Air-curing
- Air-curing is the process of curing tobacco leaves without the use of artificial heat by hanging the leaves in a ventilated barn until they have no sugar remaining.
- This is a natural process and curing is done in a wooden barn under normal atmospheric conditions.
- The leaves should be yellow before it dries out and after that the rate of drying is gradually increased by increasing the ventilation.
- The best temperature for early wilting is 21-24oC and it should never exceed 43oC, even in the final stages. It takes 6-8 weeks for air-curing.
- Air-cured tobacco is low in sugar (light and sweet smoke flavor) but high nicotine content.
- The main objective of air-curing is to keep the leaf alive until even the smaller quantity of sugar has been oxidized away.
- Most of the cigar tobacco, Burley and Maryland are air-cured.
- Air-curing consist of two stages, viz; Yellowing and Browning.
a. Yellowing
- The leaf remains alive and it is cured at 15.5-35oC and RH of 70-80%.
- To secure a satisfactory curve, the leaves should remain alive till the necessary changes takes place.
- The harvested leaf is rich in starch. One of the important changes during curing is the disappearance of starch.
- If the leaf is killed rapidly by drying, starch is not removed and tobacco is degraded.
- The green color changes to colorless and the yellow color appears.
- The yellow color marks the end of the first stage.
b. Browning
- Browning is the important change that takes place during the second stage, which is due to oxidation of substances (polyphenols) present in the leaf and doesn’t take place till the leaf cells are dead.
- The two essentials for the development of brown color are oxygen and moisture.
- Brown color development is completed by the fermentation process.
- After drying of the leaves, they are tied into bundles and bulked on raised platform.
- The bulks are broken and rebuilt frequently by shaking, to remove any ammonia and carbon dioxide formed during bulking and to prevent excessive rise in temperature.
- In addition, the position of leaves are also changed, the leaves on the top or border are placed in middle and the middle ones are transferred outside, resulting in uniform fermentation of all leaves in the bulk.
- Thus, it is slower and the sugars that accumulate in the leaf during the earlier period are destroyed during the later period.
- The leaf thus produced becomes rich in nitrogenous constituents. Brown color is the desirable characteristics of this type of curing.