Filtration Explored and Explained

In our recent piece on wine fining, we touched on filtration and promised a more thorough discussion of that topic. Here it is, at last. There are two principal reasons to filter wine, but some consumers are so afraid of filtration that many producers choose not to filter. We will do our best to present the facts and let you draw your own conclusions.

One reason to filter a wine is to improve its appearance. Allowing the wine to settle in barrel or tank is a great way to clarify it, but sometimes the spent yeast and other particles simply won’t settle to the bottom in the alloted time. Filtration can remove those particles and leave the wine limpid and bright. Another reason to filter is for microbial stability. No one wants to open a bottle and find it still fermenting, or nearly explosive with foul-smelling gases. A third type of filtration is performed by larger wineries only, to recover wine from the lees that settle to the bottom of the maturation vessel. This requires special equipment and enough lees to justify the investment, which most smaller producers just don’t have. We will ignore that here, but if you have questions about it, ask away.

Filtration tools and techniques—pad filtration

To understand how and why we might filter for appearance or for stability, it is helpful to understand how filtration works. There are two technologies in widespread use today. Pad filtration is the older technology. A series of cellulose (wood pulp, essentially paper) sheets are stacked in a frame and wine is forced through them. The tighter the weave of the cellulose, the more solids are left behind. One obvious disadvantage is that cellulose has flavor, and it is not a flavor that we would enjoy in our wine. This is easily overcome, however, by running (a lot of) water through the pads prior to introducing the wine. The water emerging from the pads is tasted and, when there is no more “papery” taste, it’s time for the wine. Another disadvantage is that the size of the gaps in the cellulose is imprecise. The filter sheets are sold with a nominal pore size, but there will always be a range of sizes. This can be of special concern when filtering for stability.

Typically a wine will go through several sets of pads, starting with bigger (looser) pores, and ending with tighter or smaller pores.

Crossflow filtration

A newer filtration technology is called crossflow because, rather than pushing the wine through the filter medium, it gradually passes through as it circulates. The filter medium is a membrane and it can have very precise pore sizes, making this a better choice for filtration for stability. Typically only one pass is needed, as opposed to one pass for each pore size as with pad filtration. Membrane filtration is also a necessary first step to treating a wine with reverse osmosis, which we will discuss below as a special application of crossflow filtration.

Filtration for appearance

To clean up or “polish” a wine, either technology will work. For pad filtration, the wine might first pass through pads with a nominal pore size of 5-7 microns (0.005-0.007mm) and then, if not “bright enough,” through another set of pads with 2-3 micron pores. Some winemakers who do this will still confidently claim that there wine has not been filtered, for reasons that elude us (but may be elucidated below).

Filtration for stability

Naturally, if the concern is to remove microbes, the pores have to be tighter. Wine is a wonderful beverage in that virtually no human pathogens can survive in it. Nevertheless, some microbes can survive in it and, while they pose no threat to human health, they can pose a threat to the taste of the wine. Lactobacillus spp., for instance, can metabolize any sugars left in the wine and produce acetic acid (vinegar smell) and carbon dioxide (gas, foaming). The acetic acid may form an ester with the ethanol in the wine to create ethyl acetate, which smells like nail polish remover. Brettanomyces yeast can use ethanol as fuel and transform polyphenol precursuors into nasty vinyl phenols that smell of anything from stale hay to horse manure. Not what we want to drink!

A 0.45 micron pore size is generally considered tight enough to remove bacteria (much larger yeast will have been removed at larger pore sizes). With pad filtration it usually takes 4 passes to filter to this level, but with crossflow filtration it can be achieved in one pass. Some winemakers prefer to go tighter still, to ensure that no bacterial spores can persist in the wine. 0.2 microns is considered safe for this. Again, the pore size for filter pads is nominal, and even at the tightest level there may be larger pores that could allow bad agents to pass, but at a minimum the filtration will dramatically reduce their population and thus the risk of contamination.

