Does cross-flow filtration affect wine flavor? No easy answers.

Cloudy wine (mostly) doesn’t sell. Neither does (most) wine spoiled by spoilage microbes that produce off-aromas. And so, while it is entirely possible to remove floating particles that make wine cloudy and (most) microorganisms via careful winemaking without sterile filtration, most winemakers appreciate the extra insurance it gives. Filters are essentially membranes punctuated by lots of little pores: all good stuff should flow through the pores; bad stuff you want to keep out of the bottle should be too big to pass. To reliably remove yeast and bacteria, filters need to have really tiny pores, and the question always is: are those filters excluding stuff other than the microbes, good-tasting stuff that I want the wine to keep? We’re talking .45 μm pores here, an order of magnitude smaller than the diameter of a red blood cell; anything bigger risks allowing bitsy bacteria through.

Pro-filtration folks say that all molecules important to wine quality are much to small to get caught in even these super-stringent filters. Filtration-shy folks say that wine tastes different post-filtration; no matter what molecular measurements say should happen, something is happening. For an excellent discussion of the arguments on both sides, see Tim Patterson’s excellent review in Wines and Vines (updated here, but this one is still behind a pay wall). Bottom line: scientific evidence suggests that while some important molecules could get stuck to the surface of the filter, all the important stuff can pass through safely; dissenters can still taste a difference, and maybe that has something to do with big conglomerates of molecules.

So it stands for conventional filtration. But what about cross-flow filtration, they shiny new-ish solution to some of the conventional process’s major hassles? Conventional filtration points a stream of flowing wine directly at the filter membrane and waits for it to percolate. Enough push needs to be behind that wine stream to keep things moving, but too much push and the force of the flowing wine will rip right through the filter. And the wine must be almost entirely free of particles in the first place, else biggish stuff will cover the surface of the filter, block the holes, and slow down flow. Either way, the filter membrane will need to be replaced, and they’re expensive. Cross-flow filtration instead points the wine stream across the surface of the membrane. Liquid still flows through but with less direct pressure on the membrane, and the constant stream sweeps pores clean of junk, too.

So cross-flow is better than traditional filtration for a few technical reasons. Is it also better for wine quality?

A study addressing that question has just been published (as a provisional draft; it’s not yet appeared in the print journal) with work done by a team from UC Davis. A few published articles have shown chemical analyses of cross flow-filtered wines, but this study is unique and helpful in two ways: 1. The emphasis was on whether a trained tasting panel could detect flavor differences in filtered wine; and 2. Wines were tested not just immediately after being filtered, but from bottle samples taken at intervals out to eight months post-filtration. Kitchen-sink white and red blends were included.

Though the group’s experiments aimed at looking for sensory differences following filtration, their results uncovered something else. The flavor of the unfiltered red wine changed more over time — more earth, less fruit beginning at two months and continuing to the eight-month end point — while the flavor of the filtered red remained more or less constant; in other words, the filtered wine was more stable. The obvious explanation is that the unfiltered wine suffered from some kind of microbial growth after bottling, even though the idea of a UC Davis-crafted experimental wine having microbial spoilage issues does seem strange.

More to the original point: even though chemical analyses showed that filtration decreased phenolics in the wine — filtered reds had lower pigmentation and up to 26% lower tannin levels — the tasting panel didn’t pick up corresponding differences in astringency. That’s surprising. The only explanation offered in the paper is that the magnitude of the change mustn’t have been big enough to be detectable.

In the end, then, this study probably does more to fuel the filtration debate than to help resolve it. Pro-filtration folk can point to filtration’s apparent lack of sensory impact, and to the likely spoilage of the unfiltered wine. Filtration-caution folk can point to the color and tannin changes and say that, even if those changes didn’t affect flavor in this wine, similar changes might indeed be important in other wines. So instead, we have one more example of what may indeed be Rule No. 1 in winemaking: there are no easy answers.

