What’s the purpose of filtering? To separate the grounds from the coffee beverage after the brewing cycle has concluded. Filtering coffee usually consists of two parts: 1) the structure of the device and a shape that contains the bed of coffee grounds as well as the water that passes through them; and 2) the filter medium, typically pre-formed paper, wire mesh or woven fabric, that prevents the grounds from passing through to the finished beverage. Proper filtering achieves the following objectives.
Provides a level bed of coffee: this is important to ensure uniform extraction from each coffee particle. Upon placing ground coffee into the filter or device, you may be required to level off or shake the grounds in order to create a level bed. Unlevel beds won’t allow water to pass at an even rate, and some areas will be more highly extracted than others.
Ensures a proper depth of the coffee bed: the filter device should ensure a bed depth of 1.5 inches, plus or minus 0.5 inches.
Supports grounds so they don’t touch the finished beverage: when grounds are left in contact with the finished brew, they continue extraction. Extraction at this phase only removes undesirable elements into the brew.
Provides multiple drainage points: necessary to promote uniform flow of the water through the bed.
Allows water passage in the proper amount of time: fine grinds can request 1 to 4 minutes to brew, drip grinds 4 to 6 minutes and coarse grinds 6 to 8 minutes.
Allows the passage of the desired amount of undissolved materials (sediments and oils): this is perhaps the most important role in filtering. Removing the oil and undissolved material (often called sediment) is known as clarification. The degree of clarification is largely a matter of personal preference.
The type of filter selected plays a vital role in the development of body in the finished brew. The body is created by the insoluble materials, principally oils and micro-fine pieces of bean fiber that are rinsed off the coffee particles. These insoluble materials create brew colloids, which trap soluble flavoring materials and gases in the brew. The colloids break apart as the beverage is consumed, simultaneously releasing the aromas in one’s palate as the brew reaches the taste buds. Thus, because the filter controls the amount of brew colloids present in the finished brew, it has a direct bearing on the formation and retention of beverage flavor.
Types of Filters
Metal Filters
Wire screens from 100 to 200 mesh are able to completely separate the grounds from the finished brew, yet still allow both oil and bean fibers to pass through and form large numbers of brew colloids. Think about the nature of French press coffee, which contains a great deal of brew colloids and a much more developed body that its paper filtered counterparts.
Cloth Filters
The weave of the cloth affects the flow rate and the degree of beverage clarification. Flannel offers the slowest flow rate and the greatest clarification. The constant and high temperature used in brewing can break down the cloth fibers and increase the amount of chemicals that are absorbed into the cloth.
Paper Filters
Paper provides a high degree of beverage clarity and controls the flow rate of water through the coffee bed. They don’t clog or permit excessive brewing times, and they are very easy to handle (dispose of, store, etc.).
But paper filters aren’t without their drawbacks. Paper is cable of shedding its fibers into the coffee brew. To avoid this, premium paper filters have been carefully developed to reduce shedding, pinholes and increase wet-strength properties.
Paper should not impart an aftertaste; unfortunately, most filter papers are not taste free. The nature of paper causes it to easily absorb foreign odors that are readily transmitted to the coffee beverage. To prevent this, filter papers must be properly packaged and stored.
