Physical-mechanical process for separating substances such as liquids or gases using technical filter devices. Various methods are frequently used in winemaking. Even in ancient times, the Egyptians, Sumerians and Romans used a technique in which wine was filtered using cloth or similar material. In the Middle Ages, a piece of muslin cloth was used as a filter to remove the added flavouring spices from the wine. This piece of cloth was known as the "Manica Hippocratis" (sleeve of Hippocrates). It was used to make hypocras, a drink similar to mulled wine, which is still produced today.
Purpose of filtration
Today, filtration mainly concerns must and young wine. It is used to remove unwanted substances and thus stabilise or sterilise the must or wine. This usually takes place in several steps. Firstly, the rather large lees (pulp) produced during pressing are removed from the must (see under degumming and clarification). Only then is the wine ready to be filtered 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, which can be achieved by filtration or suitable fining processes.
Negative effects
However, filtration does not only have advantages, as it means a greater or lesser mechanical load on the wine. The fine particles and suspended particles are also flavour carriers, and removing them can result in excessive losses of aromatic substances or carbon dioxide in the worst case. The method must therefore be used with care. If used incorrectly or too harshly, this can lead to filter shock.
Improper, unhygienic storage or handling of the materials used can also result in a so-called filter flavour. This unpleasant wine taint is reminiscent of wet paper, cardboard or cellulose and is caused by odour-active residues in poorly rinsed or contaminated filters. Organic filter aids such as cellulose, diatomaceous earth or activated carbon are particularly affected.
No filtration
The words "unfiltered" are increasingly found on the label. Often the fining is also omitted. This is to prevent the loss of flavour-forming aromatic substances. A longer ageing period is a prerequisite. In barrel ageing in particular, the lees settle naturally and are removed by racking (decanting) several times if necessary. Unfiltered wines generally form more sediment in the bottle.
Time of filtration
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, filters do not only retain particles that (as one might assume) are larger than the filter pore size. This is only one of the effects.
Other mechanisms that may occur during filtration are particle inertia, diffusion effects (thermal movement of particles), electrostatics with adsorptive effect or barrier effect. Particles that are much smaller than the pore size of the filter are therefore also separated. In practice, filter pore sizes of ≤ 1 µm (millionth of a metre) are required; for comparison: a human hair has a diameter of around 40 µm.
Alternative to filtration
Starting in the USA, the term "unfiltered" (or similar terms) has increasingly been used on labels in Europe in recent years for marketing reasons. The alternative to filtration is racking (racking) from one wine container to another, which is certainly much gentler, but considerably more time-consuming. Beaujolais Nouveau is basically unfiltered wine. Wines that have been aged in barrels for a longer period of time automatically become more stable during storage. Unfiltered wines form a...
