Effects of grapevine leafroll disease on wine quality (and when is a disease a disease?)

Gut reaction: Viruses cause disease. Disease is bad. Viruses are bad.

Gut reaction muted by a lot of recent genetics research: Viral DNA seems to be embedded in genomes all over the place. We’re not sure why a lot of it is there, or stayed there, or what it does while its there. Some viruses cause disease. Some don’t. Viruses are complex, and we probably don’t know the half of it yet.

A name like “grapevine leafroll-associated virus” gets you thinking about negative consequences. Rolled leaves don’t collect light efficiently, which means that they won’t contribute to the plant’s photosynthetic metabolism efficiently, which means that the plant may be malnourished, grow slowly, and/or not have enough energy to ripen fruit. Rolled leaves are bad. A virus that’s associated with rolled leaves is bad. But the virus is only associated, not causative. Some viruses in this general family of leafroll-associatedness aren’t associated with vine symptoms. And infected vines only show symptoms post-veraison (the stage of ripening at which grapes change color), even though they carry the virus in detectable quantities year-round.

Ergo, a group of vine and wine scientists headquartered in eastern Washington state designed an experiment to ask (published in PLOSOne, and therefore open-access to everyone): do grapes from vines with grapevine leafroll disease, and carrying one of these viruses (GLRaV-3), lag behind their undiseased counterparts throughout ripening, or only when vines show symptoms? Being particularly conscientious*, they also improved on existing studies of grapevine leafroll disease by collecting data for three consecutive years from a commercial vineyard, sampling grapes throughout the season but also harvesting grapes at the typical time and making wine from diseased and undiseased pairs, and subjecting those wines to (limited) chemical and sensory analysis. They also used own-rooted rather than grafted vines, which eliminates some potentially confounding variables.

Their conclusions, after collecting data over the 2009, 2010, and 2011 growing seasons:

  • Grapevine leafroll disease decreases vigor (as measured by cane pruning weights) and fruit yield in own-rooted Merlot vines in Western Washington.
  • Grapes from diseased/infected vines have lower total soluble solids (TSS) and higher titratable acidities (TA) (and, to a less dramatic degree, lower anthocyanin concentrations) than grapes from undiseased/uninfected vines, but only after vines begin displaying symptoms post-veraison.
  • Wines made from diseased grapes were browner and less intensely colored, earthier and less fruity, and more astringent compared with their undiseased counterparts.
  • However, panelists only correctly distinguished diseased from undiseased wines when served side-by-side in black glasses, removing the notable color differences from consideration and forcing them to differentiate on smell and taste alone.
  • Soluble solids, TA, and pH were all more dramatically affected than anthocyanins in diseased vines, which reflects the decoupling of anthocyanin development and sugar accumulation that happens late in ripening during which environmental conditions play heavy in anthocyanin development.

These conclusions probably do more for plant scientists than for commercial growers: data from one Merlot vineyard near Prosser can’t be precisely extrapolated to you, wherever you are, and thresholds for usable fruit are always a matter of context (the authors note that future studies should document the effects of grapevine leafroll disease on specific sensorily-important compounds). The study does add data points to a collection of statistically robust data that might help large companies make judgments about what they can include in their generic red blends before pH or some other parameter becomes a problem. But maybe the most interesting line of thinking here has to do with the nature of disease, and of relationships between viruses and diseases and symptoms. Do vines have leafroll disease before they exhibit symptoms? Where do we want to draw lines between normal or acceptable variation and disease symptoms? If a vine looks sad but makes grapes that make wine indistinguishable from happy-vine wine, and if genetic testing says that the plant also happens to have a virus, does that mean that the vine has a disease, or is it healthy?

Disease” can mean something different to the plant pathologist who looks at a vine, a geneticist who looks at the DNA of a vine, a commercial grower who looks at the fruit of the vine or a winemaker who looks at the juice it makes. That vine may be infected with viruses. Is the virus bad?

 

*Full disclosure: I know and think highly of several of the scientists on this team.

Spice and sulfur: Recent research news from Australia

Wine Australia’s “research, development, and extension” arm periodically releases briefings on research they’re funding. This month, two of those briefings promise particular interest for folks outside Oz.

Spice (Rotundone)

Short: How rotundone (and the peppery flavors it yields) develops in grapes still lacks good scientific explanations, but scientists are working on it and suspect that it’s a multi-factorial process involving sunlight and air exposure as well as enzymes.

Longer: Not necessarily positive for wine quality, but stellar for talking about how chemistry produces flavor. Rotundone is the “impact compound” behind the peppery flavors prominent in some Australian shiraz (and some other red wines), and rarely does a single compound correlate so clearly with a single and very easily identifiable wine sensory note. The sensory correlation seems simple (caveat emptor: expect it to become more complicated as scientists spend time studying it). How rotundone forms is anything but. A lot of scientific activity (and not just in Australia) over the past year or so has been working out both the chemical pathway (enzymes and intermediates) responsible for rotundone and the viticultural parameters describing where it forms.

A major rotundone conundrum has been why its concentrations seem highest in cooler sites – within the plant, the vineyard, and a region – and yet rotundone characterizes ripe Australian grapes. As is so often the case in scientific conundrums, the confusion may stem from trying to pin causality on the wrong set of variables. Rotundone is formed by an oxidation reaction. Recent work says that that oxidation may be motivated by enzymes, sunlight, or oxygen. Or all three. And so, even if rotundone = peppery flavors is a simple equation, solving X + Y + Z = rotundone is shaping up to be a good deal more complicated.

