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**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!*

- Another bit that changes more than you might expect is the water loss ratio. It's variable enough that I switched from MojoToGo to a spreadsheet. Your 19.1 might be quite different.Jan 13, 2012
- +Collin Moody If you wanted that same ratio, then yeah, I guess it'd be 963g since that's 1.0L at 200°F.

+Jeff Kilpatrick Yeah Jeff, for sure. It's adjustable in the MojoToGo, though it takes two screen-taps to get to. I don't see how that effects the 19.1% though, unless you're not taking G.W.R. (grounds water retention) into consideration (which I am).Jan 14, 2012 - excuse the ignorance, but how does one account for grounds water retention? are we counting it as part of the total water or are we adding more water to compensate? please enlighten.Jan 14, 2012
- Will, If you weigh your brew so you know how much water went in, then when you're done, weigh the resulting brew. The difference between the two is your G.W.R. If you want to be precise, you'd want to subtract the mass of coffee solids in the brew:

total brew water - water loss = resulting brew - (resulting brew x TDS%)Jan 14, 2012 - I suppose I didn't word my question the way I intended... I can gather what GWR actually is, but my real query is practical: what to do with the data? If my ratio is 17 to 1, and I'm brewing 20g of ground coffee with 340g water, would I want my resulting brew to weigh 17 times that of the dry grounds or would the water in weigh 17 times that of the grounds? (the GWR would usually be twice that of the dry grounds, for my brew it would be 40g, not an insignificant amount)Jan 14, 2012
- Will, this is ultimately a question of traditional convention. I'll put it this way: for a
*brewing ratio,*it makes sense to use mass of coffee to mass of water-in. For*solubles ratio*(TDS), it's obviously going to be mass of solubles to mass of solution (with mass of solution = mass of solubles + mass of H2O in the solution). When using TDS, mass of water-in, and mass of coffee to calculate extraction yield, G.W.R. is used to calculate mass of solution. That's why G.W.R. is so important. Imprecision in G.W.R. means imprecise calculation of any resulting math. It does make it important to either make sure you're dripping-dry when you brew (so that G.W.R. will be more constant), or you have extra math to do, including weighing your resulting brew and doing backwards-math.Jan 14, 2012 - Thanks for the thorough answer, Nick.Jan 14, 2012