The downsides

Filtration sounds great, doesn’t it? So why are some consumers afraid of it, and why do some producers proudly claim they forego it? Much of the hostility towards filtration seems to be based in romantic ideas of a wine’s integrity, or even soul. Wine certainly inspires mystical thinking and that is part of its charm. But do odd bits of grape pulp and particles of dead yeast really comprise an integral part of the beverage? Dead yeast actually can contribute pleasant flavors and textures to a wine. That is properly the subject of another piece, but in brief, as the yeast cells die and autolyze, they release compounds into the wine that can enhance mouthfeel, making the wine feel more silky and rich on the palate. Still, that can and should happen before the wine goes into bottle. Leaving the spent yeast in the bottled wine just makes for a cloudy drink.

Another downside of filtration is the risk of oxidation. Anytime a wine is handled there is the chance for oxygen pickup. At best oxygen can make a wine seem prematurely “tired” or flat. Alcohol is oxidized to acetaldehyde, which smells like stale apples. The risk of oxidation is greater with pad than with crossflow filtration, not least because of the multiple passes required for pad filtration. With a skilled operator, however, the oxygen pickup is minimal.

Perhaps the biggest concern with filtration has to do with its affect on a wine’s structure. But this, we will see, is based on a misunderstanding. Many elements contribute to a wine’s structure, which is how it appears on the palate, or its “three-dimensionality.” Tannins give the wine its grip, and ethanol can make it seem somewhat thick, as can glycerol and any residual sugar.

Colloids also contribute to wine’s structure and mouthfeel. Colloids are associations of large molecules, and they cannot pass through a filter intact. Tasting a wine that has just passed through a filter can be a shock. The mouthfeel is completely changed, usually for the worse. We suspect it is this mistake—tasting just after filtration—that contributes most to filtration’s bad rap. However, the components of the colloids do pass through the filter, and with time they re-form the colloids. All better now.

Why we filter

We started filtration with the 2013 vintage. We have had no reports or complaints regarding the appearance or stability of our earlier, non-filtered wines, or for that matter, of our later, filtered wines. We decided to filter because we wanted to be sure that the wines you enjoy are as we intended them, with nothing nefarious happening before you open that bottle. We hope that this piece has helped you to understand our rational for filtering, and what the process entails. Please chime in with any comments or questions you have on the subject.

Post-script—reverse osmosis

Crossflow filtration can be so tight that it can discriminate among molecules by size. The principal components of wine: water, ethanol, and organic acids such as tartaric and malic acids, are all quite small by molecular weight. So are most volatile compounds, which contribute aromas. When bad things happen to good wines, unpleasant volatile compounds are often produced. Some examples are acetic acid and ethyl acetate, the volatile phenols mentioned above, and compounds related to “smoke taint,” such as 4-methylguaiacol. Wines can pass through a crossflow filter and then over resins that selectively bind these compounds. With the bad actors gone the treated path is reunited with what did not pass through the filter to re-create the now restored wine. This technique can also be used to adjust the alcohol content in the wine. The permeate (the portion that passed through the filter) is distilled to remove the alcohol before being rejoined with the retentate (the portion that did not pass through the filter). Pretty crazy stuff!

2017 Wildfires Update

Last night a gentle rain began to fall over Napa Valley. In Calistoga we got a welcome 0.36″ of precipitation. I cannot remember the last time rainfall was so welcome (which shows how quickly I forget the 5+ year drought that just ended).

When we returned to Calistoga from our evacuation on Monday, the valley was still ablaze. Our Cabernet vineyard in the hills above Oakville was surrounded on three sides by towers of flame. A trio of helicopters made endless loops scooping water from Robert Mondavi Vineyard (thank you!!!) and dumping the water on those burning hills just above.

I drove down-valley yesterday and the flames are completely extinguished, revealing burn scars on the hillsides. Thank you firefighters, first responders, and everyone who came to save our homes and livelihoods.