Expect big rotundone news from Australia in upcoming years. Sussing out a viticultural recipe for maximizing (or minimizing) pepper flavors in shiraz could set up Australian shiraz to do what the Marlborough sauvignon blanc industry has done, creating an international brand around a distinctive flavor profile, fueled by scientific research into how to make those flavors ever more obvious. What that research means for smaller producers who aren’t aiming for those sorts of flavor profiles is a different, and interesting question. 

Sulfur (and copper)

Short: Adding copper to finished wines to remove or prevent sulfur aromas may not work the way everyone hopes it does.

Longer: Wines made with very little oxygen exposure and bottled under screw cap don’t have much chance to blow off smelly sulfur compounds produced via this sort of reductive winemaking. (Why sulfurous aromas are a problem in reductive winemaking involves some complex microbiology that’s summarized well here.) A standard prophylactic against eau de cabbage or rotten egg in your freshly unscrewed bottle is adding some copper before bottling; copper binds to the smelly sulfur compounds and acts as a heavy anchor of sorts, keeping the malodorous molecules from volatilizing, entering sniffable air space, and registering as an undesirable aroma. Adding a copper penny to a sulfurous wine glass is a common parlor trick for confirming that particular wine fault; if you’re really smelling sulfur, in theory, the penny should mitigate the problem.

A nice, simple solution to nasty sulfur aromas would seem to be adding copper to bind to and “lock away” sulfur compounds, then counting on pre-bottling filtering to remove the copper-sulfur compounds.

Problem #1: It seems that filtering doesn’t reliably remove the copper.

Problem #2: The copper-sulfur binding isn’t always stable or permanent, so the copper may go off and do other (undesirable) stuff no one was counting on.

It seems likely that copper additions are useful under some if not all circumstances. The future work – of researchers and winemakers working together, one hopes – is defining “some if not all circumstances” more precisely.

When a wine is salty, and why it shouldn’t be

Salty is not a common wine descriptor. That it’s also not a positive one probably goes without saying. As a consumer, it’s also not a fault you’re likely to fret over (I don’t think I can recall ever hearing anyone say something like “Hey, Sarah, does this wine taste salty to you?”) But the fact that wine-producing countries have (widely varying) legal maximums for sodium chloride in wine should tell you something. Salinity is a concern in dry locations when frequent irrigation increases soil salinity, which increases wine salinity, which may add one more to the list of western American winemakers’ concerns. Soil composition often doesn’t translate in the way you’d expect into grape composition; salt is, unfortunately, an exception.

An article in the American Journal of Enology and Viticulture last year mentions Australian growers’ and winemakers’ experience that grapes that taste salty may clock in under the legal sodium chloride limit and vice-versa. Law or no law, obviously no one wants “salty” to show up in their product’s tasting notes. The article reported on an effort to quantify when wine saltiness kicked in and how best to measure it. Most of the non-sodium chloride salts that show up in wine – potassium chloride is notable – register as bitter more than salty. Sodium chloride registers as salty, obviously, but also appears to convey soapy sensations. They were interested, then, both in how much salt it took for a taster to call a wine salty and in the negative impacts of defined amounts of salt on wine flavor.

Their cadre of tasters – enology students at the University of Adelaide with some specialized tasting experience – were able to first identify saltiness in Australian Shiraz and unoaked Chardonnay at .36 to 1.76 g/L with a median of .8 g/L and a lot of individual variation (values were lower for the white wine, higher for the red). The Australian legal maximum of .606 g/L, then, means that some of these folk may sometimes encounter a salty wine; the Swiss limit of .06 g/L, on the converse, seems unwarranted at least in terms of sensory concerns. The researchers also spiked the Chardonnay with several concentrations of NaCl and asked a smaller group of specially trained students to rate their sensory qualities. Those experiments confirmed that at .5 and 1 g/L, added salt dampened perceptions of fruit and added a salty flavor and soapy mouthfeel.

To the Australian researchers, the utility of their findings was in recommending that Australian growers could probably rely on their (quite possibly a bit more sensitive than average) taste perceptions to gauge grape saltiness in the field, in terms of acceptability for the Australian market, but not for meeting more stringent international guidelines. They didn’t comment on the implications of their findings for the reasonableness of those guidelines, though perhaps they go without saying. It does seem plausible that saltiness perception thresholds might vary among people of different nations accustomed to different diets, though this study’s Australian-based results were about on par with previous studies including a few conducted in Japan.

One other interesting implication, for household use. Obsessive molecular gastronomist Nathan Myrhvold has recommended that people try salting their wine as they would salt any other food. This is the same gentleman who suggests that a blender is an efficient tool for oxygenating wine, a more aggressive version of “letting it breathe” in a decanter. Myrhvold suggests a tiny pinch per glass. If one teaspoon of salt weighs about six grams, then 1/10th teaspoon per liter of wine amounts to the Australian limit of .6 g/L. A standard glass of wine is about 150mL. In other words, any realistic pinch will send your glass over the technically established edge. But it’s worth noting that Myrhvold is recommending this as a tactic to make a wine taste more savory.

I tried this with an exceptionally ordinary glass of Australian shiraz. The salt did, indeed, make the wine taste more savory. Frankly, that was neither difficult nor especially unwelcome for something that started off as a bit of a fruit bomb. But – and keep in mind that I was not tasting blind – the potential for that to be a benefit was outweighed by the kind of soapiness you get from having added a bit too much baking soda to your biscuits. In this wine, where the fruit was pretty much what it had going for it, I wouldn’t do it again, but I’m interested to see what happens with the next glass of reasonably lively Chardonnay I come across.

For producers in California, Washington, and other devastatingly dry locales, unfortunately, adding salt isn’t going to be that kind of easy option.