Double filtering homebrew

The first question that really should be answered is why do brewers filter beer? Some brewers filter beer to make it clear and pretty, some filter beer so that they don’t have to wait weeks for yeast to settle out and want the yeast out for flavor reasons, others use sterile filtration to guard against beer spoilers that may be in their beer and other brewers filter because most beer is filtered and they are simply going with the flow. All of these reasons, except for the last, are good reasons to filter as long as these reasons are aligned with your needs. But if you want to filter just for the sake of filtering, you really should reconsider your reasoning.

The fact about filtration is that it can both frustrate you and quickly damage your beer in one fell swoop if performed improperly. Large volume filtration is often conducted in stages where the first stage is performed using diatomaceous earth or perlite as a filter aid in a type of a filter commonly referred to as a powder filter. In this sort of filter, the filter aid (diatomaceous earth or perlite slurry) is metered into the beer stream as the beer enters the filter. This allows the filter aid and the yeast to develop a filter cake on the filter during the course of filtration. A secondary sheet filter is used to then “polish” or make the beer very bright, and sometimes a third filter pass using a membrane, sterile filter is used prior to packaging. At home, powder filtration is extremely uncommon.

The most common filters used at home and by smaller scale commercial brewers are sheet filters and wound, hollow-core filters. These filters do have limited sludge capacity because they have some thickness and solids can be trapped within the filter depth, like powder filters. Starting out with well-settled beer is very important when using low-capacity filters because blinding the filter with excessive yeast is a real problem that requires the filtration process to be terminated and the filter replaced. In the process of replacing the filter, beer loss is also pretty common. Yeast cells are about 10 microns in diameter and can be removed with filters in the 5–10 micron nominal size rating. Nominal rating means the filter removes particles in the size range, with pores that are actually larger than the nominal size. In filter jargon, depth filters function by trapping particles in a “tortuous flow path.”

Secondary filters are typically used to either polish beer or to remove bacteria. Polish sheet filtration can be accomplished using filters with a nominal rating of 1–5 microns. The primary purpose of polish filtration is the removal of haze, since almost all of the yeast is removed by primary filtration. The thing to consider here is that you are likely to strip the beer with filtration and stripping is increased as pore size decreases. Bitterness, mouthfeel, foam, flavor intensity, hop aroma, and color all can be affected by filtration. Beers that are always filtered are often “tweaked” so that the filtered beer has the attributes desired by the brewer.

Finally there is membrane filtration that can be used for sterile filtration. In the beer industry the term sterile filtration means that the pore size of the membrane is no larger than 0.45 microns. Sterile filtration can be tricky with all-malt beers because gums, such as beta-glucans, can quickly blind sterile filters. Unlike filter sheets, membrane filters have no sludge capacity and quickly go from flowing to not flowing when the pores are plugged.

The easiest way to filter your keg is to use a hollow-core filter with a nominal pore size in the 5-10 micron rating. A sheet filter system using a plate filter housing can also be used, but these are more expensive and not as common for small-scale, periodic filtration. You can set both of these filters up by packing the system with water and blowing the water out with carbon dioxide before filtration. This step is really important; if you are not thorough with your flushing you can very easily oxidize your beer and convert excellent, cloudy beer into oxidized, disappointing, clear beer. Once the filter is set up, all you need to do is push the unfiltered beer from one keg to another using carbon dioxide pressure. If you are going to use a secondary filter, then you repeat the process.

This whole filtration process works best if you allow your beer to settle in your primary for at least a week after fermentation is complete before racking to a keg. And filtering the beer as cold as possible results in the clearest beer.

Response by Ashton Lewis.