds
The analogy that popped into my mind from the sugar/aroma discussion was the role of modest amounts of volatile acidity, which can often amplify or elevate wine aromas. But that's a bad analogy, because volatile acidity is, well, volatile, while glucose and fructose aren't exactly known for that. So, what mechanism could come into play?

Peter McCrea sugar
 
A pinch of sugar made aromas "come to the forefront when you uncorked the wine." --- Peter McCrea

Peter McCrea at Stony Hill in Napa isn't sure what the chemistry is, but he does think there's something to the connection. Known for its long-lived Chardonnays, Stony Hill has been in the Riesling business for decades as well. In the early 1970s, the team observed that it often took its dry-style Riesling a few years in the bottle to develop interesting aromatics, something consumers wouldn't wait for. With some experimentation, they found that leaving a pinch of residual sugar (slightly less than 1%) made the aromas "just come to the forefront when you uncorked the wine."

At Chehalem winery in Oregon's Willamette Valley, Harry Peterson-Nedry says that often he can identify the presence of residual sugar simply from smelling a wine--but he can make mistakes, too, with wines that smell sweet and taste dry. "Even if I am a chemist," he says, "I don't necessarily understand it."

One of his speculations is that perhaps converting the last few Brix of a fermentation to complete dryness may burn off a fraction of esters that hang on and make it into the bottle with a stopped fermentation.

Winemaker Jim Klein, at Navarro Vineyards in the Anderson Valley, has his doubts about whether sugar affects aromatics--simply through the nose--in reliable ways. He cites the surprise many people have when they sniff the heady Navarro Muscat and then discover it's entirely dry, confounding expectations.

Gavin Sacks, enologist at the Cornell/Geneva Experimental Station in the Finger Lakes, N.Y., does have a handle and some opinions on the chemistry. Whether a particular compound becomes volatile or not often involves its interaction with water. And if that water/wine includes sugar, that could change the polarity and decrease some interactions, leaving the ethanol to increase volatility. Direct interaction of potential aromatic compounds with sugar, in other cases, can decrease volatility. So even if the sugar is not volatile itself, it can alter the environment for volatility.

But mainly, Sacks says, these subtle effects are not likely to make it up to the normal sensory threshold. They can be detected by a machine, but probably not by a mere human nose. "Even if you got up to 20% residual," he says, "it might not be clear."

It's all in your mind
The kind of expectations Jim Klein mentions--mental constructs that kick in after a sensory stimulus--are key to what happens when wine aromatics are perceived through the mouth and the retronasal machinery. When some sugar is part of the mix in the mouth, repeated studies have reported that panels of tasters find an increase in fruitiness, sometimes specific fruit characters, even though there are in fact no more fruit and nor more aromatic compounds.

This is not just a wine phenomenon, it's widely known through the food and beverage industry. Sacks notes some recent work by his wine chemist colleague, Terry Acree, in which the addition of sugar to coffee led to the perception of more caramel and similar descriptors--without any caramel being added. This connection between slight levels of sugar and heightened fruitiness takes us well beyond the delicate realm of Riesling and straight into the nouveau California style of big reds, 15% alcohol Cabernets with a splash of residual sugar and fruit-bomb credentials.

Prevailing opinion among researchers is that the sugar-fruit-in-mouth link is almost entirely psychological, or perhaps evolutionary thinking. "We did not evolve to decouple what we smell from what we taste," Sacks says. "We want to integrate all that information."

This creates the effect of synesthesia, experien ce in one sensory pathway because of stimulation of another--sometimes in unexpected places, as when numbers take on color-coding. Here, the sweet stimulus triggers the perception or stepped-up search for fruit character--a response that occurs in the brain, not in the taste or aroma receptors.

This is one reason a lot of Riesling is made off-dry--even Navarro's gets a pinch of residual. That fruit association in the mouth, plus improved mouthfeel, is no doubt part of why Peter McCrea's Riesling sells better than the initial dry style, and why discussions of "dry" in the Riesling world are often a little different from the same discussions for, say, Sauvignon Blanc.

There's sugar and then there's sugar
The most intriguing results I tripped across while looking into this came on a slightly tangential subject--the differences between sugars, which can apparently have their own aromatic effects.

Gavin Sacks notes that sugar degradation can produce powerful volatiles, even in tiny, parts-per-million concentrations--like caramel, for example. So a wine back-sweetened with sugar, like the C&H cane I've been known to use in my garage wines, could strike a new note--not from the sugar itself, but from a spin-off compound.

Along the way I discovered a 2004 American Journal of Enology and Viticulture article from a team at Missouri State University that compared the aromatics of wines sweetened in different ways. I caught up with Karl Wilker, one of the researchers. On two vintages of Vignoles, a French-American hybrid variety typically vinified with some RS, batches got their sugar through multiple methods: stopped fermentation, reintroduction of unfermented juice after dry fermentation, and a muté addition (partially fermented juice/wine) after dry fermentation. Acidity, pH and ethanol were adjusted to make the wine chemistry similar. Tasting panels reported that the wines made through stopped fermentation showed fresher and more intense fruit aromas.

Since Vignoles usually gets the off-dry treatment, the researchers, alas, didn't have a fully dry ringer wine in the smell-off and taste-off. Drat.

Research agenda
Since some of us are bonkers about these wines--no, not the fruit bombs, the off-dry aromatic ones--it seems there is some research yet to be done.

Sacks is quite confident he could take a dry Riesling to a tasting panel, have it sniff and sip, get grapefruit as a descriptor, add some sugar, and start hearing sweet citrus and tropical terminology. While describing this would-be experiment, he floated the idea of including little packets of sweetener with bottles of such wines, letting consumers dial in their favored aromatic profile--a project that will make us both rich some day.

Meantime, I want someone to do the very basic comparison I couldn't quite find in the literature, though it may be there. An aromatic white with and without 1%-2% RS, evaluated in triangle tests by trained panels, just by nose. In fact, I may just get started on that right now: Go down to my cellar, get a bone-dry Riesling from the Pfalz or the Finger Lakes, pour three short glasses, dose one with some simple syrup, have my wife scramble the order, and sniff away. I foresee many replicates.

Tim Patterson writes about wine and makes his own in Berkeley, Calif. Years of experience as a journalist, combined with a contrarian streak, make him interested in getting to the bottom of wine stories, casting a critical eye on conventional wisdom in the process. Contact him through edit@winesandvines.com

 
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