Monday, May 18, 2009

coffee-to-water ratio

What's the proper grounds-to-water
ratio for brewing excellent coffee?

The most common recommendation floating around on the internet is the two tablespoons of ground coffee per 6 fluid ounces of water. But what if you want to brew a gallon of coffee, or even a half gallon? According to "Ratio" is defined as "a proportional relationship". However, when brewing larger volumes of coffee, the proportions of grounds and water are not scalable. For example, based on the 2 per 6 rate, 42 tablespoons of ground coffee would be required to brew 1 gallon of coffee.

Here's the math:
128 fluid ounces per gallon / 6 fl ounces = 21.3 fl ounce cups per gallon * 2 Tbl = 42 tablespoons of ground coffee.

Before dissecting the 2 per 6 rate any further we need to establish the amount of ground coffee per tablespoon. Even though the units of measurement are switching between mass and volume here, if we take into account the bulk density of ground coffee of 22 lb/cu. ft. (Sivetz, 1979), then we can calculate that 1 tablespoon of dry coffee grounds = approximately 0.38 dry ounces.

To obtain the bulk density of ground coffee we can take the average between coarse ground coffee (19 lb/cu ft) and that of fine ground coffee (25 lb/cu ft) resulting in an average bulk density of ground coffee to be 22 lb/cu ft.

We also know (from google) that there are 915 tablespoons = 1 cu. ft., then the number of ounces per tablespoon is of ground coffee is:

22lb/cu ft (16 ounces) (1 cu ft)
1 cu ft (1 lb) (915 Tbl) = 0.38 ounces/Tbls

Having established the amount of ground coffee per tablespoon (0.38 ounces) we can look deeper into the 2 per 6 ratio. The 2 per 6 ratio is equivalent to using 0.76 dry ounces of ground coffee for each 6 fluid ounce cup of coffee. Or, slightly more than 1 pound of ground coffee per gallon.
Furthermore, if we assume an absorption rate of 1 quart of water per lb of coffee, then the total volume of final brewed coffee is reduced to approximately 3 quarts. So, we would have used 1 lb of coffee to brew around 3 quarts of coffee. Seem like too much? That's because it is.
What do the authorities on the subject suggest? The Specialty Coffee Association of America (SCAA) recommends the following standards for certification as an SCAA approved "brewer".

  • The "standard" ratio of coffee to water for comparative purposes is 3.75 ounces per half gallon; 55 grams per liter; or 2.25 gallons per pound.
  • If coffee weights are measured in terms of "cups" (6 fluid ounces of water), the ratio of coffee to water for comparative purposes will be 10 grams or .36 ounces per cup. This equals 45 cups per pound.
Thus, instead of 2 per 6, or 0.76 ounces per 6 fluid ounces, the SCAA recommends 0.36 ounces per 6 fluid ounce cup. In other words, a little less than half of the 2 per 6 recommendation.

There are several key factors that affect flavor extraction, or more specifically brew strength, during the brewing process. Coffee freshness, grind size (and uniformity), water temperature, and grind bed dynamics (saturation), all play a role in the resulting cup quality. So, what's the take home here? The take home is that ratios, though proportional by definition, are not always scalable. This means that with regards to brew strength, the proportion of ground coffee to water do not maintain a consistent relationship along a graduating scale.

General References:

Sivetz, M.; and Desrosier, N.W. 1979. Coffee
Technology. AVI Publ. Co., Westport,
Connecticut, USA.

SCAA website.

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