There are two reasons to filter wine: aesthetics and microbial stability. On the aesthetic side, filtration can make a wine more polished both in the glass and in the mouth; often creating a rounding effect that softens the wine’s edges. If your wine is sound with no flaws, then you can decide if you want to further shape your wine by filtering it. However, if you have residual sugar or Malic acid left in the wine, or there was a problem with Acetobacter or Brettanomyces during the ageing/storage period, then filtration is no longer an artistic decision; it becomes the only way to guarantee microbial stability for the wine.
Pore sizes of filters are measured in microns. Typical winemaking sizes are 5, 3, 2, 1, and .45 micron media. The smaller the holes, the “tighter” the filter is said to be. Filtration’s guarantee of microbial stability comes from the fact that the pore size of filters can be made smaller than the actual yeast and microbes themselves. As the wine passes through the filter the larger microbes become stuck and are removed from the wine. Note: 2-micron filters are used to remove yeast, and .45-microns are needed to remove bacteria.
Filters are rated as being “Nominal” or “Absolute”. A nominal filter will remove most particles that are equal or greater than the rated micron size. An absolute filter will remove all particles larger than the micron rating. Nominal filters are cheaper than absolute ones, and if you are only doing a general cleaning up of the wine, a nominal filter may be all you need. However, if you are filtering to remove either yeast or bacteria, you will need to rely on an absolute filter. Note that an absolute filter is only needed at the final filtration of the wine (usually during bottling, right before the filler to minimize exposing the sterile wine to contamination).
The effect that filtration has on wine becomes more pronounced as the micron-size becomes smaller. Filtration does remove certain elements from a wine; however, these are often elements that are worth losing. Filtration can stress a wine and cause it to temporarily “fall apart” right after the process. However, just as with “bottle shock”, filtered wines put themselves back together just fine over the following weeks.
Filtration set-ups are based on the two different forms of filtration media: cartridges and pads. Cartridges use housings, whereas pads require a “plate and frame” set-up. Both require a pump to move the wine (note that small lots can also be done without a pump using a keg and pressurized gas if you have this equipment). Cartridges are more expensive than pads because they are more intensive to produce, but they can be cleaned and stored for future use. Pads are cheap but they can only be used one time. Both pads and cartridges are tried and true, and choosing between the two technologies just comes down to personal working preferences: cartridges are clean to work with but they are more expensive and time intensive for maintenance. Pads are economical and somewhat messy, however when you’re finished you just toss them.
Note: Only cartridges can provide .45 Absolute ratings. In other words you cannot achieve a sterile filtration using a plate and frame set-up with pads.
In the end, filtration is a very effective winemaking tool that can be used to gently polish a wine or to make sure it is microbiologically stable. However, the initial investment for the housing(s) or the plate and frame system make it a bit of an economic hurdle for the beginning winemaker. Fining requires no equipment and offers a cheap way to clarify a wine and have control over its tannin profile. Fining is the only way to achieve heat stability in a wine- as filtration does not remove the proteins responsible for heat instability. The only caveat is that fining is not very selective. You need to be careful about preserving the balance of all of the elements. Finally, keep in mind that the two actions are not mutually exclusive and a light fining is often done to improve a wine’s filterability.
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