Physical-mechanical process 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). This was used to make a drink called hypocras, similar to mulled wine, which is still produced today according to an old recipe.
In today's viticulture, filtration mainly concerns grape must and young wine. The purpose 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 wine ready for filtration as a prerequisite for the respective type of filtration. This means that the wine must not be too cloudy. A subsequent step is the removal of microorganisms such as yeasts and bacteria. Filtration does not only have advantages, however, because it means a more or less strong mechanical strain on the wine. The fine particles and suspended particles are also flavour carriers, and removing them can also result in (excessive) losses of aromatic substances or carbonic acid. Therefore, the method must be used carefully. Improper or too sharp application can lead to filter shock
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 carried out in combination. However, filters do not only retain particles that are larger than the filter pore size (as one might assume). 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 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 (the human hair has a circumference of approx. 40 µm).