CO2 mixes with air, if heaviness was a big determinant in all gases, then we'd not have a mix in the atmosphere. Mixing is promoted by concentration gradient, movement (temperature related, too) and individual properties of the gas. Take Argon, for example - that's a gas that is said to tend to settle.

At any rate, I didn't do much on gas physics but this came up on a search and I thought it was interesting: http://www.dummies.com/how-to/content/u ... ir-mo.html

aecletec wrote:CO2 mixes with air, if heaviness was a big determinant in all gases, then we'd not have a mix in the atmosphere.

Blink. Blink. (face palm).

Of course, this makes perfect sense. And it also kind of explains why my husband was looking at me a bit strangely when I was talking about some of my ideas.

Oh, don't worry!
This idea comes up reasonably often and I hear it all the time... it's how those valve bags work, doncha know!

You can add the gas to the top, but then you get some mixing...by adding gas to the bottom of the container, you can gently lift out the air by displacement. In terms of getting 0% oxygen and using crude measurements like a match, no Schlenk line thinking required here!

In the lab, we do simple flushes, like this, using argon within open boxes......

aecletec wrote:This idea comes up reasonably often and I hear it all the time... it's how those valve bags work, doncha know!

Well, I think that means that if coffee is packed in a valve bag, in the presence of air, that means a mixture is trapped inside the bag. And it also means, that as the beans outgas, that a mixture of air and CO2 will be coming out. Probably over time, given the amount of outgassing, the proportion of O2 will drop, but likely never be zero. Do I have that about right?

Ground coffee at room temperature that has been stored in a effectively pure N2 or CO2 pressurized environment will oxidize rapidly (stale) when opened. I suspect the Illy book goes into painful detail as to why but part of the reason is the coffee has been partially dehydrated due to the storage technique.

Purging one gas with another is a very complex process. Think of it as trying to empty a cylinder of water by pouring in fine sand. When finished and the cylinder is full of sand there is still a fair percentage of water left in the cylinder. To remove most of the undesired gas requires multiple fill, pressurize, vent cycles to effect complete removal of the undesired gas.

Single dose storage is probably the only way to avid the rapid staling issue.

That outgassing is the result of the coffee oils and bean cellulose structure oxidizing, going stale by degrees. This is why it is bagged immediately post roast so the initial CO2 has a chance to partially purge the bag. You can sample valve bagged coffee for CO2 content if you want to spent the $$ for the equipment to do it.

CathyWeeks wrote:Do I have that about right?

This I can't confirm - I'm sceptical as to the effectiveness of the valve seal in preventing the concentration gradient from causing mixing. I've not seen tests to confirm or refute hypotheses here I'm afraid!

Headspace sampling doesn't seem to involve CO2 measurements from what I can see, but this was interesting and somewhat relevant
http://www.dbpia.co.kr/Journal/ArticleD ... DE06235802

CathyWeeks wrote:Huh. It seems I cannot delete my own posts.

Happened to me. Clean out the text and add the word "deleted"