Flue curing is the most rapid of the curing processes for tobacco and is used for Virginia or Flue Cured Tobacco which is used in the manufacture of cigarettes. The objectives of flue curing are:
To make sure that most of the carbohydrate in the leaves is broken down to sugars. The carbohydrate which is mainly in the form of starch, has to be broken down to sugar without the process going too far allowing the sugar to be further broken down to carbon dioxide and water. Flue cured tobaccos have the largest sugar content of all tobacco types and lose the least amount of dry matter during curing;
To achieve a bright colour in the lemon‐mahogany range without the leaf becoming dark brown as the dark brown colour is of air cured leaves; and
To remove most of the moisture in the leaves.
The conventional barn used for flue curing measures 5m by 6m and is high enough to allow for 7 or 8 tiers of leaves with at least 750mm between each tier. This type of barn holds 800 strings which holds about 80 000 leaves. This number is the maximum for a single furnace.
Figure 1: A conventional barn with 8 tiers
The tobacco barn has a heating system usually wood or coal fired combined with a system of ventilation so that the flow of air can be controlled inside the barn. Important factors are the heating area, the amount of heat provided by the furnace, the ventilation area, the density and moisture content of the leaf in the barn and the moisture content of the air in the barn.
Air moves by convection inside the barn. The air is heated by the flues or radiators on the floor and expands and rises. As the air rises up through the barn heat is lost to the leaves of tobacco hanging
from the tiers. While it travels up the barn moisture is drawn from the leaves into the air. Moisture will be removed from the leaves until the air in the barn has become saturated with moisture vapour and fresh air is allowed in through the ventilation system. A delicate balance between heat and fresh air is necessary and forced air systems using fans are much more efficient for maintaining this balance. Metal flue pipes on the floor of the barn are still the main conductors of heat but on some farms have been replaced by radiators used in steam curers or by heat exchangers used in tunnel curing.
Although it is a relatively simple matter to know what the best conditions to achieve the chemical changes required in the leaf and to dry the leaf at the end of the cure, however these are not always easy to achieve because of the physical limitations of the standard barn. In most cases the cure has to be carried out within the limitations of the curing unit, rather than in the conditions most favourable for producing the best leaf. This difficulty is not serious with leaf that is of good quality at harvesting. Leaf that is of poor quality and style at harvesting cannot be improved during the cure. The quality of the cured leaf is set at harvest.
2. THE COLOURING PHASE
The leaf is alive during this phase and must be kept alive to allow the chemical changes to take place. The main chemical change is the breakdown of starch in the leaf to sugars, some of which are, in turn, broken down to carbon dioxide and water. These two compounds are given off into the atmosphere and account for the loss in dry matter in the leaf during curing. In a good cure about 15% of the dry matter of the leaf is lost but much more is lost if the colouring phase is allowed to go on for too long. A freshly harvested leaf contains about 30% starch and this is reduced to 5% at the end of the curing process. At the same time the sugar content increases from 5% to between 15 and 20% depending on the type of leaf. Carrying the colouring phase on for too long causes a loss of sugar and increase in nicotine. It can also produce a leaf which is too brown.
The most obvious change in the leaf during the colouring phase is the disappearance of the green colour and development of the lemon to mahogany colour which is characteristic of flue cured tobacco. This colour change is caused by the breakdown of the chlorophyll in the leaf and unmasks the yellow and darker coloured pigments. This colour change takes place at the same time as the other chemical reactions and provides a basis on which to judge the progress of the colouring phase. The time taken for the leaf to change colour is also the time taken for the starch to be broken down in a good cure.
The colouring changes that occur during curing are all caused by the action of Enzymes and these will only function if the leaf is kept turgid or semi‐turgid. If the leaf dries out completely it dies and no further chemical changes will occur. Each enzyme involved in the chemical changes in the leaf will function only at a certain temperature and temperatures which are too high or too low will stop them from functioning. The enzymes which control the breakdown of chlorophyll and some of the carbohydrates work best between 32 and 38°C. Their activity is reduced at 49°C and stops altogether at 54° C. The enzyme called Amylase which controls the breakdown of starch to sugar works best at 49 ‐ 60°C, and stops working at 72°C.
The colouring phase should begin at a temperature of 30° C as soon as possible after the barn has been filled with leaves and the ventilators in the barn should be kept closed. This temperature should be maintained until a definite colour change can be seen in the leaves on the bottom tier of the barn. The temperature should then be increased to 33°C and the ventilators opened. The amount of ventilation can be judged by the condition of the tobacco leaves on the bottom tier and will depend on the experience of the operator. A temperature of 33 ‐ 35°C should be maintained
until colouring is well advanced and the temperature should then be increased to 38°C and remain there until colouring is complete. Only a trace of green should now remain in the midrib, main veins and the shoulders of the leaf, and at full colour the leaves should be well wilted.
