[Homeroast] OT: Gases, was Re: Storage right after roasting

Andrew Thomas adt0611 at yahoo.com
Tue Aug 19 22:03:39 CDT 2014


Mike, what happens in a closed container with 2 gases, say O2 and CO2, where one gas is heavier than the other? I assume they do not remain completely separate, correct? Do they eventually diffuse and become a uniform mixture? Or do they become a graded mixture, with the heavier gas more concentrated at the bottom and the lighter one more so near the top? And if the top of the container is open, will the heavier gas diffuse into the atmosphere?

> On Aug 16, 2014, at 11:46 AM, Michael Koenig <koenig.mike at icloud.com> wrote:
> 
> I've ranted on this topic in the past before, so hopefully the old timers on the list will bear with me. 
> 
> Gases are funny things and mix very readily. It's easy it assume they behave like oil and water, but the reality is they they mix readily. Therefore the CO2 given off by the resting beans will ALWAYS be mixed with O2.  You may reduce the concentration of O2 but it's my contention that in order to exclude meaningful amounts of O2 you need to either pull a vacuum on your container (pulling your volatiles out with it), or extensively purge with another gas.  On the timescale we drink our coffee it simply is not worth jumping through these hoops.  (Nitrogen flushed bags might be worthwhile for a commercial roasting operation who wants longer shelf-life). 
> 
> Back in my lab days we had a glovebox that was flushed with nitrogen to work with oxygen sensitive compounds. It required several pump downs and nitrogen flushes to get out all the oxygen. This is something that's just not practical or worth worrying about for us home roasters. 
> 
> --mike 
> 
> Sent from my iPhone
> 
>> On Aug 16, 2014, at 1:31 PM, Mike Davis <mldavis2 at sbcglobal.net> wrote:
>> 
>> I think the big unknown in storage techniques is the effect of oxygen (if any) to the resting process.  Over a long enough time period, coffee will go stale of course when exposed to oxygen.  But on the other side, we know that "resting" can enhance the flavor. So the real question would seem to be what happens during the "resting" time and if the presence or absence of oxygen is responsible for the improvement.  Logically, if CO2 is a result of chemical process taking place during "resting" then oxygen should be nearly non-existent.  It is possible that cooling could pull oxygen back into the beans.  But storage immediately after cooling would seal them from additional oxygen as the beans exuded CO2 which would expel oxygen from the container for a while before oxygen begins to work its way back in when CO2 pressure is reduced to equal.  So I assume (perhaps incorrectly) that the favorable chemical processes that enhance coffee during resting are a "coasting effect" of heating and continue to completion in the (nearly) total absence of oxygen.  In other words, they are not an oxidation process as such requiring oxygen.
>> 
>> So my system (such as it is) is based on those assumptions.  Put the beans into a light-tight container (AirScape canister) with minimal holes just sufficient to allow exit of CO2 overnight, yet so small that the greater CO2 pressure probably prevents incursion of oxygen as the CO2 exits.  Then seal the canister once the CO2 has stopped. There should be little or no oxygen present except what little might remain in the beans, and the coffee should complete the chemical "coast" and then more or less stop, waiting for the lid to be unsealed.  I think this is the idea behind coffee that is weeks old (or older) on store shelves - one way CO2 valves that exclude oxygen.  Once my beans are loaded into the hopper, then of course, there is abundant oxygen and staling begins.
>> 
>> That's just what seems to work for me.  I'm always open to better suggestions and ideas.
>> 
>> Mike Davis
>> 
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