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.

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2 thoughts on “Does cross-flow filtration affect wine flavor? No easy answers.

  1. Some believe the extra pumping and the real small pore size of the wine going through a cross flow filter may break up the colloidal structure of a wine, pretty much changing it and not for the better as far as aging and structure. A bacterial issue is likely a wine that wasn’t microbiologically stable to begin with. Another better made wine would be a good idea to make the comparison. Even a trained tasting panel the people have different sensitivities and preferences. If like most people today they seem to be more fruit driven in their predilections and not as much on structure and complexity. Much of training I also believe gives too much preference to the fruit character.

    • Indeed, Keith. Tim gives a good recount of the colloids discussion in his Wines and Vines piece, so I skipped giving it short shrift here so as to not make this post too unwieldy. But while even trained tasters will have different sensitivities — the training process should normalize out a lot of that, though it’s certainly never gone, but the statistical analysis done afterwards should account for it — you’ll have a hard time convincing most people that microbial instability is a useful source of structure and anti-fruit forwardness. It’s also useful to observe that the end point of this study wasn’t which wines the panel preferred most — we can’t say that either the trained tasters or untrained consumers liked the filtered wines better or vice-versa — but merely that the difference exists.

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