Rate of coffee staling - Page 2

Discuss flavors, brew temperatures, blending, and cupping notes.
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another_jim
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#11: Post by another_jim »

It's true that chemical change and diffusion rates are always a simple exponential function of temperature; but each reaction or diffusion process has a different rate constant, and most of the many reactions that occur in staling are unknown. The rub here is that there is no closed form solution for the inverse of a sum of exponential terms. This means that there is no method for generalizing about staling rates, the numerical rates you observe in your experiment does not lead to knowledge of what it would be in other circumstances.

Trust me, I know all about this gotcha.
Jim Schulman

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AndyS

#12: Post by AndyS »

another_jim wrote:there is no closed form solution for the inverse of a sum of exponential terms.
Thanks, Jim, that's what Ian and I were trying to say all along! :-)

But seriously, part of what makes staling (or perhaps, "aging") so interesting is related to what you say: "most of the many reactions that occur are unknown." It's particularly intriguing to me that at least in espresso, a coffee may be peaking in high note flavors only a few days out of the roaster. And then, after a decline for week or even two, it may hit another sweet spot of more mid-range and low notes.
-AndyS
VST refractometer/filter basket beta tester, no financial interest in the company

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RapidCoffee (original poster)
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#13: Post by RapidCoffee (original poster) »

another_jim wrote:It's true that chemical change and diffusion rates are always a simple exponential function of temperature; but each reaction or diffusion process has a different rate constant...
This is a big part of what concerns me with the 10C rate doubling. There are many compounds in coffee that contribute to flavor, each of which undoubtedly stales at its own rate. For example, Illy's Complexity of Coffee article (Scientific American, 2002) states:
The aroma of green coffee contains some 250 different volatile molecular species, whereas roasted coffee gives rise to more than 800.
Another concern is phase change. In particular, some lipids will transition from liquid oils to solids over the range of temperatures involved (water too, of course). I'm sure there is dramatically different staling behavior in different states of matter.
cafeIKE wrote:Ok, I should have said ...slows the chemical processes of staling by about a factor of...
This is not a triviality. For a 40C temperature difference, assuming 2X per 10C yields a factor of 16X. Assuming 1.5X per 10C yields a factor of only 5X. Regardless, for the reasons mentioned above, I still don't buy into such a simple model.
John

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cafeIKE
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#14: Post by cafeIKE »

RapidCoffee wrote:Regardless, for the reasons mentioned above, I still don't buy into such a simple model.
Doesn't Ignoring boffins with spectrographs, say the same thing? :?

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AndyS

#15: Post by AndyS »

RapidCoffee wrote:This is not a triviality. For a 40C temperature difference, assuming 2X per 10C yields a factor of 16X. Assuming 1.5X per 10C yields a factor of only 5X.
It leads to some interesting possibilities. Normally, coffees are aged at room temperature (say, 21C). If one aged them at 1C, theoretically the degassing would proceed at ~44% of the room temperature rate while the chemical staling reactions would slow down to ~25% of their room temp rate (again, the scenario is oversimplified, ignoring the possible phase changes you mentioned and a host of other factors). *

But still, 7-10 days aging at 1C might be about right. If one properly protected the coffee from moisture, we might get a different (and improved) flavor profile: degassed coffee with less chemical staling.

Or maybe not. :-)

* [edit] To clarify, one could try aging the coffee at 1C, for 2.25x longer than one's typical room temperature aging period. At that point, it would theoretically be degassed about the normal amount, but would have experienced only about 56% of the usual chemical staling reactions.
-AndyS
VST refractometer/filter basket beta tester, no financial interest in the company

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cannonfodder
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#16: Post by cannonfodder »

I am just a computer guy, not a PhD so I have a much simpler set of guidelines for aging. Dave's rules of coffee aging...

1. all coffee ages
2. freezing slows aging
3. different coffees have different prime flavor windows
4. to prevent coffee from aging beyond its prime flavor window, drink it faster
5. coffee past its espresso prime will often still make a good drip/press pot
6. don't order more than you can drink before rule #4 is enforced
7. if your coffee is all stale and the next batch is not ready, drink tea or beer and plan better next time.
Dave Stephens

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RapidCoffee (original poster)
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#17: Post by RapidCoffee (original poster) »

I suppose I am a "boffin" (had to look that one up :wink:), but I agree with Dave's guidelines. I might add:

8. Any coffee that you will not consume within roughly two weeks should go into the freezer.
(Personally, I believe that freezing does impact the taste of coffee... but it's much better than the alternative of letting it go stale.)

