Red Wine Filtration: Beneficial? Detrimental?

Filtration can remove microbes that might spoil the wine. Some think it removes color.  A few years ago a California winery started putting “Unfined, Unfiltered” on its wine labels to imply superiority.
I toyed with the idea of putting “Uncentrifuged” on my labels, but thought better of it.

In St. Louis, in the 1990’s, I had an outstanding example of one of those unfiltered wines.  The reek of Brettanomyces from the glass poured from the $80, 375 mL bottle was intimidating at 3 feet.  Pad filtration might have captured the Brett microbes, but 0.45 micron membrane filtration would surely have.

Why do some winemakers eschew pad filtration?  I have seen the deposition of color on filter pads and I agree that it seems to be proof of color stripping.  But it is not.  The particles caught on the surface of fibrous filter pads are fairly large, and not the molecules that give the liquid wine its color.  These larger particles do have color molecules attached, but they are particles that will eventually settle out either on the closure or on the glass of the bottle.  The trial that few winemakers care to perform is the comparison between a filtered wine and an unfiltered wine after six months or a year in bottle.  If you doubt the premise, do the trial.

Pad filtration generally goes as far as removing 99.9% of the particles sized down to 1 micron.  0.45 micron membranes can assure removal of all particles greater than 0.45 microns, giving rise to the term “Sterile Bottling”.  Good work has been done showing that color stripping does not occur even with this stringent filtration technique:

From Abstracts from Presentations at the ASEV 63rd National Conference 20–21 June 2012, Portland, Oregon

Evaluating the Effects of Membrane Filtration on Sensory and Chemical Properties of Wine

1. Luke P. Bohanan, 2. David E. Block* and 3. Hildegarde Heymann

     Author Affiliations:
2.  Department of Viticulture and Enology, University of California, Davis, CA 95616

It is a long-held belief in the wine industry that membrane filtration, specifically sterile filtration below 0.45 μm, will strip a wine of aroma and color. For this reason, many winemakers avoid the use of sterile filters in wine production, which can cause uncertainty in microbial stability of the finished wine. There are currently no studies connecting sterile filtration to a significant sensory effect on finished wine. To assess this, two red wines, a Cabernet Sauvignon and a Merlot, and one aromatic white wine blend were filtered through 0.45 μm polyethersulfone (PES) and polyvinyl difluoride (PVDF) membrane filters. Sensory and chemical characteristics of these wines were compared to unfiltered control wines. Treatments were expanded with the Merlot and white wine blend to also examine the effects of a pad filter and a cartridge depth filter used as prefilters. Possible changes in dissolved oxygen were monitored during bottling, while tannin concentration and color were examined through the course of filtration. There were minor differences in tannin and color after pad filtration, but there was no significant variation during the course of the filtration. Descriptive sensory analyses were conducted for each wine immediately following filtration and on a regular basis for up to 24 weeks. While all three wines exhibited significant variation in sensory descriptors over time, a decrease in astringency between control and filtered Merlot wines was the only significant variation among treatments. Overall, there was no significant impact of sterile filtration on the sensory or chemical properties of the wines tested, regardless of the type of filter material used.
  • ©2012 by the American Society for Enology and Viticulture
Brett will wake up and grow in the bottle, which is why I strongly encourage the use of 0.45 micron membrane filters when bottling.
Dr. Tom Cottrell   7/6/2015