The oenology program that I’m attending in Toulouse recently took our class to the INRA wine research center that is located on the Mediterranean coast near Montpellier. INRA (the French Institut National de la Recherche Agronomique) is the largest academic agronomic research center in Europe.
Besides being a repository of over 30 different grape varieties, the center’s engineers and technicians test and develop equipment and techniques used in fermenting, filtering, distilling, pressing and stabilizing wine. In May 2010 I wrote about the center’s Cité de la Vigne et du Vin (“City of Vines and Wine”) public exhibition space, which is now, unfortunately, closed.
This return trip was an academic exercise, with the hands-on use of a variety of filtering equipment used to remove heavy particles from wine. We had the chance to use three different types of filtration: depth filtration, using diatomaceous (alluvial) earth; surface filtration, using cellulose-fiber pads; and tangential, or cross-flow, filtering, where the wine flows parallel to the filter surface, avoiding the clogging that is the chief problem of depth and surface filtration.
We had the opportunity to use two different depth-filtration systems—the first uses a series of rotating hollow metal plates with a mesh surface, while the second consists of a rotating, horizontal drum with a cylindrical, stainless steel mesh surface. Both systems use a vacuum pump that attracts the slurry formed when you pour in various grades of the diatomaceous earth (also called diatomite or kieselgur) into water. Diatomaceous earth consists of the skeletal remains of diatoms, tiny sea creatures that inhabited ancient seas. After being ground into a fine powder, they are treated with acids and alkali until all that remains is silica. A porous barrier with numerous, tortuous channels is formed within the beds that build up on the rotating plate and drum mesh surfaces. As the wine passes through the layers, the solid particles are trapped within the channels.
The technician demonstrating the filters explained that both systems are used for a first filtration, immediately after fermentation has finished. He also told us that the drum filter, while having the advantage of being able to cope with extremely heavy solids—including the wine lees, had the disadvantage of having a large area of the filter exposed to the air. The rotary drum filter is totally encased, protecting the wine from oxidation, and it can even be flushed with nitrogen before use, avoiding any contact with oxygen.
We also were able to see a surface filtration system that uses a membrane to filter out yeast and bacteria before bottling. These filtering systems are quite inexpensive, we were told, but the membranes are costly, and they can become blocked very quickly.
Unlike depth and surface filtration, where the wine passes perpendicularly to the filtration surface, tangential (or cross-flow) filters use a process where the wine flows across the surface of a ceramic or organic tube. This helps to sweep the surface clean, and the wine can circulate repeatedly past the filter membrane. The one that we saw demonstrated was set up to filter tank after tank of wine sequentially, avoiding the preparation and clean-up required for a depth-filtration system. The tangential filters can also be used right after fermentation and right before bottling. They are expensive, however, and would be found more often in larger wine cellars or cooperatives. Several winemakers whom I have interviewed use mobile tangential filtering services that charge by the volume of wine filtered.
Wine filtration is a controversial subject, with some saying that filtering can ruin a wine. The demonstrations that we saw are by no means the definitive “last word,” but it appears that the filtration of commercial (versus fine) wines, properly used, can speed up the production process and allow better quality control.