19.1 is the new 20.0

The prior literature on coffee brewing tends to use mass units for coffee (grams or ounces), and volume for water (liters or fluid ounces, sometimes gallons or cups). Granted, you'll see teaspoons or tablespoons used sometimes, but none of those are really trying to be scientific.

Lavoisier's Law of the Conservation of Mass teaches us that mass is a constant. Volume depends on density. If density is a constant, then you can effectively treat volume as a constant in that particular case. In the case of coffee brewing, the density of water is not a constant. Water density decreases at higher temperatures. I have this particular web page bookmarked for when I need to calculate water density at a particular temperature: http://antoine.frostburg.edu/chem/senese/javascript/water-density.html

So when you say "I'm brewing coffee with one liter of water," if you want to be precise and/or want to use this data to do some coffee brewing math, you need to know what temperature that water is. At room temperature, let's say 20°C (68°F), one liter is 998.2 grams per milliliter. At 93.3°C (200°F), it's 963.1 grams. The density decreased, and a given mass of water will expand in volume as it's heated. This is true, and undisputed.

This is a fact that Vince Fedele has pointed out to the world by integrating it into the ExtractMojo (and MojoToGo) software. Both pieces of software, therefore, uses mass for water instead of volume. If you plug in a volume measurement, it will use its own temperature-density calculator to convert it to mass, before it does its calculations. This a great thing!

So what's the problem? The problem is, with new units, you have to adjust the chart.

Everyone is still using charts that all read 18-22% as the Gold Cup extraction yield zone. But the 18-22% zone was developed with calculations using volume, not mass, of water. Therefore if you change the units to mass of water, since there's a density-based Δ (delta, or empirical change), you have to adjust the results of any calculations accordingly.

If using volume as your water number, the extraction yield zone of desirable taste characteristics "by the book" was 18.0-22.0%. Using mass and 93.3°C (200°F), the new corresponding zone is 17.2 to 21.1%. The "sweet spot," if you're trying to nail the middle point of that zone, is 19.1% extraction.

Therefore, 19.1 is the new 20.0!﻿
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