jpender Senior Member Joined: 11 Jul 2011 Posts: 695 Location: California Expertise: I like coffee
Grinder: OE LIDO Vac Pot: S/S Moka Pot Drip: Aeropress
Posted Fri May 11, 2012, 12:56pm Subject: Re: Coffee Extraction Discussion, Questions for the membership:
I was also concerned about the commenter's observation, but honestly I don't think I see that happening. If I have the scale stabilized, I have to make sure any additional mass takes it out of the stabilized zone (i.e. if you drop a few grains of coffee onto something that reads 100.00, you should tap the scale and let it re-stabilize, and usually it will register 0.01 changes). Same thing if you remove the thing being measured and re-place it.
(1) Sure, a brewing chart is a rough "predictor" of how much beverage you will end up with, given a measured dose, a measured amount of brew water, and a decent guess at the absorption factor. (2) But a brewing chart is a poor "predictor" of what the beverage strength and extraction yield will be, since changing the brew ratio will change the amount of solids that are extracted from the dry coffee. (3) I agree with jpender that your equation Extraction = Strength * ((1/Brew Ratio) - Absorption) is only approximately correct, since it ignores the extracted solids as part of the beverage mass. The fact that it matches up well with a 50 year old chart doesn't mean it's correct; we know the creators of those charts made errors and/or omissions.
I believe you have a typo there, and you meant to say:
Coffee Produced = Brew Water - (Absorption X Dry Coffee)
But since you haven't defined "Absorption" (or at least I don't see where you have), the equation is ambiguous.
It appears you are defining Absorption as: ((weight of wet grounds/weight of dry grounds) - 1)
A more intuitive (and perhaps more accurate) definition of Absorption might be: (amount of brew water absorbed by the grounds/weight of dry grounds)
Here's an illustration of the difference with numbers: say we start with 10 grams of dry grounds. After extraction our wet grounds weigh 30 grams, and our extraction yield is 20%.
Using your definition, Absorption = (30/10) - 1 = 2
Using the second definition, we take into account that 2 grams of soluble solids have been removed from the grounds. Absorption = (30-8)/10 = 2.2
The second definition is the one that ExtractMoJo software is using. Tellingly, it is referred to as a "water lost" ratio (as in: brew water absorbed by the grounds, "lost" to the beverage).
I'm not saying your definition is "wrong." By choosing an appropriate absorption number, you can make the figures come out OK. But since the whole point of extraction theory is to focus on how much material is removed from the dry coffee, your definition seems like an inappropriate oversimplification to me. I haven't worked it all through, but I believe it creates some problems down the line.
All I did was normalize the equations into brew ratio, but the equations are all standard coffee extraction theory.
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