Modern Bourbons are filtered to remove unwanted vegetable oils and other particles that can cause “flocking” in cold weather or when the Bourbon is poured over ice. Flocking is simply the condensation of these oils that will give the whiskey a cloudy look. It does no harm and in fact it contains flavors that are lost in the filtration process. This is not always a bad thing but in most cases the oils add a deeper flavor and a better mouth-feel to the whiskey. Even the so called non-filtered Bourbons have some filtration to remove the larger particles of charcoal from the barrel but this is a minimal invasive filtration and on the whole a good idea.

For most of the 19th century Bourbon was sold directly from the barrel. The customer would come to the liquor dealer with a flask or jug and have it filled from the barrel and pay the dealer for the pint or quart or gallon that was needed to fill the container. The only filtration would be that of the liquid passing through the barrel tap keeping out the larger pieces of charcoal. If the customer did not have a flask or jug the merchant would supply a filled bottle but they were most likely filled in the same manner with no filtration. Even in saloons the barkeep would fill a decanter to use to pour the drinks and would purchase his or her whiskey by the barrel.

In the last decade of the 19th century bottled whiskey started to become more common as machine blown glass made it economical to sell whiskey bottled at the distillery. With bottled whiskey came the whiskey label and on the label was usually the proof of the whiskey. The whiskey would be dumped from the barrels into a vat and the proof was often adjusted, especially after the Bottled-in-Bond Act of 1897. With this process it was important to make the whiskey look as pure as possible for the consumer so filtration became an added step to selling whiskey. This filtration was a simple system that filtered the whiskey through cloth filters to catch the char. The distillers did notice that when they bottled in cold weather the whiskey would get less cloudy in the bottle.

After Prohibition with standard bottle sizes and label requirements there came lower proof whiskey – as low as 80 proof. Technology had improved and filtration systems became more sophisticated with paper filters. The distillers were also using activated charcoal thrown into the whiskey in the cistern followed by an agitation of the liquid. The vegetable oils would collect on the charcoal before the whiskey was run through the paper filter system. This is what many brands refer to as being “Charcoal Filtered”. As the technology in refrigeration improved the distillers decided that cooling the whiskey in the cistern would be added to the process to imitate the fine clarity of the whiskey bottled in the winter months. The technology continued to improve to today’s chill filtration systems.

Not all filtration is bad. When done right it adds a clarity of appearance and can be used to remove some unpleasant flavors and allow other flavors to shine through. This is particularly true in older whiskeys that are dominated by wood tannins flavor. A proper filtration of the whiskey allows caramel and vanilla flavors to come out that would be hidden by bitter tannins if not filtered. On the whole, the more you filter a whiskey the more flavor you remove. The lower the proof in the bottle the more you have to filter the whiskey to prevent flocking.

Not every distillery treats filtration the same. Some have a one size fits all filtration system where companies like Michter’s have multiple systems and determine how to filter based upon the brand they are bottling. Filtration should be viewed as simply another step in the recipe for making the Bourbon in the bottle you are purchasing.

Photos Courtesy of Maggie Kimberl