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Rate of coffee staling

Postby RapidCoffee on Sat May 17, 2008 4:51 pm

cafeIKE wrote:For every 10°C change, the speed of a chemical reaction changes by a factor of 2.

Ignoring boffins with spectrographs, coffee staling and freezing is simple chemistry :
    - Changing the temperature from +20°C to -20°C slows staling by a factor of 16. In laymans terms, two weeks in your typical refrigerator freezer equates to one day at room temperature.
    - Changing the temperature from +20°C to -30°C slows staling by a factor of 32. In laymans terms, one month in your typical chest deep freeze equates to one day at room temperature.
Dave's [and Ken's] freezer @ -15°F is midway between, so let's say 3 weeks = 1 day.
Ken's coffee, frozen immediately after roast for 4 to 8 weeks, accumulates ~1.5 to 3 days.

Ian, thanks for your feedback. But I doubt that the rate of coffee staling has any simple relationship (exponential or otherwise) to temperature... Is there a substantive basis for this statement?
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Postby cafeIKE on Sat May 17, 2008 5:36 pm

RapidCoffee wrote:But I doubt that the rate of coffee staling has any simple relationship (exponential or otherwise) to temperature... Is there a substantive basis for this statement?

"Staling of coffee occurs gradually as the result of numerous chemical processes affecting the coffee at different rates. The actual amount of time in which these processes take place will depend upon the state of the coffee (whole bean or ground) and conditions of storage (amount of oxygen contact, heat, moisture, and light)." from Coffee Analysts -> Packaging.

If temperature did not affect coffee staling, it would be a chemical anomaly.
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Postby RapidCoffee on Sun May 18, 2008 12:44 am

cafeIKE wrote:If temperature did not affect coffee staling, it would be a chemical anomaly.

Sure. I didn't question whether higher temperatures accelerate staling (of course they do). I just wanted to know if there was a substantive basis for your claim that every 10C accelerates staling by a factor of two.

I assume from your response that there isn't.
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Postby cafeIKE on Sun May 18, 2008 2:03 am

RapidCoffee wrote:Sure. I didn't question whether higher temperatures accelerate staling (of course they do). I just wanted to know if there was a substantive basis for your claim that every 10C accelerates staling by a factor of two.

I assume from your response that there isn't.

It's not my claim. It's basic chemistry.

Turning your argument on it's head, are you saying there are special coffee properties that cause the chemical processes of staling to ignore the laws of nature?

Ken's short term and Dave's long term freezer tests are close enough to what one would expect from the science to validate the assertion.
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Postby RapidCoffee on Sun May 18, 2008 2:24 am

cafeIKE wrote:Turning your argument on it's head, are you saying there are special coffee properties that cause the chemical processes of staling to ignore the laws of nature?

Ken's short term and Dave's long term freezer tests are close enough to what one would expect from the science to validate the assertion.

Kindly refrain from misquoting me. I'm simply asking for substantiation of what seems to be an extraordinary claim: coffee stales at twice the rate for every 10C rise in temperature. Neither Ken nor Dave's results come anywhere close to proving this assertion. I don't buy it, but I'm willing to be convinced...

BTW, reaction rate doubling for every 10C is not a "law of nature", it's merely a rule of thumb in chemical kinetics. :P
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Postby AndyS on Sun May 18, 2008 9:18 am

cafeIKE wrote:"Staling of coffee occurs gradually as the result of numerous chemical processes affecting the coffee at different rates. The actual amount of time in which these processes take place will depend upon the state of the coffee (whole bean or ground) and conditions of storage (amount of oxygen contact, heat, moisture, and light)." from Coffee Analysts -> Packaging.

If temperature did not affect coffee staling, it would be a chemical anomaly.


Of course temperature affects coffee staling. I believe the point RapidCoffee is trying to make is that it's probably an oversimplification to say that coffee neatly follows the 10C rule of thumb.

Here's one reason why your Coffee Analysts quote is an oversimplification: coffee staling is not only a chemical process, it's a physical process as well. As the coffee ages, CO2 and aromatic compounds gradually work their way out of the coffee and are lost into the surrounding space.

