Filtration is a winemaking technique that has been used for many years in both Old and New World winemaking. While it has benefits for the aesthetic appeal of a wine, it is also important for the structural integrity and ageability of a wine.
There are many methods of filtration (not to be confused with fining*) but they should not involve any chemicals or extra additives. I’ve outlined a few of the main methods below:
Plate-in-Frame, or Pad Filtration
This is a common type of filtration technique used for smaller scale producers. Like the name suggests, it involves plates that sit vertically in the metal frame of the filter mechanism. Pads made from cellulose (tree or plant fiber) are inserted between each plate, and the wine travels through the filter mechanism by moving through the pads. Any sediment, microbes, tartrate crystals, etc. are left adhered to the pad and at the outlet you have your clean*, filtered wine ready to be bottled.
The cellulose pads come in many different pore sizes through which the wine moves, depending on how much a winemaker wishes to filter his or her wine. Wine may pass through multiple filtration passes or even multiple types of filters, one to clarify the wine from bigger sediment pieces and another one to reduce microbial content in the wine. Plate-in-frame is the method of filtration with which I am the most familiar. It is technical and time consuming, but requires skill and precision to operate.
With a membrane filter you can “sterile” filter your wine. The word sterile sounds scary, but it just means that the wine passes through a membrane with a pore size of less than 0.45 microns (smaller than what we can see) so that spoilage bacteria or yeast cannot get through. Filtering your wine at this level quite literally removes most of the potential for spoilage and thus increases the potential to age the wine for many years. It drastically decreases the likelihood of having any harmful yeast or bacteria in the bottle that could have harmful effects on the wine’s integrity, such as altering the taste or smell. This is the type of filter often found at the beginning of a bottling line and the last line of defense against spoilage before the wine is bottled.
Cross-flow is very common in medium to large-scale commercial wineries. It is more straight-forward to hook up and run than plate-in-frame and can also be used to remove Brettanomyces yeast, which can yield sensory characteristics that in America may be undesirable.
Not every producer filters. Some wines don’t need to be – they may have settled in barrel/tank for quite some time so that when they are racked off the lees there is hardly any sediment. Some wines are meant to “drink now” and the winemaker may not deem sterile filtration necessary. Like most things in winemaking, there is no right or wrong way to do it, it’s mostly winemaker and consumer preference. There is nothing bad or wrong about a wine that has some sediment at the bottom. On the converse, just because a wine has been filtered that is not a bad thing! Filtration is just another step in the winemaking process like crushing or de-stemming the grapes. It doesn’t add any “chemicals” to the wine or make the grapes any less organic. You can always ask the producer what their filtration method is to gain a better understanding of it as well.
*fining entails adding an agent to clarify wine by binding to tannins, sediment, color, proteins, etc. and then the unwanted compounds are either precipitated or filtered out.
*by clean I mean filtered and rid of sediment, debris, microbes, or anything unwanted in the wine. However the term “clean” when used in wine marketing has no one, clear meaning. Marketing ploys use this broad term to suggest that the wine is natural or doesn’t contain additives, but since there is currently no legal definition of “clean wine” it does not actually guarantee that it is. It doesn’t guarantee the consumer anything in terms of the quality of wine or where it came from.