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.
Principles of filtration
The filtration mechanisms or application techniques are divided into the following basic principles:
According to the location of the trub retention, a distinction is made between surface or membrane filtration and depth filtration. In surface filtration, the filtered solids form a layer on the filter medium, the filter cake. In depth filtration, they are retained in the depth (inside) of the filter medium. There are the variants precoat filtration, layer filtration and depth filter modules/candles. According to the liquid flow, a distinction is made between static and dynamic filtration.
Static filtration (also frontal or dead-end filtration): The direction of flow of the liquid is vertical from top to bottom (vertical) through the filter area. All particles are retained either on the surface (surface or membrane filtration) or in the depth of the filter medium (depth filtration).
Dynamic filtration: The liquid is not passed vertically through the filter medium, but parallel or across a membrane surface. There is no dynamic depth filtration.
Sharp filtration can therefore also be used to fractionate wine. In this process, different substances such as alcohol and aromatic substances are separated and combined to form a "new wine". However, this application is prohibited within the EU or limited to very few applications such as alcohol reduction, but is common in the New World (see Spinning Cone Column).
Filtration forms & techniques
The most important of the many forms with different filter techniques and in detail different variants, often used in combination:
Membrane filtration: The static MF is mainly used for sterilization. This requirement is fulfilled when all potentially harmful microorganisms can be removed. Due to the low trub absorption capacity, large surfaces are required, which is why the membranes are usually folded (pleated) like an accordion.
Cross-flow filtration (also known as cross-flow filtration or tangential flow filtration): This is a dynamic form of filtration. The name is derived from the flow direction of the liquid. This is guided across a membrane surface (crossflow). This allows the size range of the particles to be selected specifically. Depending on the particle size of the solids to be separated, a distinction is made between micro-filtration (0.5 to 0.1 µm), ultra-filtration (0.1 to 0.01 µm) and nano-filtration (0.01 to 0.001 µm). For sizes from 1 to 0.1 nm (billionths of a meter) one speaks of reverse osmosis (see under osmosis).
The wine is pumped through the membrane at high speed. The filter cartridge consists of many asymmetrically constructed, hollow plastic membrane fibres in a vertically arranged filter housing. With newer models, filter aids are no longer necessary. The wine is pumped repeatedly through these hollow fibres in the circuit. A small, filtered part flows continuously sideways out of the housing, the larger part remains in circulation. The advantage of this technique is that the clarification (compared to all other techniques where at least two operations are necessary) is carried out in a single operation and filtration is possible even when the degree of turbidity is very high, such as with a young wine still fermenting.
Precoat filtration: In the static depth filtration form, filter aids such as diatomaceous earth, perlite and cellulose as well as pulverised resins are used as ion exchangers. These are first suspended (dissolved) in a liquid and then washed onto the filter carrier. As soon as a sufficiently thick filter cake has built up, which then functions as the actual filter, the filter is switched over to the liquid to be filtered. This enables the extensive removal of even the smallest undissolved particles. In addition, adsorptive effects can also be achieved depending on the auxiliaries.
Layer filtration: The static depth filtration form is mainly used in Central Europe; in Austria it is used to about 90%. In contrast to the precoat form (where a filter cake is first built up), there is a prefabricated filter bed. Depending on the application, this consists of a special mixture of filter aids such as cellulose, diatomaceous earth and/or perlite as well as resins. Depending on the mixture, special filter properties result. The layer, which is only 3 to 5 mm thick, results in a labyrinth-like, close-meshed space sieve with the finest, countless branched channels. The wine flows through these channels relatively slowly. The mode of operation results from a mechanical screening effect and an additional adsorptive component.
Drum vacuum filtration (vacuum rotary filter filtration): In this form, a rotating drum is mounted in a trough, which is surrounded by a close-meshed stainless steel sieve mesh. A mixture of water and filtration aid (diatomaceous earth or perlite) is added to the tank. This is only suitable for must lees, not for wine.
The filtration residues (lees, yeasts) are returned to the nutrient cycle in the vineyard, because organic matter is predominantly of a nutrient humus character with a high nitrogen content. Larger quantities of filter aids reduce the nutrient content. However, since it is difficult to spread the residues over a large area in a fresh state, they are usually composted beforehand after mixing with straw, tree bark, wood chippings or marc. This does not include the fining substrate produced during blue tinting, which has to be disposed of because of the cyan compounds it contains (see under tinting).
Possible alternatives to filtration using various mechanical devices are flotation and centrifugation. Further processes are described under the keyword Schönen. Complete lists of the numerous cellar techniques, as well as the types of wine, sparkling wine and distillate regulated by wine law are included under the keyword vinification. Comprehensive information on wine law can be found under wine law.