Measuring the moisture content of green coffee - Page 3

#21: Post by **AndyS** » March 26th, 2010, 8:06 pm

Thanks, Gary. I guess this is why the big coffee companies are confident that they can pack coffee at normal weight and send the product into supermarket distribution. When the consumer buys it -- eighteen months later -- the net weight will still be OK!

#22: Post by **gscace** » April 23rd, 2010, 2:12 pm

weight is mass times gravitational acceleration, so the two are directly related and 1.5% of substance by mass or by weight are the same damn thing.

I have a little trouble swallowing Illy's number and think it's prolly a typo or plainly unexplained improperly (haha). CO2 molecular weight is 46 gm/mole. At 1.5% by mass, the mass of CO2 degassed by 1 kg of roasted coffee is 15gm, which is 0.326 moles. At 22.4 liters per mole at 20C and atmospheric pressure, the volume is 7.17 L. If you store the roasted coffee of mass (weight if you wanna talk about kg force) 1 kg in a 2L jar, you'll get that gas at pressure of 7.17/2 atmospheres. At 101kPa per atmosphere you're talking about 363.4 kPa, which is nearly 53 psi (gauge pressure, not absolute) for you english unit weenies. Ain't no way there's 53 psi of degassed CO2 in any jar of coffee that i roast.

-Greg

#23: Post by **JohnM** » April 23rd, 2010, 8:40 pm

Greg,
Great thought process.
Maybe the numbers are not too huge based on some extremely crude observations from my roasts?
I put ~0.2 kg of fresh roast into 1 qt mason jars and evacuate them to ~0.3 atm. After 2 days the lids are 'free' i.e., no more vacuum. If I did the math right, 0.7 quarts of gas from 0.2 kg of beans is ~ half of the 7.17L your calculation shows. Maybe >1% mass loss due to degas is feasible?

John

#24: Post by **AndyS** » April 24th, 2010, 9:42 am

gscace wrote:weight is mass times gravitational acceleration, so the two are directly related and 1.5% of substance by mass or by weight are the same damn thing.

Well not exactly the same damn thing. A given mass weighs about 0.28% less on Mt Everest than it does at sea level. And it weighs about 0.5% less on the equator than it does at the poles. Because of these measurement inconsistencies, I never bother brewing coffee atop Everest or at the North Pole anymore, I just go with Starbucks Via.

gscace wrote:I have a little trouble swallowing Illy's number and think it's prolly a typo or plainly unexplained improperly (haha). CO2 molecular weight is 46 gm/mole. At 1.5% by mass, the mass of CO2 degassed by 1 kg of roasted coffee is 15gm, which is 0.326 moles. At 22.4 liters per mole at 20C and atmospheric pressure, the volume is 7.17 L. If you store the roasted coffee of mass (weight if you wanna talk about kg force) 1 kg in a 2L jar, you'll get that gas at pressure of 7.17/2 atmospheres. At 101kPa per atmosphere you're talking about 363.4 kPa, which is nearly 53 psi (gauge pressure, not absolute) for you english unit weenies. Ain't no way there's 53 psi of degassed CO2 in any jar of coffee that i roast.

I like bashing Illy as much as the next home-barista, but your analysis is flawed (which I'd chalk up to the fact that you probably made a quick post from work)

(1) You'd have to put the beans in a totally sealed jar immediately after roasting; I doubt the typical mason jar holds pressure all that well.
(2) It takes two or three months for the beans to complete degas; you probably don't wait that long.
(3) A kg of roasted beans occupies a volume of about 1.3 L, so the head space in the jar isn't 2 L, it's really only 0.7 L.
(4) 1 kg of roasted beans won't ever completely degas in a 2 L jar because the CO2 will come to an equilibrium pressure inside the beans and in the jar. So you don't get the full 15g of gas.