I am reading “Everything But Espresso” by Scott Rao and am in the chapter “The Coffee Brewing Control Chart”.

In the section “How to Calculate Extraction without a Chart”, he says you need three values:

  1. Brew strength (total dissolved solids)
  2. Dry coffee mass
  3. Brewed coffee mass

You plug those values into the following two equations to get extraction percent:

TDS% x Brewed Coffee Mass = Extracted Mass

Extraction% = (Extracted Mass) / (Dry Mass)

So, my question is: how do you get brewed coffee mass? Do you have to take out your mug before you start everything and then weigh the solution afterwards?

Also, is it necessary to have a refractometer to calculate the TDS?

Without knowing how to get those two variables, I obviously can’t solve the whole equation.

Any tips on any of this is greatly appreciated.

up vote 2 down vote accepted

You have everything right. If you want Extraction % you need to measure:

  1. Total dissolved solids: You will have to measure this using a refractometer. There are many out there, for commercial settings they start at around 300€. I have no experience with cheaper, "home use" refractometers but I'd assume they lack accuracy.

  2. Dry coffee mass (in g): You have to weigh the ground coffee before brewing, that's all there is to it.

  3. Brewed coffee mass: As in 2. you will have to weigh your brew. Tar your kitchen scale with the cup on it, brew coffee and put the cup back. Voila.

Now you just all pluck it in the equation and you are good to go. Suppose you have 15g of ground coffee, yield 190g of brewed coffee with a TDS of 1,3%, which is great for a pour over.

That gives you an extracted mass of 2,47g.

2,47g/15g = 0.165. So the extraction % would be 16,5%.

The extraction percent of coffee could be calculated with a Petri dish, stove and a precision scale, if you don't have access to a refractometer.

I assume, the original question here is: How does one calculate extraction percent? If this is the question, the answer is simply a refractometer as it directly reads the extraction percent.

However, the main problem is what could you do if you don't have a refractometer, or you are not assured with the quality of your refractometer. This boils down the subquestion: is it necessary to have a refractometer to calculate the TDS?

Actually, before the invention of refractometers (the device that checks the opacity of a liquid, so it could reason how many irregularities exists in it), chemists were able to calculate total dissolved solids in a solution by simple methods. I will explain one here. That, you can make at home without precision. And repeat at a lab environment with high precision.

The methodology

  1. Find a very thin, light heat-resistant glass, preferably a Petri dish. Put it on (precision) scale and zero your scale.
  2. Put a fair amount of (say, 10 grams) brewed coffee in the Petri dish while it is on the scale.
  3. Place the Petri dish in the stove, set it to 100 °C. Wait until all water vaporizes and coffee remains dry out completely.
  4. Put the Petri dish on the scale again. You have weighted the total dissolved solids.

So, you now have all the required mass you need for your formulas.

Notes

  • Be careful to be clean at all points. E.g. your sweat prints, etc. may affect the weight as the mass will be fairly small.
  • Your accuracy increases while the weight of Petri dish decreases and the amount of brewed coffee used in this method increases.
  • Use a scale as precise as possible.
  • This methodology cannot weight the aromatic oils as everything vaporizes. However, by definition TDS does not include aromatic oils. So, I don't care about them that much.

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