The Oxford Companion to Spirits and Cocktails Definition of filtration

filtration is an important, though less visible, aspect of spirit production in which unwanted compounds are removed to improve a spirit’s appearance or flavor. Indeed, along with oxidative maturation and fractional distillation, filtration is one of the chief methods of taming raw spirit and rendering it pleasant to drink.

Most spirits undergo a simple mechanical filtration to remove particulate matter, especially if they have been stored in wood. Some undergo additional mechanical or chemical filtration to remove unwanted organic compounds or color from the liquid. Finally, some spirits are filtered to remove compounds that come out of precipitation at low temperatures, as when ice is added, and cause them to become hazy.

There are many types of filtration media. The most common are cloth, paper, and other fibers such as cellulose, followed by sand or other granular materials (such as diatomaceous earth and limestone), and precious metals such as gold, silver, and platinum. There are also adsorption filters, such as activated carbon and zeolites, that remove unwanted compounds at the molecular level and membrane filters that have specific pore sizes.

The most basic method of filtration is to pass a liquid through a woven material, such as cheesecloth or muslin, to remove coarse materials. This is often done when producing bitters or infused spirits. These coarse methods of filtration are a starting point for further filtration because they are relatively fast and help prevent finer filtration systems from being clogged with the coarser material.

Pressure or vacuum filtration is used to improve speed and maximize the efficiency of the filter. Gravity-based filtration is a slow method, especially when filtering small particulate through microscopic filtration media. The addition of pressure or a vacuum to push or pull the liquid through the filter significantly improves the efficiency of the operation. It also maximizes the life of a filter. As a filter removes materials from the filtrate, it also becomes more efficient as particles clog its pores, making it harder for the liquid to pass through but also capturing even smaller particles. Those in turn make it yet more difficult for the liquid to pass through. The use of pressure or a vacuum forces the filter to work for longer, but eventually the filter will become encumbered and need to be replaced. These physical methods of filtration have limited effect on the flavor of spirits and are done for production or cosmetic purposes.

Activated carbon or charcoal filtration is a method that is used to remove organic compounds from a liquid. This is done to improve the taste or color of a spirit. Its use dates back to at least the seventeenth century, when it was employed in eastern Europe in making vodka; by the end of the eighteenth century, it was also used in western Europe for rectifying spirits. See rectification and vodka.

Carbon filtration works through a method called adsorption where molecules, like congeners, adhere to the carbon surface, removing them from the liquid. Filtering through carbon or charcoal is an extreme form of filtration that removes flavor compounds as well as color from a distillate. It is often used in products that are targeting consumers who want a smooth-tasting product, such as vodka and white rum. See Bacardi. It can also be performed on distillate before it is placed in barrels to age, as Jack Daniels and George Dickel do with the Lincoln County process, which involves slowly passing the fresh distillate through a large quantity of sugar maple charcoal to remove impurities (many American whiskies used charcoal filtration in the nineteenth century). See Lincoln County process.

Membrane filters are a specialized filter made from polymers that have a specific pore size. They are commonly used to filter the water that is used to dilute and mix spirits. The filters are found in reverse osmosis filtration systems. The key benefit of membrane filters is that they can filter dissolved minerals from water. Previously, removing minerals required distilling the water, which was a more expensive and time-consuming method.

Because of the specific pore size of membrane filters, there is some interest in this as a method of separating alcohol from water. This would be akin to the distillation and production of grain neutral spirits without the distillery.

In some cases, filtration can refer to the practice of using a material like a limestone (calcium carbonate) to remove iron from water. Iron compounds impart an unpleasant bitter and metallic taste to a spirit and may affect the color of the final product. Many distillers prefer to remove iron from their water source, bourbon distillers in particular. Other filtration methods have difficulty removing specific minerals, but limestone makes the water slightly alkaline and causes iron to precipitate out. The use of limestone also adds trace amounts of calcium carbonate to the water, which can improve its taste.

Originally a Russian technique first used in making vodka, precious metal filtration—usually done by forcing the spirit through silver- or platinum-impregnated charcoal—can soften a spirit and give it a creamy texture due to their catalytic effect on the ethanol/water mix, helping to break congeners down into esters and pull some minerals out of the water. Gold, however, mostly does its work in the press release.

Vodka, the most highly filtered of spirits, often uses multiple filtrations—such as charcoal, quartz sand, and silver, each designed to remove different impurities. The same vodka, filtered differently, can taste remarkably different.

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