Drying procedure for determining extraction percent

by gt on Sat Sep 17, 2011 12:41 pm

What is the correct procedure for evaporating brewed coffee to determine extraction percent?

I've attempted this several times in the past but I've found significant differences in the results depending on the temperature of the initial ground coffee and the temperature of the evaporated residue.

In other words, at what temperature should (1) the initial ground coffee and (2) the evaporated residue be at when the weight measurements are made?

Thanks Gary

by **asicign** on Sat Sep 17, 2011 2:59 pm

at what temperature should (1) the initial ground coffee and (2) the evaporated residue be at when the weight measurements are made?

I wouldn't think the temperature would matter in this measurement. What would matter is that the residue is dried the same way each time, and that humidity was fairly constant.

I've never done this with coffee, but there are probably proscribed methods to follow.

by **jbviau** on Sat Sep 17, 2011 3:12 pm

This thread over on CG seems relevant. At the very least it might give you a lead on someone else who's done what you're describing and presumably would be good to talk to.

http://users.rcn.com/erics/ · by **erics** on Sat Sep 17, 2011 3:32 pm

1. Obtain a representative portion of brewed coffee - about four ounces.

2. Filter the sample through a coffee-type paper filter to remove any insoluble sediment or fine grounds. (It may be necessary to filter the brew two or three times to obtain a sediment-free solution.)

3. Using an analytical balance, weigh a small, clean, and dry aluminum dish to the nearest tenth of a milligram. Transfer a small portion (exactly 10 milliliters) of the clarified solution to the dish.

4. Put the dish, with its contents, into a drying oven that maintains a constant temperature of 221F to 230F. Let three hours elapse. During this period, the heat drives off all the water; only the non-volatile material that was extracted from the coffee remains.

5. When the drying period has ended, transfer the dish to a desiccator - a small vessel, containing a drying agent, that can be tightly sealed. In the desiccator, the dish and the solids cool to room temperature without picking up moisture from the air. This requires 15 minutes.

6. Reweigh the dish. The difference between the first and second weighing represents the amount of soluble solids in the brewed coffee expressed in grams per 10 milliliters of beverage. To obtain the concentration of soluble material in the beverage expressed in terms of "percent solubles concentration," multiply the weight of the residue by 10, which now represents the number of grams per 100 milliliters of beverage that can be directly interpreted as percent. (Note: the metric system defines a milliliter as the volume of 1 gram of water at 4C and 760mm pressure.)

by **peacecup** on Sat Sep 17, 2011 4:27 pm

I went about it the other way - I weighed the puck before brewing, then dried it out after brewing, and subtracted the difference.

PC

by gt on Sun Sep 18, 2011 9:50 pm

Thanks jbviau, andys describes in the thread you referenced exactly what I've seen when removing either the grounds and/or the evaporated residue from the drying oven.

AndyS wrote:Also, when I removed dried beverage from the oven, the residue absorbed water back from the air very quickly. On a .01g scale I could watch the numbers steadily spooling up. So there was a rush each time to get the measurement made asap, and I didn't have a lot of confidence in the result.

Moisture rushing back into the dried sample also brings up the question of the moisture content of the fresh ground coffee. It just makes me wonder if the dried sample is weighed at a drying temperature, should the fresh ground coffee also be dried at the same temperature.

I thought maybe there was a standardized procedure for weighing the starting grounds and weighing the dried sample but it sounds like there isn't such a procedure.

by **Nick** on Mon Sep 19, 2011 4:20 pm

peacecup wrote:I went about it the other way - I weighed the puck before brewing, then dried it out after brewing, and subtracted the difference.

Problem with this method for espresso is that depending on your particular machine, a decent amount of solubles can get sucked UP and out of the puck through a drain solenoid or similar.

by **shadowfax** on Mon Sep 19, 2011 5:34 pm

Nick wrote:Problem with this method for espresso is that depending on your particular machine, a decent amount of solubles can get sucked UP and out of the puck through a drain solenoid or similar.

Not to mention that it also would measure the total suspended insoluble solids.


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