Tobacco leaf harvesting takes 70 to 130 days after transplantation. One way of harvesting is to cut the plant, split or spear the stalks, and hang them on a tobacco stick or lath. The other way is to remove the leaves as they mature. The mechanical process of making the harvested tobacco ready for consumption is called Green Leaf Threshing.
Green Leaf Threshing (GLT):
GLT is the most important process in the tobacco industry. The process entails several stages that require scrupulous handling. The process separates lamina and midrib, which go through further stages, and finally, pack for consumption. Here are the different processes involved in a Green Leaf Threshing factory.
Firstly, the leaves lay on a conveyor belt. Automated knives cut the leaves into two parts; tips and butts. These two parts carry on separate conveyors. Tips, containing thin stems, are cut up to a specified measurement of stem diameter. The tips normally bypass the threshing process and mix with the lamina coming out of the threshing stage. Tipping is a very important stage as manufacturers can get different blends of tobacco on the tipping table.
Another important stage, the freshness of tobacco and overall experience depends a lot on the conditioning stage. The butts pass through the conditioning cylinder, which is a rotating drum having tobacco pads at the front. Hot air and steam force the tobacco pads to open. Steam jet nozzles at either end of the cylinder control the steam pressure.
There are two stages of conditioning, primary and secondary. The aim is to gradually increase the temperature and moisture to preserve the freshness of butts. The moisture content at the primary stage is around 15% to 17% at 45 to 50 degrees centigrade. At secondary conditioning, the objective is to retain a moisture content of 17% to 22% at a temperature of 60 to 70 degrees centigrade. The conditioning makes the leaves ready for threshing. Conditioning of tips allows them to match the final threshed lamina, breaking the pads.
A series of three to four threshers take in the conditioned leaves. Thresher entails a cylindrical body with a rotor having blades at equal intervals. At the front and rear end of the thresher, drums are fixed teeth. At the bottom of the thresher is a removable basket to cater to the threshed tobacco. The leaves fall on the rotor and pass through the fixed teeth. The lamina gets away from the stem. This product goes down to the basket. Factors that influence the efficacy of the threshing process include: –
- The level of conditioning,
- machine setting factors such as clearance between basket and rotor, basket size, blade speed, and roughness,
- feed rate,
- tobacco distribution into threshers.
Air classifiers use the density difference to separate threshed lamina from unthreshed leaf and stem. Counterflow classifiers are very common in the process. The classified lamina comes through the conveyor to the final sieving section. The unthreshed leaf and stem go to the next thresher. The threshing and classification process continues at least 4 to 5 times till the stem and lamina completely separate. Tips blend with the threshed lamina before sieving. Then they pass onto the drier.
As the lamina comes from different classifiers, it contains various moisture levels. Redrying helps maintain standard moisture content. This allows for a longer shelf life while preserving the freshness of the tobacco product. A redrying facility has three chambers: –
- a drying chamber
- a cooling chamber
- a conditioning chamber.
The mixture of the threshed lamina and tips spread onto an apron conveyor with an automated feeder. This allows for a uniform thickness, close to 80 to 120 mm. Hot air blows alternately up and down in the drying chamber for a uniform drying effect. As the mixture leaves the drying chamber it has 6 to 8% of moisture. In the cooling chamber, the fresh atmospheric air exhausts the hot air to cool down the mixture. High humidity in the conditioning chamber allows an optimum moisture level, of approximately 11 to 12%.
Maintaining optimum moisture content is critical to get good-quality tobacco. Too dry leaf can cause the tobacco to shrink and fall out of cigarettes, cigar leaves to crack, or paper misfeeding onto machines when manufacturing cigarettes, causing production delays. On the other hand, too high moisture content makes way for mold and musty odor, spoiling the tobacco goodness. It deteriorates the shelf life of the product. Green Leaf Threshing helps maintain the best moisture level, delivering a premium quality tobacco experience.