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Magazine Mesa y Negocios
April 2010
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| VINOBLE |
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Wine can only be enjoyed in moderation. |
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NEWS |
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THE USE OF FILTRATION IN WINEMAKING
Filtration, although controversial, it is one of the procedures available at the winery that can be used by the winemaker at different stages of the production chain. If applied in a graded fashion it can be of tremendous benefit in obtaining wines that have a good clarity, excellent colour and no micro organisms that could spoil the finalised product.
Filtration can be performed based on two principles, depth filtration and surface or absolute filtration. Depth filtration, also known as sheet filtration involves the use of a relatively thick layer of a porous material such as Kieselguhr or diatomaceous earth, cellulose powder or perlite. It is good for liquids heavily laden with solid particles, such as juice directly from the press. As the wine passes through the layers, small particles are trapped in the tortuous channels.
Kieselguhr or diatomaceous earth is the skeletal remains of diatoms, tiny sea creatures that inhabited the North Sea millions of years ago. The deposits are ground into a fine powder and treated with acids and alkali until all that remains is Silica. When these particles are formed into a bed they produce an effective filter by forming a porous barrier with numerous and tortuous channels. Kieselguhr forms the basis of two types of filter which are used for a first filtration, right after fermentation has finished. In both cases the diatomaceous earth is made into a slurry, with water or wine, and is then dosed into the cloudy wine to be filtered. The first system is the rotary vacuum filter which is the choice for the first filtrations since it can cope with very thick liquids. It consists of a large horizontal drum that has a cylindrical surface formed from a stainless steel mesh and it is connected to a vacuum pump. Filtration occurs because the Kieselguhr is trapped by the mesh forming a layer that will act as a sponge. This type of filter is used primarily for filtering the lees. However, it has the disadvantage that a large area of the filter is exposed to air.
The earth filter was developed to overcome the problem mentioned above of possible oxidation. It consists of a series of rotating hollow disks with a mesh surface which operates exactly as the rotary vacuum filter. It is totally enclosed and can be flushed with nitrogen before use, to avoid any contact with oxygen. Sheet filtration is another form of depth filtration commonly used once the gross solids have been removed. It consists of a fixed framework with a fixed back plate and a movable front plate. Between these two plates it is a chamber called plates, which distributes the wine. The wine, however, does not pass through all the sheets from one end to the other but only through one sheet. The reason for having numerous sheets is to increase the flow rate. Each sheet consists of cellulose fibres that may also contain Kieselguhr to increase efficiency. Although the sheets are not expensive it is important for the operator to know and understand the stated maximum flow rate and maximum pressure differential between the inlet and the outlet. The successful filtration will depend on the correct selection of sheets for each type of wine.
A further development in the process of depth filtration it is the understanding that many particles in suspension carry an electrostatic charge known as zeta potential. It is possible to incorporate a substance in the structure of the filter medium that also carries an electrostatic charge. These substances will attract particles with the opposite charge and will be removed. It must be mentioned that zeta potential sheets do not trap colloids, therefore, fining cannot be replaced.
The second principle of filtration is the surface filtration, also known as membrane filtration or cartridge filtration. The difference with depth filtration is that with this type of filters the particles are removed at the surface of the filter. It consists of a membrane with holes of a determined size. This type of filtration requires extra care and training because the membrane can get blocked very quickly, therefore, it should be used only in the final stages prior to bottling.
Although the structure of this type of filters is not terribly expensive, cartridges or membranes are. These membranes come in different size pores, being 0?45 cm the most widely used size for the final membrane which removes all yeasts and bacteria. A guard filter it is also used to protect the final membrane from clogging and it is made of a fibrous material.
It is important to relate pores sizes to the type and style of wine being filtered. German wines need a more stringent filtration due to their usually higher residual sugar and low alcohol, to prevent a re-fermentation in bottle. In sweet wines it is also mandatory a good filtration process, also due to the risk of re-fermentation.
A modern and effective filtration system it is known as cross flow or tangential filtration. In all the above mentioned systems the flow of the wine is perpendicular to the surface of the filter which will gradually block it. If the flow goes across the surface of the membrane it will help sweep the surface clean. The wine can circulate repeatedly past the membrane. Another advantage of this system is that it can be used right after fermentation up to bottling standards in one pass. It has only the disadvantage of being quite an expensive system.
The process of filtration is causing more controversy than and other winemaking procedure in today?s wine industry. There are those who maintain that filtration ruins the wine and there is the school that claims that, if properly applied, there is no harm to the wine. The fact is that filtration properly used, with care and expertise, in a graded manner, should have no negative effects. The greater danger with filtration is being over-zealous, using sheets that are too tight or membranes that have too small pores. Properly applied it gives the wine maker a satisfactory system for the bottling of good quality commercial wines.
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