A urine smell is likely ammonia, which is generated by the ongoing breakdown of albuminous proteins within the leaf lamina. (By contrast, a fecal, barnyard smell is likely due to the growth of
E. coli bacteria, and should not be there.) Flue-curing should eliminate proteins. It is always a fast curing process, and should follow a rigid time schedule—from primed, green leaf to fully flue-cured in 5 to 9 days, depending on the stalk level of the priming.
In my own flue-curing efforts, the leaf stem (central vein) usually ends up a dark brown, and brittle. It may be that these batches were not properly flue-cured.
You should not have to kiln (123-128°F x 6-8 weeks) any properly flue-cured leaf, although, in this instance,
that would likely resolve the problem. I doubt that you would be eager to go to the trouble and expense of constructing a kiln for this one batch of leaf. [Home growers should always invest in a kiln.]
I agree with
@ShiniKoroshi. Kilning is actually accelerated aging, which depends on the intrinsic oxidase enzymes within the leaf lamina to break down any remaining protein. There are two major enzymes that accomplish this in tobacco leaf. The fastest-acting of the two is denatured by heat at 149°F, so it's likely gone in this leaf. The slower of the two enzymes is not denatured by heat until about 191°F, and is likely still present after the flue-curing process. For it to act, it needs some moisture and air.
You can try
lightly misting the leaf with water, then allowing it to rest in a plastic bag for a couple days, airing-out the bag at least daily. Every third day, allow the leaf to become fully dry, to avoid the growth of mold. If this is working, you should smell ammonia each time you open the bag. (That would be the protein going away.) Repeat the misting—resting—drying cycle for as long as your patience lasts (hopefully a week or three), then resample the leaf, to see if it is making any progress.
It's worth noting that the oxidase enzymes are simply catalysts created by living cells. As with all catalysts, they speed up a reaction
that would happen regardless. That acceleration can be tens to hundreds of times faster than the natural, unaided reaction. The speed of the chemical breakdown of albuminous proteins is always dependent on the ambient temperature (within limits), which is why a kiln is used. If you just store the tobacco for long enough at ambient temperature, it will mellow, but that might be 5 or more years.
Bob