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Filtering Myths


Sometimes it seems like filtering is the final frontier of home winemaking. For every winemaker who filters, there are dozens who don’t. Some because the wine is just fine without filtering (and it is!), others because they don’t want to bother with the extra step or the expense, and others because they’re concerned that filtering will hurt their wine rather than help it.

But helping is what filtering is all about. Not only does filtering make a finished wine look better (think of the difference between a freshly washed car and a freshly waxed car—that’s the difference filtering makes) but also it promotes stability in the finished wine because when you removed suspended solid material with a filter it won’t drop out later or mutate into crusty goo over time.

Filtering is great and cool, and most people who are concerned are really frightened about nothing. However, that’s not to say you can’t mess things up with a filter.

Actual Danger

The only really issue is the same as for any processing operation in winemaking, from racking to fining and stabilizing: the chance that you’ll introduce oxygen into the wine.

Filtering agitates wine as it travels through pumps, hoses and filter media, but doesn't necessarily introduce oxygen into it. Most filter set-ups are positively pressurized, meaning they use a pump to force wine down a hose and through the system. If there is a leak somewhere in the filter between the pump and the carboy the wine is going into, it’s going to squirt wine out, not suck air in.

The potential issue comes in if you run the output hose down the side of the receiving carboy, where it can fan out and expose an enormous surface area to oxygen pick up—gently place the output hose directly into the bottom of the carboy instead, and allow the tip to submerge as it fills, keeping everything as quiet as possible.

This won’t even be an issue if you’ve followed the instructions for maintaining the sulphite levels in your wine and kept your carboys topped up during storage. I usually do a sulphite test on my wines before filtering and if they’ve dropped, I top them up to 35 PPM FSO2. If that sounds like a lot of bother, measuring and testing, you can simply add a (precise!) one-quarter teaspoon of sulphite powder to every six gallons of wine and that will top it up by about 12-15 PPM, a good buffer for the extra handling.

Mythical Dangers

Boun Vino Mini Jet Filter
Does filtering strip color, flavor or aroma? Yes and no. But actually 100% no, because any reduction in aroma and flavor is strictly temporary. The kinds of filters available to home winemakers operate on the micron scale, with the tightest, most efficient filters stopping somewhere above 0.2µ, about two-tenths of a micron. Your typical yeast cell is around 0.45µ, and a freshly budded daughter cell (they grow up so fast, sniff) is down at the 0.2µ mark. It’s far more common to see filters that allow the passage of material as large as two to four microns in size.

Color molecules, the aforementioned anthocyanins, are not on the micron size. They are so very much smaller that their structure can’t be seen with a microscope. They’re so tiny that in fact they will sail straight through a filter pad or cartridge entirely unimpeded—you can’t filter them out.

Which begs the question, for anyone who has ever used a filter on a red wine, why do the pads come out stained with color? Those stains aren't pure, happy color compounds: they are color compounds that have already bound to other kinds of goo in the wine. When bound to tannin, they’ll fall out later as a deposit (mentioned above) and when bound to a colloid, they’ll fall out as sediment. This is a vast over-simplification (I specialize in those) but the core truth is that you cannot filter out color with civilian filter pads—not any color that wouldn't fall out on its own anyway. The goo on the pads was never going to stay, never going to contribute, and made the wine look crappy and hazy.

What goes for color compounds goes for flavor and aroma molecules: they’re just too dang small to stick to filter pads. And yet anyone who has ever filtered a wine has almost certainly noted that it tastes notably less distinct and aromatic post-filtering.

The good news is that this condition is temporary. Give the wine a few weeks rest, and often only 24 hours will do it, and the aromas and flavors snap back into focus, good as new, with filtering not to blame after all.

Do I Have To Use Fining Agents If I’m Going To Filter Anyway?

YES! You can’t filter a wine that isn’t already really, really clear. The amount of yeast and good in suspension would clog a filter up so badly that you’d spend more than the cost of the wine kit itself in filter pads before you got to the end.

If we go back to the car analogy, you can’t wax a car that hasn’t already been washed thoroughly: waxing isn’t to remove dirt, it’s to put a final polish on the car. Filtering isn’t intended to clear wine it’s to put a final polish on it right before bottling.

Summing Up:
  1. You don't have to filter, but your wine will benefit from it.
  2. Filtering doesn't harm wine in any way.
  3. Filter wines only after they've been fined.
  4. You can't clear up cloudy wine with a filter. You can polish clear wine with a filter.
  5. Filtering is a handling step that introduces oxygen. Make sure your free SO2 levels are appropriate.
  6. Wine judges like shiny clear wine, as do most casual drinkers.
One last word on whether or not filtering harms or helps a wine, I heard an interview with Christian Moueix, the winemaker who made Chateau Petrus for decades. Petrus retails for thousands of dollars per bottle and is pretty much the byword for crazily desirable French wine. The interviewer knew that Moueix sterile filtered the wine, and was clearly in agony, because conventional wisdom says 'filtering bad'. He asked Moueix why he dared to filter such a beautiful wine.

"Because", Moueix replied, "I don't hate my customers.”

I’ll drink (filtered wine) to that.

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