The term for a large pile of tobacco, arranged for fermentation. Burros can be enormous, weighing 3,500 or even 4,000 pounds or more. After curing in a curing barn, or casa de tabaco, the tobacco is brought into a warehouse and assembled in bunches of leaves called hands, and made into a burro.
The leaves sit flat in a burro, one on top of the other, with boards, cardboard or old tobacco stems beneath. The weight of the tobacco and the moisture in the leaf (as well as moisture that is added by workers before assembling the burro) creates heat, which causes fermentation to begin, changing the chemical structure of the tobacco, removing impurities such as ammonia and rendering the tobacco smokeable. When the desired temperature is reached, workers break down the burro and rebuild it. This process is repeated again and again, and can last for months or longer.
There are many terms to describe the same thing. Others call the piles pilones (pee-LONE), bulks, or trojes (TROH-heys). Another variation on is to call a pile of tobacco that has been removed of its stems a burro, and the pile with stems a pilón.