a. Risk of mold
Cured, finished tobacco can support mold growth in a humidity range somewhere between that of leather and that of cheese.
If the relative humidity (shown
as percent in the chart) is kept below 76%, the risk of mold growth is quite
low. As the humidity is increases above that (in typical home temperatures),
the risk increases dramatically. Tobacco that is damp will usually not
mold over a span of a few days, but after that may suddenly bloom with common
molds. Since mold spores are ubiquitous, their control is generally limited to
manipulating the humidity, in order to inhibit them (in the absence of chemical
Mold typically appears on the thicker parts of the stem first, since this region of whole leaf is most likely to hold the most moisture, and release it slower than the leaf lamina. In stemming a leaf for use, if there is slight mold only on the stem, then it can be ignored once the stem is discarded.
b. Mold concerns
Mold on the lamina creates two issues. The most obvious is its impact on the smell of the tobacco. If it smells moldy, it will be unpleasant to smoke or use in a smokeless preparation.
A more important issue with mold is its potential to create toxins within the tobacco on which it grows. Of greatest concern is aflatoxin (commonly produced by species of Aspergillus, which may be white, black, gray or yellow). Aflatoxins can permanently damage human tissues, including the liver—and can cause liver failure.
When aflatoxin is burned, during the combustion of tobacco, no toxin is detectable in the smoke. By contrast, aflatoxins that are in non-burned tobacco (e.g. snus, snuff, chew, and the wrapper of a cigar in the mouth) are absorbed into the tissues of the mouth and nasopharynx.
So if there is any concern about using tobacco that might be moldy, its use in cigar filler or a cigarette or in pipe tobacco is probably safe. Such tobacco should not be used in smokeless products or as a cigar wrapper.
c. Control of mold
Very slight mold can me inhibited by misting the tobacco lightly with diluted (50:50) hydrogen peroxide. But by far the wisest policy is to prevent mold in the first place, by controlling the ambient humidity.
With regard to controlling humidity, one factor that is often ignored is storage of a closed container of tobacco in ambient temperatures that swing back and forth. If, for example, a sealed bag of tobacco is exposed to a 20°F increase in temperature, the internal relative humidity drops by half. If the ambient temp decreases by 20°F, then internal humidity will likewise rise.
That in itself is not a major issue under most circumstances. But a container of tobacco exposed to swinging temperatures does not alter its internal temperature uniformly—some parts of the contents becoming warmer than other parts of the contents. This causes a heat pump effect, by driving moisture from the warmer areas, and then condensing that excess moisture within the cooler areas. The result of this heat pump effect caused by swinging ambient temperatures is to create favorable conditions for mold growth in portions of the contained tobacco. The easiest way to avoid this is to store tobacco containers (bags, tubs, etc.) in living areas of a home that are the most stable with respect to ambient temperature. In large tobacco containers (5 to 10 pounds), periodically inspect the tobacco withing the most interior regions.
2. Moisture Control
Whole leaf shipped by WLT
usually comes in sealed bags. Their poly-nylon bags are entirely vapor-proof,
and their heavy gauge polyethylene bags are nearly so. It is sealed within the
bags at medium case (fully pliable, but not damp) at controlled, warehouse
temperature—like a typical home. If the sealed bag is shipped or stored at a
significantly lower temperature, the internal relative humidity will increase.
These shipping bags should be carefully opened with scissors, cutting a clean line across the seal at the top, so that the top edge can subsequently rolled, flattened and clamped, to retain humidity. Each time the bag is opened, for inspection, or to remove some leaf, the humidity of its contents drifts toward that of the room environment in which it is opened. If the ambient environment tends to be more humid, then the leaf’s moisture content may increase toward it. If the ambient environment tends to be drier, then the leaf will gradually dry.
If the leaf in a container becomes too moist (risking mold), that can be reduced by gently warming the open bag, to drive off some of the excess moisture, then sealing the bag again. If the leaf is too dry, its contents can be lightly misted with non-chlorinated water, then sealed again. Light misting will disperse its humidity into all the leaf in the bag over a period of a few hours to a day.
3. Tobacco Beetles
Lasioderma serricorne, the tobacco beetle, first became a
pest of cured tobacco over a century ago. It initially was a problem in
tobacco warehoused in the Philippines as well as other areas of the Far East. But
subsequent world trade, coupled with poor pest control measures, led to its
spread throughout the world. It is now a ubiquitous pest.
The tobacco beetle is a brown, pinhead-size (2-3mm) beetle that can fly. It is its larval forms that like to tunnel through cured, finished tobacco leaf—both lamina and stems—leaving easily identifiable holes as well as "dust". Tobacco warehouses everywhere today regularly inspect for it, and fumigate their tobacco to eliminate it. But it nonetheless persists. The adult beetles can be successfully wiped out, but then return when its eggs hatch and mature.
Lasioderma serricorne can hitch
a ride into a house in any tobacco or tobacco product (including commercial
cigarettes), as well as in purchased, dried grains and cereals (including
breakfast cereals), flour, commercial bread crumbs, and a host of other items.
If they get into a cigar humidor, they may tunnel through wrappers on expensive
cigars, and otherwise create damage to tobacco, and to foods in the pantry.
Once tobacco beetles have been detected within a home (in a humidor or pack of cigarettes, or in stored tobacco or pantry foods), they are quite difficult to eliminate. The beetle and its eggs can be killed in stored tobacco by placing the closed bag in the freezer and leaving it there for a week to 10 days. (The adults die within 6 days at 4 °C [39.2° F], and eggs survive 5 days at 0–5 °C [32°F].) Individual bags, cartons or boxes of pantry foods can be kept within closed Ziploc bags, and tossed if seen to be infested.