The fires began the night of Sunday, October 8. We held on until we faced mandatory evacuation on Wednesday, October 11. In the interim, the fires laid waste to much of Santa Rosa (our nearest big city), the eastern and western sides of the city of Napa, and so much more.

As we sat in exile, we could only hope that we would have a home–and a town–to return to. We are so grateful to all who made that hope a reality.

We are deeply aware of how fortunate we are, and of how much our friends and neighbors have lost. If you would like to help those in need, we recommend donating to the Napa Valley Community Foundation:…/38…

For other reputable suggestions, please look here:

Our community will take a lot of love to be made whole. Thank you for sharing yours.
Smoke filled the valley on Monday, October 9

Another smoky scene from Wednesday, October 11, the day we evacuated.

This picture does not convey how red the sun was through the smoke.

Vegan? Vegetarian? Fining? Filtration?

Are our wines vegan or vegetarian? Are they filtered or fined?

This apparent hodgepodge of topics is no mistake—these topics are related. It may never have occurred to you that a wine might not qualify as vegan or vegetarian. Wine is made from grapes, after all. But sometimes the resulting wine needs a little tweaking to meet the maker’s expectations. That’s where fining and, to a lesser extent, filtration come into play, and some of these techniques involve animal products.

Our wines are vegan-friendly

For those of you who wish to have no part in the exploitation of animals, take heart that we do not use any animal products in the production of our wines. You can sip easy, friends. Whether that is a concern or not, read on to learn how, when and why animal products can be used in wine production, and why we eschew them (gesundheit!).


Fining is subtraction by addition. If a wine has something undesirable, or an excess of something desirable, the addition of a fining agent can remove or reduce that something. In most cases, the fining agent binds to what it is removing and precipitates out of the solution, leaving no trace of itself in the wine.

Fining in red wines

The principal reason to fine a red wine is excessive tannin. All red wines contain tannins, which come from the grape skins and seeds and, in some cases, from oak barrels as well. Tannins are compounds that, by definition, bind with proteins. In your mouth, the wine tannins bind to your salivary proteins, which lubricate the mouth. They therefore have a drying or astringent effect. Too much tannin can make a wine rough, coarse or austere. The solution is to fine out the excess tannin.

Since tannins bind to protein, adding a protein to a red wine will reduce the amount of tannin in that wine. The traditional protein used is albumin, from egg whites. Pure albumin, derived from egg whites, can be purchased, but most winemakers just add egg whites, since they are almost pure albumin. Other proteins that can be used include casein, which is derived from milk, and gelatin, which is derived from animal hooves, etc.

Vegetables do not contain as much protein as animal parts, but that has not stopped one company from bringing to market a fining agent derived from potato protein. Tannin reduction by fining can, therefore, be done in a vegan manner, but most winemakers continue to use albumin or other animal-derived proteins.

We prefer to manage our tannins in the fermentor. Of course all winemakers do that. I guess we are just happier with the results than others who fine before bottling. Again, we do not fine our red wines with any fining agents—animal-derived or otherwise.

Fining in white wines

White wines are principally fined for the opposite reason as red wines—too much protein. Protein in white wines does not affect the flavor and is present in quantities too small to affect the wine’s nutritional value. However, if a white is exposed to excessive heat, which can happen during transportation, for example, the heat can denature the protein. Instead of remaining invisibly in the wine, the protein is said to throw a haze, which can look like wavy bands of stuff or solid chunks. Whatever it looks like, the customer is going to recognize that something is wrong with the wine. White wines are fined to avoid this problem.

But the real problem remains unsolved. A fined white wine exposed to excessive heat will not throw a haze, but it will still be damaged if not ruined by the heat. The customer simply won’t know that this has happened. That is why we do not fine our white wines and instead work with our storage facilities and our shipping windows to ensure that the wines are not overly heated before they reach you, our customer.

