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We had an Italian made chamber purge/vac machine at the roastery I used to work for and it would do a nitrogen flush then very deep vacuum to all of our coffees sold in valve bags. Most coffees were packaged no more than a couple of days out of the roaster and none of them, no matter how much of a brick they had become during the vacuum/sealing operation, ever maintained the vacuum more than a few hours due to CO2 outgassing. Afterward most of the valve bags would end up in a neutral state (neither brick nor puffed).

The question was raised whether or not a CO2 gas environment was detrimental to the flavor of coffee as it rests in a container. In all of my experience in the industry and in picking the brains of the pros in the packaging industry, I've never heard anyone make this claim. Since CO2 is a natural by-product of roasted coffee beans and they are being bathed in it continuously from day one till time of consumption, it would be a very bad thing if it were the case. Now, I have heard that if fresh roasted coffee is kept in a non-venting container and is allowed to reach the maximum possible pressure and if kept at that pressure, then it will prevent any further outgassing and can cause the time needed to degass to extend much longer than if allowed to only build to a low pressure environment. A low pressure CO2 environment in a loosely capped mason jar is a good thing and being an inert gas, helps displace freshness killing oxygen.

It would be of interest to me to know if at a certain pressure from CO2 outgassing, that the post-roast improvement from resting could be retarded or compromised by slowing down its release. I've never read anything definitive on the subject. There's obviously the difference in the quality of extraction comparing an aggressively foaming bed of coffee during brewing versus one where water can readily contact all of the grounds equally but this is not a primary reason for the improvement had during the rest period.

Regardless of how we're going to bag or store our freshly roasted coffee, the object is to get the coffee into a sealed container as soon as physically possible to minimize exposure to oxygen since the staling clock starts immediately.

Allen