CURING is a process by which the harvested tobacco leaf is made ready for the market. It is a well standardised process to achieve the desirable qualities in the cured leaf along with the removal of moisture. The process of curing has an intimate bearing on the quality of cured leaf. A good quality leaf from the field can be made poor by improper curing. The curing operations followed in India are dependent on type of tobacco, the mode of consumption, local preference, convenience, market value and production economics.
Depending on the type of the tobacco principal methods of curing can be distinguished as flue-curing, air-curing, fire-curing and sun-curing. Though some of the characters that govern quality in tobacco are genetic, the way of practising and following standard curing methods play a significant role in determining the quality of leaf.
Uniform, well-matured and ripe leaves should be harvested. Ripe leaves have greenish-yellow colour with velvety feel and less sticky. Matured leaf lie horizontally or bend slightly down with tips slightly dry. As a general rule, the leaves are harvested from the bottom primings slightly on the green side, the middle leaves when they are ripe and the top leaves when they are fully ripe.
On an average, not more than three leaves should be harvested at a time. Harvesting must be done on a clear weather day. Immediately after rains or irrigation the crop should not be harvested and it is to be delayed by 2 – 3 days in such cases. Under normal condition, priming is done once in a week.
The leaves should be plucked against the direction of the sun for better judgment of matured leaf colour. While picking, the midribs should be bent side-ways and a well-matured leaf will snap crisply with a characteristic sound. The leaves are to be carried carefully without pressing in a wide basket with tips upward and shifted to tying shed immediately to minimize the possible wilting.
In spite of utmost care, during the harvest some immature leaf and over ripe leaves may be picked, which have to be sorted out before tying. These over ripe and under-ripe leaves must be tied separately so that each stick contains leaves of uniform colour. Over-ripe leaves are usually yellowish white and under ripe leaves are relatively dark green.
Tying the leaves
The leaves are to be tied to sticks by handling gently in a shaded place avoiding wilting and bruising. A bruised leaf (physically damaged) does not cure well in the barn. About three leaves are tied in a bunch, back-to-back, with a jute twine loop on a stick. About 90-100 such leaves are tied in separate bunches with a series of loops on a stick approximately 130 cm long. The leaves are distributed uniformly all over the length of the stick to avoid overcrowding.
Loading the barn
For a satisfactory curing, the whole barn should be loaded with the freshly harvested leaves from a single priming. The un-ripe leaves (green) are placed on the top tiers, the over-ripe leaves (yellowish-white) leaves on the bottom tier and well-matured leaves (greenish-yellow) in the bulk of the intermediate tiers.
The sticks are placed on the tiers approximately at a distance of 20-25 cm so that the leaves from the adjacent sticks slightly touch each other without pressing. A five-metre by five-metre barn will usually be loaded with 750 sticks with such spacing. The barn should never be over loaded while curing the bottom and middle leaves since they spoil easily if drying rate is slow. Leaves from the top of the plant may be crowded slightly more by closer spacing without much detriment to grade-return. Loading of the barn should be completed by late afternoon.
Curing tobacco according to fixed schedule is not possible all the time because of the variability in green leaf due to various factors like weather condition, plant position, leaf maturity, disease prevalence and in such cases slight adjustment in the process is necessary. The schedule will only serve as a broad outline and individual judgement will be necessary in almost every case.
The early stage of flue-curing should permit continuing biological activity in the leaf permitting destruction of chlorophyll, conversion of starch to simple sugars and leaf proteins to soluble nitrogenous constituents.
These cellular reactions take place in fully turgid leaf cells in aqueous medium and for complete enzymatic reactions, thermal inactivation of these enzymes must be prevented. This means maintaining high humidity and low temperature in the barn for these favorable reaction sequences.
l Produced by the Central Tobacco Research Institute