6.1.3.b What are the existing filtration systems and their features?

Filtration is used to reduce a wine's cloudiness, and the risk of precipitation and microbiological and organoleptic deviances (presence of microorganisms, plant debris, precipitates such as potassium bitartrate). The filter makes it possible to retain certain particles according to their size. It is a mechanical operation that aims to replace or complement natural clarification, which is not always sufficient depending on the objectives set for the wine (microbiological stability, clarity, etc.).

In this sense, there are different types of filters that allow smaller or larger particles to flow through.

Two types of precoat filtration exist:

Press filters: This device operates with a piston pump and consists of several plates. The plates are covered with a cloth on which the impurities mixed with the additive (often perlites or diatoms) become fixed. The pressure increases as the chambers fill.

Rotary vacuum filters: These operate by suction of the fluid to be filtered due to rotation under vacuum. The wine passes through a pre-layer of diatoms before being sucked into the drum. A scraper removes any additive or impurity accumulation during the filtration process, allowing a continuous renewal of the filtering layer. The flow rate is higher than for press filter presses.

Plate filters

These systems allow the filtration of less charged liquids and are well suited to bottling, for example. Impurities are retained by sieving and adsorption. The plates (made up mainly of cellulose and then diatomaceous earth or perlite) can be of various sizes, with 40, 60 or 100 cm sides. The flow rate is generally lower than with diatomaceous earth precoat filtration (500 l/h/m2). Depending on the amount of retention of microorganisms, two types of plates can be defined:

Clarifying plates : not as close together as sterilizing plates, they have a better flow rate

• Sterilizing plates: they are more constricted, filter a larger amount of small microorganisms (such as bacteria) and have better retention than clarifying plates but therefore have a lower flow rate and a higher risk of clogging.

Lenticular filtration is also possible with plates, but the particularity of this filtration is its enclosed mounting in a hood to avoid leakage. Lenticular filtration is commonly used at the Eugenie estate and was also used at Château Latour before cartridge filters were introduced.

Membrane filters

Membrane filters enable sterile or very low-germ filtration and are used for the final filtration prior to bottling. This type of filtration consists of two filters: cartridges used as prefilters to reduce clogging and protect the membranes, and a final filtration through a membrane. The pre-filter cartridges and the membranes are attached to the base of a housing and then covered with a hood. To avoid the risk of clogging, the wine should be clarified beforehand (prefiltered or fined), which greatly improves the flow rate (about 400 l/h/m2).

The membranes can be of different sizes depending on the filtration required (sterile or not). For example, the membranes can be 1.2, 0.6 or 0.45 µm to retain yeast or even bacteria. The pre-filter cartridges can be 3 or 5 µm for example.

This type of filtration is used prior to bottling at Château Latour.

Crossflow ultrafiltration (0.1 to 0.001 µm) can be distinguished from crossflow microfiltration (10 to 0.1 µm) by pore size, although during filtration, clogging reduces the distinction between the two methods. Unlike the frontal filter types which are used either on a charged liquid or on an already clarified liquid, crossflow filtration can be implemented on any type of fluid, charged or clarified (prefiltered or not). This adaptation is made possible by the choice of membranes (0.2 or 0.4 µm).

The advantage to employing this filtration method is that, despite the high cost, it is not always necessary to repeat clarification operations on the wine, which could cause it to lose substance (even though some crossflow filtered wines still require preparation to reduce clogging, for example). The flow rate is often lower than with dead-end filtration (about 50 to 100 l/h/m2).

To summarize:
Type of filter/filtration for different particle sizes Source: Romat H., 2014. Filtration en oenology.