Tobacco doesn't need preservatives. The curing process is sufficient to ensure that tobacco, if stored properly, will last indefinitely. It *can* mold, if too wet, and ultimately rot, again if too wet. But it doesn't putrify, spoil, rot, become septic or anything else which warrants a "preservative." The reason some manufacturers put humectants [chemical preservatives] in their tobaccos is to prolong the "shelf-life" of the "fresh" (read moist) tobacco. For some reason, people don't want to find their own personal "humidity index" for their tobaccos, so the manufacturer second guesses, loads the stuff with propylene glycol, and packs the stuff in little pouches. Not all tobacco is so adulterated, but in some cases, the tobacco in pouches is, while that in tins is not. Pouches have a shorter "shelf-life."
G.L. Pease, 1997-12-22
In terms of quantity sold: most tobacco mixtures are flavored with chemical additives. This includes almost all 'dime store' blends, most premium continental European blends (including most of the traditional English blends being produced on the continent now), and many premium US & UK blends. These blends will most probably not improve with age & are probably best smoked as close to 'off the store shelf' as possible.