From the Tobacco Aging FAQ:
http://www.scribd.com/doc/92384204/Tobacco-Aging-FAQ
Once a tin is open, store it in a cool place, sealed as best you can seal it. [Aging is] an amazingly complex medley of carefully choreographed chemical and microbial dances. But,the bottom line is, when you open the tin, it's over. Other changes will take place, but it's never going to be the same again.
GL PEASE,
2001-10-17
Once air is reintroduced, the anaerobes snuff it. If there are endospores forming, they'll sprorulate, and the process *could* conceivably be restarted, *if* the correct environmental conditions were made present. But, one of those correct conditions is the absence of oxygen. So, either the tin would have to be fully evacuated, or some aerobic bacterial process would, once again, have to consume the O2 that is now in the tin. It's all a delicate dance of tiny life forms. Once you mess up the ecology that's been carefully crafted over thousands, even millions of generations of bacteria, it's tough to get it back "the way it was."
GL PEASE
2001-10-17
If you buy tobacco with significant age already, the picture changes. As soon as you open that tin, or high-barrier bag, you introduce significant changes to the environment within the container. From that point, all bets are off. Perhaps I shouldn't say that the blend will no longer age, but it will age differently from that point forward.
GL PEASE, 2003-10-28
The aging process is a series of both biological activities and organic reactions, some of which can be very slow. Many of reactions tend to happen sequentially, so once the environment is radically changed by introducing fresh air, all bets are off. Further, all those lovely aromas that emanate from that freshly opened tin are volatile organic products that, once shared with the angels, is lost and gone forever. There's no way to revers time and return things to the original state. Once you open it, smoke it. It will certainly change, but it will not likely improve in the same way that it will once it's sealed up.
GL PEASE, 2003-11-11
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I don't take GLP's word above all on such matters but I am more inclined to go with his observations than general heresay. I respect Harris' experienced observations about cigars and I am interested by this information , but I have to say that I am not convinced by the general condemnation of the theory in this thread. Tobacco could stop fruitfully aging when the seal is broken after a set period of time - easily.
None of us here (to my knowledge) are chemists so I think that we should be more open to unforeseen possibilities rather than to dogpile on theories because we think they cannot be confirmed by common sense or the naked eye or some other form of unaided perception. That, my friends, is foolish.
Experimentation is likely required but we will not likely receive many satisfactory answers as there are so many factors at play. Best of luck to whoever tries to get down to the facts!