Based on the red wine tannin-protein interactions described above, you might imagine that white wines are fined by adding tannin to remove the protein. Although that is done sometimes, it is actually trickier than it sounds. Any tannin left over after the protein precipitates out will add unexpected qualities to the wine that most winemakers would wish to avoid. Instead a kaolin clay known as Bentonite is added to the wine. The clay has numerous charged sites that attract the charged protein particles. The clay-bound protein then settles to the bottom, and the clean, protein-free wine is racked off the sediment.

All well and good, and all vegan. But there is another agent used to clarify white wines (beer, too): isinglass. Isinglass is used to remove yeast (which are fungi, not animals), living or dead, from a wine to clarify it. Isinglass is a collagen-type protein derived from fish bladders, so a no-no for vegetarians and vegans. Very little of the material remains in the wine post-fining, so vegetarians who are more concerned with what they actually consume than with what was used in its productions may be okay with isinglass, but no vegan would. be  Again, we do not use Isinglass or any other fining agent in our wines.


“Unfiltered” and “unfined” are often seen on wine labels together. There is really not much of a link, however. TTB—the agency that regulates wines and their labels in the US—requires that statements on labels be true, but does not have much of an enforcement mechanism for these terms. For other items on the label, such as the origin of the grape or the grape variety, the validation and enforcement methods are more rigorous. It is unclear if there is even a legal definition of the terms. Take them with a grain of salt when you see them on a label.

We do not fine our wines, but we do filter them. The filtration cleans them up in the visual sense, and also ensures that no microbes are left behind. The microbes that can survive in wine are harmless to humans, but they can produce off flavors in wines if left unchecked. Filtration means we don’t have to worry about that.

If you would like to know more about filtration, why we do it, and how it may affect the wines, I’ll be happy to write about that in a future posting. I think this current post has grown long enough for now! Feel free to ask for more information or clarification on any of the points above, and remember to let all your vegan buddies know that our wines are safe to consume.


The People’s Viognier 2013

2013 The People’s Viognier

Salem Ranch, Dry Creek Valley, Sonoma County

The 2013 People’s Viognier is 100% Viognier from the Salem Ranch in Sonoma’s Dry Creek Valley. It is hauntingly perfumed, with delicate and subtle notes of jasmine, white peach, apricot, and sweet almond. The beautifully balanced wine is medium-bodied. The aromas dance and weave throughout the long finish.

We harvested the grapes on September 28, 2013, at 23.6° Brix, pH 3.47 and 5.8 g/L titratable acidity. The grapes were gently pressed and the wine fermented slowly in a neutral vessel. We bottled 275 cases on February 21, 2014. 13.5% alcohol, pH 3.50 and 5.87 g/L titratable acidity.

Download the PDF here.

Bea’s Knees Petite Sirah 2008

2008 Bea’s Knees Petite Sirah

El Dorado County, California

Our 2008 “Bea’s Knees” is a blend of Petite Sirah (86%) and Zinfandel (14%) from the Sierra Foothills, a region unlike any other in California due to its elevation and granitic soils. Petite Sirah and Zinfandel are two of the varieties that excel in these hills and this is a beautiful blend. The naturally abundant tannins of Petite Sirah are tempered by the Zinfandel, producing a wine with a commanding presence, but not one that will frighten the horses! The Zinfandel also adds bright bing cherry and black raspberry flavors to the rich blackberry, blueberry and mocha flavors from the Petite Sirah.

The Petite Sirah is from 10-year-old vines planted 2700 feet above sea level near Placerville in El Dorado County. The Zinfandel is from 20-year-old vines at 1800 feet on decomposed granite near Fiddletown.

The grapes were harvested at 25.2 ºBrix, pH 3.60 and 6.2 g/L titratable acidity. The wine was barrel aged in mostly (90%) neutral wood. until bottling on April 25, 2012.

Download the PDF tech sheet here.