filtration, fundamental but controversial winemaking process, a means of clarification involving the removal of solid particles from a wine. At the end of fermentation, wine contains a vast amount of particulate matter, including yeast cells, bacteria, grape cellulose, proteins, and pectins. All of these have to be progressively removed in order to produce a clear wine.
There are two principal categories of filtration: depth filtration and surface or absolute filtration. Depth filtration involves the use of a relatively thick layer of a finely divided material such as diatomaceous earth, or pads made of cellulose fibres. As the cloudy wine passes through the layer, small particles are trapped in the tortuous channels and clear liquid passes through. This form of filtration is useful for removing large quantities of solid matter because the thickness of the filtration layer presents a large volume of medium in which the particles are trapped. The rotary drum vacuum filter is the extreme example of depth filtration because it can be used to separate wine from the thick deposits at the bottom of a fermentation tank. A sheet filter fitted with thick pads of cellulose fibres is used for removing fine particles in preparation for the final filtration prior to bottling. The risk involved in using depth filtration is that by increasing the pressure or flow-rate beyond the specified maximum, particles can be forced through the filter to the clean side.