Microbial terroir? The media gets it wrong again (surprise)

As has by now been widely publicized in wine circles and elsewhere, Dr. David Mills and graduate student Nicholas Bokulich of UC Davis have just published a journal article (in the Early Edition of the Proceedings of the National Academy of Sciences here) demonstrating that populations of bacteria and yeast associated with wine grapes vary geographically in organized and predictable ways. Bokulich collected samples across California, isolated bacteria and yeast from those samples, sequenced bits of their DNA, and then looked for patterns.

This is a beautiful, extremely strong study with useful implications. Unfortunately but unsurprisingly, the news headlines are getting much of it wrong. The New York Times article on the subject* sets up the long-standing American skepticism about terroir, then proclaims that “American researchers may have penetrated the veil that hides the landscape of terroir from clear view, at least in part. They have seized on a plausible aspect of terroir that can be scientifically measured – the fungi and bacteria that grow on the surface of the wine grape.” This is the kind of unscientific media hogwash that contributes to people on the street having balderdash-worthy ideas about how science works. I can’t blame the article’s author for being one more journalist who doesn’t understand science (or who knows better but still needs the sexy story). I can blame the NY Times for not having the sense or taking the effort to get someone with enough research knowledge to cover the story well (Inside Scoop SF did, though they do have Jon Bonné). You’re the NY Times; you folks can do this.

Problem #1: Bokulich and Mills’ findings don’t actually say anything about terroir as we typically talk about it: in terms of sensory impact. The paper describes regional variations. The paper doesn’t connect those variations with any element of wine quality, perceptible or otherwise.

Problem #2: This is not the first time researchers have attempted to quantify some element of terroir. Not even close. Geologists and pedologists (soil scientists) have done a lot of good work looking at soil structure, depth, aspect, and so forth. Other microbiologists have looked at differences in bacteria and yeast across space. Bokulich’s study is exceptionally strong, but it’s not as earth-shatteringly unique as the media are making it out to be.

Problem #3: A technical point, but Bokulich and Mills didn’t actually look at microbes on the surface of grapes. They collected grape musts, which for them meant “destemmed, crushed grapes, representing a mixed, aggregate sample of all grapes from an individual vineyard block” collected after ordinary stemming and crushing operations at the winery. The strength of looking at musts is in having a sample that reflects averaging across a vineyard block and does away with potentially idiosyncratic variations between individual grapes. The main weakness is that we’re a lot less sure of where the microbes came from. What if some of the microbes came from the winery equipment or from handling operations instead of being present on the grapes in the vineyard? Significant regional patterns correlated with environmental factors – precipitation and temperature, for example – but we still can’t actually pinpoint where those microbes are originating.

The NY Times article does a pretty good job of summarizing the original PNAS paper. Kudos to it’s author for talking a bit about the methods behind the findings, for observing near the end of the piece that “the Davis scientists still need to prove that these microbes affect the quality of the wine,” and for calling up Dr. Thomas Henick-Kling at Washington State University for a second opinion. The problem is in the headline and the first few paragraphs which are, of course, what get picked up and misconstrued by everyone else.

This is a fine example of a frequent pattern in news science coverage. Researchers publish a paper on a sexy topic like wine or cancer, and – being like other humans – their conclusions about the implications of their findings might take a few steps over the bounds of reasonability. The university and/or the academic journal puts out some kind of press release, highlighting the sexy bits and the in-our-dreams implications. Journalists pick up on the sexy bits and elaborate even sexier hooks and headlines around them. The hooks and headlines get picked up by less reputable news replicators and on Facebook and Twitter. And by now we’re wandering around smack-dab in the middle of unreasonableness territory.

A chicken-and-egg problem: do we get headlines like this because national science literacy is bad, or does poor science literacy stem (in part) from the uncritical quality of our media? Either way, there’s improving to be done here.

  • Wine-Searcher’s coverage has to get an honorable mention, not only for its especially unreasonable tag line – “New research suggests that bacteria and fungi could be as important in the expression of regionality as soil and climate” – but for referring to what we all know and love as Botrytis cinerea or Noble rot by the show-worthy name of its anamorph (another form of the same fungi), Botryotinia fuckeliana.