An often-discussed topic in winemaking is filtration technology and the pros and cons of using it. We know wine goes through several stages to turn from grapes on the vine into the clear “nectar of gods” in our glass. Clear wine is most often the result of filtration. However, some producers eschew it and claim that a non-filtered wine is much better in quality and flavours. Nonfiltration can benefit wine, but also has risks.

Filtration means running liquid through a porous surface and separating the particles that are bigger than the pores from the liquid. The younger the wine stage, the bigger these pore sizes usually need to be. During ageing wine autoclarifies itself meaning that during its time in barrels or tanks certain physio-chemical reactions cause the precipitation of particles that cannot remain in suspension. Filtration during ageing accelerates this auto-clarifying step. Therefore, the longer wine ages, the less filtration it needs later on to reach a brilliant/clear stage.

Winemakers can actually avoid complete filtration under the following conditions: the wine had enough time to auto-clarify itself (temperature, initial grape quality and tank material dictate the length of this process) and the wine is residual sugar-free, or better, completely dry and without any malic acid. If residual sugar or malic acid is still in the wine there’s a risk of a second fermentation in the bottle. Finally, to avoid filtration the bottling process needs to be done under highly sterile conditions.

Generally, red wines benefit more from nonfiltering as they are usually more accepted to be dry and they usually do not have any residual malic acid. White wines, on the other hand, are more difficult to handle and non-filtering usually impairs flavours and consistency of the product. A non-filtered wine is richer and rounder in the mouth, which makes sense since filtration takes out particles that would otherwise contribute to the mouth feel of wine. Non-filtered wines do risk developing off-flavours due to microbiological contamination or unwanted second fermentation in bottle.

Keeping handling of the wine to a minimum and giving it maximum time to vinify benefits the development of flavours and structure. However, different wine styles do require different treatment. Wine that is launched a few months after its actual harvest time usually needs to run through several filtration steps in order to be clear in the bottle. These wines usually show a young and fruity character that is actually wanted. The filtration helps these wine types to open up easier, to release more fruity characters and to appear cleaner on the nose.

Another factor is where the wine is going to be consumed. Hot-climate countries have a risk of re-fermentation in the bottle for wines that are not dry and therefore a sterile filtration is crucial to guarantee consistency. Hot climates also accelerate ageing and the development of undesired flavours due to temperature influence. If the precursors for those undesired flavours have been taken out previously by filtration there is less risk of it developing.

It’s hard to decide when to filter wine, especially when it impairs the mouth feel and flavour. But filtration technology nowadays is highly advanced and manufacturers of the machinery continually do research to preserve the integrity and consistency of wine. The skill and bravery of the winemaker to choose the right filtration technology is aimed at keeping the need for it to a minimum during the winemaking process. However, completely abandoning the filtering step is risky. The right method and necessary frequency lay somewhere in the middle.

Unfiltered wines can be very exciting, but are recommended only if the winemaker knows the risks associated with it. Additionally, it helps to know the producer’s track record in producing non-filtered wine in order to guarantee full enjoyment without unpleasant surprises. Stories have been told about bartenders in France needing to shake a bottle of unfiltered wine before serving it to the client in order to remove the CO2, which is the product of an undesired second fermentation in the bottle.