Thanks to Ian for bringing up this interesting topic. It's certainly worthy of further exploration.
John

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cannonfodder
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#18: Post by cannonfodder »

I keep a small air tight jelly jar of beans in the freezer. That is my 'plan better next time' stash. I let a coffee age to within two days of its flavor window, put some in the jar, date it and put it in the chill chest. If I run short, I have two days worth in storage ready to go the next day. I cycle that stash every month so it does not sit around getting stale.
Dave Stephens

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welone

#19: Post by welone »

Here's a long answer where that rule of thumb comes from and for what it can be used. And after that an alternative calculation method, which is imo more useful when dealing with gases escaping the bean.

It is derived from what is known as van't Hoff rule (which is also the basis of the Q10 temperature coefficient and known to the german speaking by Reaktionsgeschwindigkeit-Temperatur-Regel). The assumption it makes is that the state change of the compound of interest involves an energy of about 50'000 Joules per mole (=12 kilocalories (kcal)). When making a small adaptation to it (related to the energy involved) it gives a good estimation of the temperature dependence of a whole bunch of biological, chemical and physical processes. The exact relation is described as "temperature dependence of equilibrium constants (K) or rate constants (k) as a function of the corresponding Enthalpy Changes (delta H) or Activation Energies (Ea)" (from Environmental organic chemistry, schwarzenbach et al, 2003). The adaptation you can do requires some knowledge of the kind of reaction your dealing (you have to look-up the 'delta H' or 'Ea' from it) with that you can estimate the average factor for a change in temperature (of 10°C for instance). A few examples: a reaction with a 'delta H' or 'Ea' of 10'000 Joules has an average factor for a change in temperature of 1.2 per 10°C (basis was 0-40°C) one with 30'000 Joules a factor of 1.5, and as already mentioned 50'000 Joules= factor of 2.0).
To estimate the change in volatilization during staling of coffee when cooled, you can calculate the change in vapor pressure of the compound of interest depending on its Enthalpy of Vaporization (for liquids) or Enthalpy of Sublimation (for solids). Generally there's a clear trend that the bigger the molecule the lower its initial vapor pressure and also steeper it diminishes with decreasing temperature. Especially for solids the extra amount of energy needed to sublimate it (equal to the amount to first melt and then vaporize) accounts for a stronger dependence of the vapor pressure from temperature than in the case of liquids (or subcooled liquids).

All this may be used for thermodynamical state changes (like the evaporation of a liquid or the sublimation of a solid), chemical processes (for example hydrolysis or redox reactions) or biochemical processes (enzyme reactions). For biological systems dealing with enzyme reactions it only holds true for temperatures roughly from 10-30°C; and doesn't apply at all for anything in the range of freezing temperatures because the proteins already denatured.

When dealing with gasses it seems more appropriate to use the 'ideal gas law'. Especially in relation to the quote from illy: "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...."
You can calculate the pressure change of gases (like CO2) inside the bean by applying the 'ideal gas law' (p*V=n*R*T). It is as easy as forming the quotient of the two temperatures of interest (in Kelvin!) to yield the change in pressure: For example the pressure change from 25°C (=298.15 K) to 0°C (=273.15 K) results in a decrease of pressure of 8 %. Or from 25°C to -20°C a decrease of 15% in pressure.
Calculated with:
pressure change in percent=(1-(273.15 K/298.15 K))*100 ; with the general form of the equation being p2=p1*T2/T1; from p/T=n*R/V=constant
The most critical assumption you make when applying it is, that you can neglect the potential 'loss of substance' and decrease in volume during the initial cool-down phase (which is why p/T is considered to be constant).

What concerns the meaning of the concentration gradients; this causes the beans to lose their CO2 less rapidly when in a closed container surrounded by CO2 instead of air.

greets

marco

Alan Frew

#20: Post by Alan Frew »

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.
Thanks, you can send the $ to my P.O. box address. I've done the experiment, ground vs. whole beans, about an hour out of the roaster, vac packed in non-valved bags. You don't have to wait 2 weeks, the ground bag inflates within 20 minutes, and stays puffed. The whole beans bag did inflate after 24 hours, but never to the extent that the ground bag did.

You haven't been following the literature if you don't already know that a LOT of CO2 is physically trapped within the cellular structure of the bean, and is immediately released by grinding. Again, simple physics will tell you that the increase in exposed surface area caused by grinding will increase the rate of diffusion of any gases trapped within the whole bean.

Chemistry was my first career, and your rule of thumb works as an approximation for dilute solutions. Complex organic solids??? No way.

Alan