6.1.3.a Overview

What is the objective when filtering wines?

Filtration is not compulsory, but it can greatly limit microbiological deviation risks or wine instability over time. If filtration is deemed necessary, it must be carefully considered and suited to the type of wine being produced. Overly fine filtration can lead to rapid clogging, which is undesirable (reduced flow, increased pressure, etc.). Conversely, a filter that is too porous does not enable sufficient particle retention. It is therefore necessary to match the type of filter to the wine's characteristics and to determine specific objectives:

• For example, if coarse filtration is the objective (with a view to generally reducing further interventions), it may be preferable to proceed with a fining and then racking and filtering prior to bottling using diatomaceous earth filtration or plate filtration. It is also possible to use tangential filtration, if the quantities are limited, since if the fluid is highly charged, the flow will be greatly reduced.

• On the other hand, if it is a wine for cellaring, or for export (transport, change of storage conditions) or if the wine has had problems in the past (microbiological instability for example), pre-filtration prior to a final sterile filtration can be carried out as a precaution. A wine is considered sterile when the microorganism population is less than 1 CFU/750 ml, the sterility is not absolute but there are no known bacteria or yeast still in the wine (however, the analysis may not reflect the entire batch).

What are the consequences of poor filtration?

Poorly executed filtration can have adverse consequences for the wine. The removal of certain particles and colloids can result in a loss of substance and reduce the wine's potential quality.

First of all, the wine's organoleptic qualities can be modified by the filter itself, if it is of poor quality or not properly cleaned, or by the filtration circuit (oxidation, foreign tastes, unwelcome odours, etc.).
When the filter medium is not suited to the wine, over-filtration can occur when particle and colloid layers accumulate on the filter medium. In this case, two possibilities can arise:

• The filter medium porosity is modified (a certain number of pores become clogged). More pressure is therefore required to obtain the same filtration rate with a reduced number of pores. This can lead to lower retention and undesirable molecules or particles passing through, such as certain yeasts or bacteria responsible for deviations.

• The porosity is unaffected (all pores are functional) but the pore size is reduced. This results in a finer filtration than initially envisaged (and the removal of certain desirable macromolecules or polysaccharides). In this case, it is possible that their removal affects the wine's organoleptic potential. The wine can seem depleted and unstructured.

Over-filtration can cause heterogeneity in constituents (desirable macromolecules, colloids, microbial populations, turbidity etc.) to occur between the initial and final bottles when bottling a batch that was initially homogeneous. This heterogeneity can show up in tests as well as in tastings. It is therefore important to adapt the filtering medium to the wine to be filtered in order to reduce the risk of clogging and over-filtration. Regular monitoring of turbidity levels at the filter outlet or in the bottles can help to check filtration quality.