Filtration is the process of removing solids from liquid by passage across or through a porous medium. The effectiveness of filtration depends on particle size and filter medium porosity. Materials suspended in liquid will remain on or in the filter medium if they are larger than the pore opening. Filtration is widely used throughout brewing, and most beers sold today are filtered to some extent. The range of particle size removal is enormous, from the molecular level (1.0 μm) in water filtration to whole hop removal (1.0 cm) after the kettle boil.
There are two basic types of filtration: depth and surface. Depth filtration, also called powder filtration, uses a convoluted labyrinth of channels in the filter media to trap particles. The media can be diatomaceous earth (DE), Perlite, or other porous media. Depth filtration is often considered rough or primary filtration, though in many small breweries, this is the only filtration performed. Examples are plate and frame, screen, and candle filters. Surface filtration uses a thin film material with pores smaller than the particles to be removed. Particles remain on the surface of the filter while clarified liquid flows through. If the pores are of a defined size (for instance, up to 5 μm), filtration is said to be “absolute” to the pore size. Membrane and cross-flow filtration are examples.
Filter media, sometimes called filter aid, is critical to final liquid clarity. Lautering uses the grain bed, especially husk material, as the filter aid. Sweet wort is drawn through the grain bed and particles are trapped in the matrix. Hop backs use the same principle. The two most popular forms of filter media in use today are DE and Perlite. DE, also known by its German name of Kieselguhr, is a naturally occurring form of silica derived from the calcified skeleton remains of minute planktonic algae. Perlite is similar in nature but derived from volcanic rock and composed of aluminum silicate. The variety and complexity of shapes of these materials form a labyrinth of channels in the filter bed. However, there are pulmonary health risks, such as silicosis, associated with these materials. Handlers, therefore, must wear dust protection. In addition, the disposal costs of filter aids are substantial, which has lead to the development of alternative filtration methods.
Plate and frame filters consist of vertical plates covered with a filter cloth or pad. Adjacent to the plates are hollow frames that contain the filter aid. These alternate the entire length of the filter and terminate at end plates. Beer is fed into the frames, where media traps solids. Clear beer flows into the adjacent hollow plates and out of the filter through collection tubes. Similar in design is the sheet filter that uses compressed cellulose pads impregnated with DE. There are no open frames, but rather two sets of channeled plates. Turbid beer flows in on one side of the pad, and clear beer exits on the other. Sheet filters have less surface area than other powder filters, and are mainly used as secondary or polishing filters. They are often used after rough filtration, before packaging, and with the proper pads with a porosity of 0.45 μm can be used to produce sterile beer.
Screen filters use hollow mesh screens (leaves) mounted horizontally or vertically along a central shaft, housed inside a cylindrical vessel. In a process called pre-coating, a mixture of beer (or water) and DE is pumped into the vessel and recirculated until all the DE is trapped on the screens, and the liquid is clear. At this point beer is introduced. Because the high solids content of unfiltered beer would quickly blind (clog) the screens or the filter would exceed safe operating pressure, fresh DE is dosed in line with the beer. This keeps the bed labyrinth open by continually creating new channels. The nominal filtration range for screen filters is 3 to 5 μm, which can trap most large particles, including yeast.
Candle filters are similar to vertical screen filters, but instead of leaves, these have hollow, round tubes with narrow slotted openings. Operation is nearly identical, but the increased surface area of the candles allows for faster filtration while using less filter aid.
Membrane filters are made of polymer materials and trap particles by virtue of their uniform pore size. They are capable of producing very finely filtered liquids but blind quickly.
Cross-flow filtration improves on traditional membrane filtration by rapidly pumping liquid across the filter surface (usually sintered alumina). Deposited solids are trapped and discharged, leaving the filter surface free from clogging. These filters are often brilliantly designed, but expensive to purchase and operate.
Various processing aids and techniques can reduce the amount of suspended solids before filtration and thus increase filter throughput. These include long, cold maturation times as well as fining agents. These can achieve acceptable clarity though natural sedimentation. Larger breweries sometimes use centrifugation as an alternative to filtration.
Filtration helps stabilize beer and give it a polished, clear appearance. It can also remove elements, such as dead yeast, that otherwise would eventually make the beer unpalatable. That said, especially smaller breweries and brewpubs often prefer to serve their beer unfiltered, because they see no advantage to a clear appearance and feel that something elemental may be lost during filtration. In fact, in several classic beer styles, including hefeweizen and in many Belgian bottle-conditioned beers, yeast turbidity is a defining element of the style.