Curing is the process used to achieve the texture, colour and overall quality of a specific tobacco type. It take place in three steps: yellowing to obtain the desired colour at low temperature, leaf drying with fairly high heat and rapid air circulation and killing the stem with high heat and little air movement. It usually involves a heat source that reduces chlorophyll content and converts leaf starch into sugar, while the tobacco changes colour from green to brown, closer to the colour and texture you will find in a typical cigarette.
While some tobacco is normally air-cured or sun-cured, most commonly, the heat is introduced into a curing barn or shed through pipes from an exterior furnace. This is known as flue-curing.
The curing process for flue-cured tobacco production is by far the greatest energy user (constitutes more than 80% of total energy requirements).

Curing tobacco leaves has traditionally involved the consumption of large quantities of wood fuel however in recent years, alternative methods and fuels have also been introduced in several countries. These fuels include gas, coal, fuel oil and liquid petroleum gas (LPG).  

LPG has become the primary fuel used in this process, replacing wood and accounting for almost 70% of production. Almost all bulk-curing barns built since the early 1970s are equipped with LPG gas burners. It lights instantly, is easily controlled, and is a constant and uniform source of heat.

Combustion gases have been found to produce tobacco-specific nitrosamines, which are deemed to be carcinogenic. Producers are now required to retrofit, or change all flue-curing barns to operate only with indirect-fired curing systems. In these systems, direct mixing of flue gases with curing combustion gases is prevented by passing combustion gases through heat exchangers or by allowing combustion to take place outside of the barn with the resulting heat being conducted into the barn via hot water or steam.

Since some heat loss is unavoidable with indirect curing systems, it is very important for growers to gain as much heating system efficiency as possible in order to control fuel costs. The single greatest reason for burner inefficiency is improper ventilation. Growers have switched from fuel oil to LPG because LPG systems are more efficient – LPG and air mix better during combustion than fuel oil and air. This allows for a lower excess air setting on the gas burner and improves the efficiency of the system. On average, a well-maintained and operated barn will cure approximately 9 pounds of leaf per gallon of LPG.