Important Factors in Brewing Coffee

Water Quality

Water used for brewing coffee should be free of most impurities, although a small amount of dissolved solids is necessary for optimal extraction. Contaminants and disinfectants in many municipal water supplies often make unfiltered tap water unacceptable for brewing coffee. Distilled water is also not recommended. Ideal water for brewing coffee will be odorless, will have a neutral pH, and will contain about 100 to 200 parts per million total dissolved solids. This ideal may be approached by using bottled or filtered water.

Water-to-Coffee Ratio

The SCAA specifies a ratio of 3.75 ounces of coffee per half-gallon (64 ounces) of water. That corresponds to a ratio of 56 grams of coffee per liter.

A two-tablespoon coffee scoop will hold about 0.35 ounces (10 g) of ground coffee, which should be combined with 6 ounces (46.9 mL) of water to make one "cup". The coffee grounds will absorb about 0.7 ounces of water yielding a 5.3 ounce one "cup" serving. However, many people prefer a "cup" of coffee at least twice this size.

The best way to ensure a proper coffee to water ratio is to weigh the coffee rather than measure it by volume with a scoop. Measuring by volume is a compromise between quality control and convenience. If you use a scoop, you need to determine how many scoops of coffee to use for your prefered serving size. To do this, weigh out the proper amount of coffee calculated from the 3.75oz:64oz coffee to water ratio and then measure how many times that amount of coffee fills your favorite coffee scoop. For instance, to find the number of scoops needed to produce a 10.6 ounce serving of coffee (which fits nicely in a standard twelve ounce mug), weigh out 0.7 ounces (20 g) of ground coffee, and then find how many times that coffee fills your scoop. I find that I can fill my two-tablespoon coffee scoop about 2 and a quarter times with 0.7 ounces of ground Kenya.

I took the same measurement as above using Costa Rica instead of Kenya and found that 0.7g of ground Costa Rica fills my scoop 2 and a third times. This points out one of the drawbacks of measuring coffee by volume. Because of variations in coffee density, and the sizes of coffee granules produced by different types of grinders, the volume corresponding to a given mass of ground coffee will vary. The error involved with careful use of a coffee scoop will likely be small enough to keep your coffee-to-water ratio within acceptable bounds. (The margin for error is in the example given amounts to about a half tablespoon or the equivalent of about 25 beans.) But because other factors are always conspiring to ruin your brew, it is best, whenever possible, to weigh out the proper portion of coffee for each brew.

Brew Temperature

Coffee's best flavors will fail to be properly extracted if the brewing temperature is too low, and some of coffee's unpleasant flavors will be emphasized if the brewing temperature is too high. The proper brew temperature range is 195°F to 205°F (90.6°C to 96.1°C). The brew temperature should stay within this range throughout the brew cycle.

Some automatic brewers in both the home and commercial markets do not produce or maintain an adequate temperature throughout the brew cycle. This may be because of poor design, inadequate power supply, or improper installation or use. Other brewing devices, such as press pots and simple one cup mug-top brewers, present the challenge of maintaining an initially adequate temperature throughout the brew cycle. Many brewing methods involve bringing the water to a boil. If the water is not allowed to cool off several degrees before contacting the coffee, undesirable flavors will gain prominence. In the worst case, the method will involve brewing the coffee with boiling water. This is what a percolator does, and this is why it makes little sense to brew good quality coffee in a percolator.

Water Pressure

Water pressure is a factor in all brewing methods, but, with the exception of the espresso method, there is usually no attempt to precisely control it.

In drip brew systems, water pressure in the brew chamber is determined by outside air pressure, the force of gravity on the water, and the geometry of the brew chamber. These environmental factors vary within a limited range, and changes have a negligible effect on water pressure.

Two chamber stove top brewers and the elegant Chemex brewers rely on steam from the brewing water supply to create air pressure differences that drive the water through a small chamber packed with coffee grounds. The water pressure in these systems does not approach that of an espresso machine, but the pressure is somewhat greater than what you would find in a drip system. This contributes to the unique brew characteristics these systems produce.

Commercial espresso machines are designed to brew coffee under 9 to 10 atmospheres of pressure. (This is roughly the pressure you would feel if a cow could pirouette on you palm.) Different models of home espresso machines vary in their capacity to produce and maintain pressures comparable to their commercial cousins.

Coffee Particle Size and Consistency

Coffee must be ground do suit a particular brewing method. A press pot requires a relatively coarse grind, while espresso requires a much finer grind. At the extremes of common grind settings, we have the coarse grind tailored to a percolator and the powdery fine grind used for Turkish coffee. Using the wrong grind for a brew method is likely to lead to over- or under-extraction, and may even damage brewing equipment.

Ground coffee ideally consists of particles that are all the same size. By varying the size of the particles we aim to control the total surface area that will come into contact with water. A given measure of coarse ground coffee consists of larger particles with less total surface area than the total surface area of the smaller particles of the same measure of a finely ground coffee.

The practical reality is that ground coffee consists of particles that have varying dimensions. Well designed, well maintained grinders keep the variation to a minimum. Unfortunately, the popular spinning-blade grinders used in many homes produces a wide range of particle sizes. For coarser grinds, this is often tolerable, but the smallest particles make for a "dust" that adds a bitter influence to the brew. This dust will also clog filters, so a blade grinder should not be used to grind coffee for an espresso machine.

Burr grinders break up coffee beans between grooved metal discs or cones, and produce more consistent particle sizes with much less dust. The consistent output of burr grinders allows for fine tuning a grind to a brew method.

Grounds Bed Dynamics

Consider how water and coffee mix and move about in the brew chamber. In a press pot, the water is added all at once and the water-coffee mixture is briefly stirred to ensure that there are no dry spots embedded in the floating "cake" of coffee grounds. In an automatic drip system, water is ideally sprayed from above in a dispersed pattern over a basket filter, or fed into a cone filter at an adequate rate to keep most of the cone filled with water for most of the brew cycle. For espresso, the fine grounds are evenly and firmly packed into an appropriately sized filter. The espresso brew head administers water through dozens of small closely spaced holes. In each case, the objective is to distribute the water evenly through the coffee grounds. If dry spots remain in the bed or cake of coffee grounds for even part of a brew cycle, the result will be weaker and possibly over-extracted coffee. In the case of espresso, improperly ground and tamped coffee may allow the pressurized water to form channels through the grounds. More water will flow through these limited regions, overextracting flavors from nearby grounds while underextracting from the rest of grounds. In the case of the drip system with a basket filter, if the spray head is clogged on one side, a significant amount of grounds may remain dry for much of the brew cycle, and the quality of the brewed coffee will suffer. When the filter is removed for disposal, there may or may not be evidence of the uneven water distribution.

Brew Time

In drip brew systems, brew time is largely a consequence of other, primary factors, and is not directly adjusted. When using a press pot, timing the brew is the primary way to control the degree of extraction. In an espresso machine, all other factors are adjusted to achieve a brew time that falls within a specific range of seconds. Thus, brew time can be an engineered consequence of other factors or a primary control factor.

Drip systems and similar brewing methods should keep the coffee grounds in contact with water for somewhere between 4 to 6 minutes. If a drip system exposes grounds to water for more than 7 or 8 minutes, a courser grind should probably be used to reduce the brew time.

Proper press pot brew times can only be determined by experimentation. You might first try a four minute infusion and then adjust the duration of the next brew to get a more or less intense flavor.

Espresso machines should be designed and tuned to specific standards so that the only variable in the daily brewing process is the grind size. The indication of proper grind adjustment will be a 22 to 28 second brew time.

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