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Physical-mechanical process (also filtration, filtering and filtering) for separating or purifying substances such as liquids or gases by means of technical filtering devices. Various filtration methods are a common process in winemaking. Even in ancient times, the Egyptians, Sumerians and Romans had a technique of filtering wine using cloth or similar material. In the Middle Ages, the added aromatic spices were filtered out of the wine with a piece of cloth made of muslin that acted as a filter. This piece of cloth was called the "Manica Hippocratis" (Hippocrates' sleeve).

In today's viticulture, filtration mainly concerns grape must and young wine, but also air and water for the purpose of disinfection. The purpose of this process is to remove unwanted substances and thus stabilise or sterilise the grape must or wine. As a rule, this is done in several steps. First, the rather large trub substances (pulp) produced during pressing are removed from the must (see degumming and clarification). Only then is the so-called filter maturity of the wine achieved as a prerequisite for the respective type of filtration. This means that the wine must not be too cloudy. The next step is the removal of microorganisms such as yeasts and bacteria.

However, filters do not only retain those particles that (as one might assume) are larger than the filter pore size. Because that is only one of the effects. Other mechanisms are particle inertia, diffusion effects (thermal movement of particles), electrostatics with adsorptive effect or barrier effect. Therefore, particles that are far smaller than the pore size of the filter are basically also separated. In any case, filter pore sizes with ≤ 1 µm (millionth of a metre) are required (human hair approx. 40 µm).

Filtration can take place several times at different stages of winemaking, especially after pressing (must), after fermentation and immediately before or during bottling. As a rule, several of the processes listed below are also carried out in combination. However, filtering always means a more or less strong mechanical stress of the wine, it can also result in (too high) losses of aromatic substances or carbonic acid, therefore this method should be used carefully. If used improperly or too harshly, this can lead to filter shock. Many producers therefore largely do without filtration.

Starting from the USA, for marketing reasons, one can increasingly find the indication "unfiltered" (or similar) on the label in Europe in recent years. The alternative to filtration is racking (racking) from one container into another, which is certainly much gentler, but much more time-consuming. Beaujolais Nouveau is basically unfiltered wine. In wines with longer barrel ageing, storage automatically results in better stability. Unfiltered wines form a deposit (sediment) to a much greater extent. For special wines such as "sur lie", no filtration is carried out as a matter of principle (see yeast sediment storage).

The unfiltered mixture of a liquid and finely distributed solids in it is called suspension, the filtered product filtrate. Depending on the filter, the particles are separated either due to size (sieve effect) and/or due to adsorptive (electrostatic) effect. There are a number of different filtration techniques, depending on the size of the operation, the technical equipment and the type of wine required.

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