Aging Tobacco

As an Assistant Professor of Microbiology, who is also an avid pipe smoker, I have long wondered what microbial processes are at work on the aging process of pipe tobacco. After doing some preliminary literature searches [...], I have come to the conclusion that much less is known about these processes in tobacco, than, say, microbial fermentations leading to other useful commercial products (e.g., those found in the food and beverage industries).
Apparently as these tobaccos age prior to the canning process itself, a number of microbiological events occur, involving a range of distinct species of bacteria and fungi. This is because of high water levels that drop during production, finally reaching their lowest states prior to canning. After canning, [...], these microbial processes continue, somewhat abated, and most surprisingly involve an entirely different population of microbes, one encompassing more fungi and less bacteria.
After sealing, of course, the oxygen levels plummet, from 20% atmospheric to levels lower than 5% eventually, over time. This is due to the metabolic process of the populations of obligate aerobes that came in with the tobacco, and are most active in the presence of high levels of oxygen. Concomitantly, the endogenous aerobic species that came in with the tobacco gradually give way to the emergence of at first facultative (~10-15% oxygen) and then obligate (less than 5% oxygen) anaerobic microbes. The biological processes that these microbes undergo (from obligate aerobes through to facultative and then eventually obligate anaerobes) result in the production of a number of different classes of volatile molecules.
As every schoolboy knows, organic molecules (hydrocarbons) are classified into four groups (sugars, proteins, nucleic acids and lipids). Interestingly, as the aging process proceeds, over years, the sugars become fully metabolized first by the aerobes early on, leaving primarily the latter three classes of molecules. The deamination (yielding organic acids) and decarboxylation (yielding volatile, smelly amines) of amino acids in the proteins and the conversion of the lipids into smaller, more volatile fatty acid chains, is what is primarily responsible for the “bouquet” upon opening the tobacco can.
Presumably, during the smoking, it is the presence of these volatiles that lend the palate of the blend. The sugars that do remain, that were not accessible to the early degredation by the obligate aerobes, is what is going on with the sugar-rich Virginias. Apparently, the packing of the tobacco (plug verses loose) is what determines how much of this sugar is freely metabolized early on. To me, it would seem, a Virginia plug would retain more sugar than a loosely packed Virginia.

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