At some point or other nearly all alcoholic beverages undergo a filtration process. Going back to the earliest production of spirits and indeed, alcohol itself, we have been filtering out unwanted debris, colour and compounds from our drink. For thousands of years we utilised natural materials, such as fine sand, felt and organic materials such as animal skin or guts. One of my favourite early examples of filtration in its crudest form was in fact not filtering at all! The ancient practice of drinking beer through straws saw drinkers simply plunge a straw through the floating layer of grist and yeast, thus filtering out the larger pieces of debris.

These days some kind of filtration is necessary for ensuring a great product, though in spirit production it is often overly credited as an essential part of the process to guarantee quality.

There are two types of filtration.

Mechanical filtration works in much the same way as a tea strainer. In the simplest possible terms, the smaller molecules of water and alcohol are able to pass through the filter, whilst larger deposits of yeast, fruit, pigment or in the case of spirits, fusel oils and congeners are unable to pass through.

Chemical filtration works by flocculating, coagulating or attracting suspended materials, fatty acids, oils and pigment, through chemical bonding processes.

Beer

Beer is can be mechanically filtered through either a thin membrane coated with polypropylene (surface filtration), or a fine powder such as diatomaceous earth (depth filtration). A membrane which is pervious to the passage of liquid can have a very fine weave of polypropylene or polyethersulfone layered on top, the tightness of the weave will afftect the passage of unwanted yeast and suspended material. Diatomaceous earth is made up from the fossilised remains of a minute prehistoric aquatic life form. Its high surface area to weight ratio means that unwanted particles are efficiently trapped on surface pores of the material.

Often the above processes are combined together into one physical application such as in a sheet filter, a 3-4mm thick panel. The outside of the panel surface filters larger material, followed by diatomaceous earth depth filtration inside the body of the sheet. Additionally the beer can be filtered through a series of sheets, leaves or cones increasing in finer grades of filtration towards the outlet.

One of the most commonly used forms of chemical filtration in beer is the addition of Isinglass. Isinglass is the swim bladder from fish such as sturgeon, as disgusting as it sounds, it does a great job of flocculating yeast sediment in a cask conditioned beer. Although yeast will flocculate on its own accord over time, the isinglass will speed up the process and bond the sediment effectively, preventing future separation in transit etc. Milk casein is another, earlier form of chemical coagulation.

Spirits

Spirits undergo mechanical filtration at one or more stages of production. However due to the nature of distillation and the fact that it is a form of purifications itself, unlike beer filtration, it is applied not so much as to improve the quality of a liquid, but to purposely alter its flavour, body and aroma. Ironically it is the filtering process of spirits, not beer that is more commonly attributed to the quality of the final product. My favourite vodkas go through a much shorter than average filtration process, but they retain more character and that’s why I like them.

The water used in spirit production is always filtered to a certain extent and more often than not, de-ionised by distillation for consistency purposes. Mechanical filtration is most prominently used in the production of white spirits after distillation, though many darker spirits are also filtered, but some spirits are not filtered at all.
White spirits such as vodka, rum or tequila will often be forced through a compacted block of granular material after distillation to remove any residual congeners (flavour) or fusel oils left over from distillation. It has the effect of neutralising any remaining characteristics and softening the edges of the spirit. Sometimes, especially in the case of rum, the spirit may be aged in oak for a short time in order to add complexity and then have the acquired colour (but not all of the flavour) removed via filtration, for aesthetic reasons – Havana Club Anejo Blanco (aged, white) is an example of this.

In most cases the mechanical filtering agent will be probably be activated charcoal because it provides an enormous surface area to weight ratio (500 m²/g – over two tennis courts). That said, any inert and finely ground material can be used as an effective mechanical filtering agent, though total surface area along with the pressure applied to the liquid are the key factors in the filters potential. Activated charcoal is carbon that has been processed by either chemical activation or physical re-activation in order to make it more porous.

The size of charcoal used can vary from fine powder to small nuggets, which will in turn dictate the pressure that is required to pump the spirit through. It could be argued that activated charcoal filtration is a form of chemical filtration, since charcoal efficiently adsorbs1 other hydrocarbons, but since both the charcoal and the impurity remain the same it is a mechanical process that acts like a chemical one, by taking the impurities out of circulation.

Smirnoff Red is filtered at 77% ABV ten times through massive columns. It takes 12 hours from start to finish and interestingly the liquid is pushed upthrough the columns, rather than dropped down. This is to avoid sediment gathering at the bottom of the filters. The charcoal in each filter has a 120 hour life span before it is saturated and it’s filtration properties depleted. As a result of this there are actually 11 filters at the Smirnoff distillery as one is constantly offline.

Chill-filtering is a method used in whisky production. It involves chilling the liquid before bottling to somewhere around 0°C, this causes certain solubles such as fatty acids, esters and proteins to bind together in large enough lumps that they may be filtered out. Some producers argue that a noticeable amount of character is removed during this process, so they advertise their whisky as non-chill filtered. It is also argued that chill-filtering only affects the aesthetic appearance, as it stops the dram from clouding up when ice is added. The process is obviously more or less effective depending on temperature, filter aperture and pressure.

References:
The Brewers Handbook – Ted Goldammer
Distilling Knowledge – Dave Broom
http://www.soulshakers.co.uk

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