It's reasonable to assume that this diffusion rate is temperature dependent, but does it follow the same 10C rule as the chemical processes? I don't know, do you?
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Postby luca on Sun May 18, 2008 11:08 am

Hi Andy,

Thanks for succinctly stating the question!

I think that it's relevant to point out that espresso roasts run the whole gamut of resting periods; from 2 or 3 days to 2 or 3 weeks, and perhaps even longer if nitrogen flushed. So I'd be a bit surprised if there was some rule of thumb that could be applied, although I'm sure that the rest times are repeatable for any given pro roast.

Cheers,

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Postby cafeIKE on Sun May 18, 2008 11:18 am

AndyS wrote:Here's one reason why your Coffee Analysts quote is an oversimplification: coffee staling is not only a chemical process, it's a physical process as well. As the coffee ages, CO2 and aromatic compounds gradually work their way out of the coffee and are lost into the surrounding space.

Let's try a famous AndyS thought experiment: Roast some coffee. Split the roast in to equal parts. Grind one part as for drip. Seal both halves for a couple of weeks. Would you expect a markedly different gas and aromatic volume? Yes, if there was a lot of CO2 and aromatics physically trapped in the bean released by grinding. : No, it's likely that CO2 and aromatic compounds are by-products of the staling process.

I don't know for certain, do you? But my money is on NO.
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Postby AndyS on Sun May 18, 2008 1:52 pm

cafeIKE wrote:Let's try a famous AndyS thought experiment: Roast some coffee. Split the roast in to equal parts. Grind one part as for drip. Seal both halves for a couple of weeks. Would you expect a markedly different gas and aromatic volume? Yes, if there was a lot of CO2 and aromatics physically trapped in the bean released by grinding. : No, it's likely that CO2 and aromatic compounds are by-products of the staling process.

I don't know for certain, do you? But my money is on NO.


Hmmm...further evidence of the futility of thought experiments? :-)

I don't understand the point you're trying to make, but yes, there is a LOT of CO2 and a lot of aromatics physically trapped in freshly roasted beans. They eventually make their way out, whether the beans are ground or not.

Illy, Espresso Coffee, 2nd edition, p.236 wrote:Carbon dioxide formed during roasting is trapped in the cellular structure of the bean and is only released over a period of weeks following roasting, resulting in a 1.5-1.7% weight loss....The driving force at the basis of carbon dioxide and volatile release from roasted coffee is given by a diffusion flow due both to concentration and pressure gradients....The few data available about volatile partition kinetics from roasted coffee packed in air indicate that the rates of CO2 and volatiles release are of the same magnitude.


It is possible that some of the CO2 released in staling is newly formed as the result of chemical reactions, but the vast majority has already been produced in the roasting process. As far as the aromatic compounds are concerned, there are hundreds of desirable aromatic compounds formed during roasting (according to Illy). In the aging process, many of these are gradually oxidized to form characteristic stale coffee aromas.

Interestingly, in looking through Illy's book to find the section quoted above, I found a paragraph that speaks directly to the effect of temperature on CO2 and volatiles release:

Illy, 2nd edition, p. 238 wrote:in the experimental conditions adopted, for any 10C increase, the rate of volatile release increased 1.5-fold


If it's true that the rate of chemical staling reactions doubles with every 10C, and the rate of CO2/volatiles release increases 1.5x with every 10C, then your original assertion is, again, an oversimplification:

Ike wrote:coffee staling and freezing is simple chemistry :

- Changing the temperature from +20°C to -20°C slows staling by a factor of 16. In laymans terms, two weeks in your typical refrigerator freezer equates to one day at room temperature.
- Changing the temperature from +20°C to -30°C slows staling by a factor of 32. In laymans terms, one month in your typical chest deep freeze equates to one day at room temperature.
-AndyS
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Postby cafeIKE on Sun May 18, 2008 3:01 pm

Ok, I should have said ...slows the chemical processes of staling by about a factor of...

mea culpa :oops:
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