The colouring time will vary according to the position of the leaf on the plant. Leaves which have been reaped from the lower half of the plant take 60 hours and leaves from the upper half of the plant will take 36 ‐ 48 hours. Leaves which have been reaped green will take longer and those which have been reaped overripe will take a shorter time. During the first 18 ‐ 24 hours of the cure the leaves should be fully turgid and this declines over the remaining time of the cure. A leaf loses about 30% of its moisture during the coloring phase but this loss should take place sufficiently gradually to allow the enzymes to complete their work.
3. THE DRYING PHASE
The changes which take place in the leaf during the drying phase are physical changes, the chemical changes having been completed and brought to a stop by the higher temperatures and loss of moisture in the leaf. When the leaf is fully coloured it must be killed as soon as possible in order to ‘fix’ the colour and prevent any loss of quality. Killing is achieved by drying the leaf out. The ventilators in the barn should be adjusted so that the maximum amount of air enters the barn passing through the tobacco and temperatures should be increased to drive moisture out of the leaf. Drying is completed at 70°C although it can be carried out at lower temperatures if plenty of time is allowed. Temperatures above 70°C will cause the loss of volatile substances from the leaf and will lead to a loss of aroma and flavour in the smoke.
The rate at which the temperature should be increased will depend on the dryness of the leaves, the type and amount of tobacco and the efficiency of the heating and ventilation in the barn. In general, the temperature can be raised at 3°C every 4 hours but, if the ventilation is good the temperature can be raised from 38°C to 70°C more rapidly and at a constant rate. The temperature of the wet bulb thermometer in the barn should be carefully watched. The wet bulb thermometer is a measure of the humidity in the air. If the humidity rises and the air becomes moister, the wet bulb temperature will rise. A wet bulb temperature of more than 38°C during the drying phase is a serious danger signal and a loss of quality in the leaf is certain unless it is corrected by increasing the ventilation and so the rate of moisture removal.
Samples of leaf from the barn should be examined for curing faults; some of these faults are:
Barn Rot: caused by poor ventilation during the colouring phase and can be prevented byventilating earlier;
Sponge or Browning: caused by poor ventilation during the drying phase. This can beprevented by increasing the ventilation or by reducing the drying rate; and
Perished or over‐coloured leaf: caused by drying too slowly and not killing the leaf earlyenough.
These faults cannot be prevented if the barn is over‐filled with leaves and are a sign that air is not moving freely through the barn.
The midrib of the leaf is finally dried out by maintaining the temperature at 70° C.
Once the cure has been completed, the leaves in the barn will contain very little moisture. If they are handled in this state they will crumble away to dust and a large amount of the leaf will be lost. Before the barn is unloaded some moisture must be put back into the lamina and midrib of the leaf. At less than 12% moisture the leaf will shatter. The optimum moisture content is 15 ‐ 18% moisture when the leaf can be folded over without breaking. There are 3 methods of replacing some of the moisture. This process is called conditioning the leaf. These are the natural methods; by the use of steam; and by the use of water.
At the end of the cure, open up the barn doors and allow the leaf to cool down and remove all heat. The next day close all the doors and ventilators to hold the moisture and that night open the doors. The next morning the midrib should be soft and the leaves conditioned. When the leaves are conditioned, the barn can be unloaded.
Water can be sprayed into the barn provided the water particles are very small and are inthe form of a fine mist.
Steam can be directed into the barn and this will condition the leaves very quickly. If thesteam is too hot it can cause the leaves to turn a red colour and lose quality.
As the leaves are unloaded from the barn they are rough bulked for 24 hours to allow the condition to spread throughout the midrib. The leaves are laid in a pile on the floor of the storage shed on either a wooden or concrete platform which is covered with some waterproof material. Once the leaves have been stored they are under great pressure and it is important that the moisture is spread evenly throughout the leaves, giving it springiness for compression.
After rough bulking the leaves are stored in bulk until they can be graded and baled. Storage before grading is important because the leaves can change colour during the storage period. The way in which the leaves are stored will depend on the space available and the system used to hang the leaves in the barns for curing e.g. strings, sticks or clips.
Systems used are:
Loose leaf bulk;
String bulk with the strings attached; Stick bulk with the sticks attached; Rough bales;
String bales; and Cubes.
Once the bulk stack has been built it will require some sort of covering material. The main thing to guard against is too much moisture in the leaves. This will cause the leaves to go mouldy and will lead to a great loss of quality.
Figure 2: Flue‐curing Tobacco barn
Figure 3: Burley Curing Tobacco barn