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Physical-mechanical process (also filtration, filtering and filtering) for the separation or purification of substances such as liquids or gases by means of technical filtering devices. Various filtration procedures are a common process in winemaking. Already in ancient times the Egyptians, Sumerians and Romans used a technique in which wine was filtered by means of cloths or similar material. In the Middle Ages, a piece of cloth made of muslin acting as a filter was used to filter the added aromatic spices out of the wine. This piece of cloth was called "Manica Hippocratis" (Hippocratic sleeve).

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

However, filters not only retain those particles that (as one might assume) are larger than the filter pore size. This 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 are generally also separated that are far smaller than the pore size of the filter. In any case, filter pore sizes with ≤ 1 µm (millionth of a meter) are required (human hair approx. 40 µm).

Filtration can be carried out 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. Filtration, however, always means a more or less strong mechanical stress on the wine, there can also be (too high) losses of aromatic substances or carbonic acid, therefore this method should be used carefully. Improper or too sharp application can lead to filter shock. Many producers therefore largely do without filtration.

Starting in the USA, for marketing reasons, the label in Europe has increasingly included the words "unfiltered" (or similar) on the label in recent years. The alternative to filtration is tapping (moving) from one container to another, which is certainly much gentler, but much more expensive. The Beaujolais Nouveau is basically unfiltered wine. For 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", filtration is not carried out as a matter of principle (see yeast sediment storage).

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

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