Tobacco curing is an artificial process that creates conducive conditions for the leaf to physiologically ripen. Curing flue-cured tobacco has two objectives: (a) to provide temperature and humidity conditions that will encourage certain desirable chemical and biological changes to take place, and (b) to preserve the leaf by timely drying to retain quality. There are typically three stages to curing:
Stage 1: Colouring (duration one to two days), the colour of the tobacco leaves is fixed by the chemical and enzymatic changes produced by the heat applied to the tobacco leaves. This process is conducted at operational temperatures of 30 – 40 ◦C at a relative humidity of about 85 %.
Stage 2: Lamina drying (duration two to three days), moisture is driven from the tobacco leaves at operational temperatures of 40 – 50 ◦C.
Stage 3: Mid rib drying (duration four to five days), the mid-rib or stem of the leaf is dried. During this stage the operational temperatures are 65 – 70 ◦C, with diminishing humidity.
The curing period varies with the positions of the tobacco leaves on the plants, the type (and thus, efficiency) of the curing barn used and the local conditions .The top leaves have lower water content than the bottom leaves and require less time to cure. After completion of the curing cycle, the barn is “conditioned” by adding 12 to 15 % moisture back into the leaf to enable easy handling.
Sprinkler, drip and centre pivot irrigation systems can be used to irrigate tobacco. Research has shown that early-planted irrigated tobacco has high yield potential under good irrigation management. Irrigation scheduling can be as follows:
Stage of development
50 – 150
Four weeks after planting
25 – 35
Five weeks after planting
15 – 20
End of October to November
Up to button stage
15 – 20 every 4 – 5 days
Button stage to second harvest
25 – 30 every 4 – 5 days
Second harvest onwards
35 every 7 – 8 days
General hints on tobacco irrigation
Almost all the different types of barns used for curing flue-cured tobacco can be found at the TRB. These include
• Conventional barns
• Bulk curers
• Continuous systems
• The rocket barn
• The plastic barn
The barn operates using natural convection where the air is forced to rise through the tobacco by the heat from the flues. The heated air reaches the top of the barn displacing the cold air at the top which then begins to move downwards towards the flue pipes to be heated again and the process continues .The inlet (bottom) vents are ducted under the flue pipes so that air enters the barn and picks the heat from the hot pipes to the top of the barn where the top vent for exhaust are found. This barn mostly uses wood as the curing fuel though coal can also be used. The barn can cure a maximum of 2.5 ha of tobacco. The energy efficiency of the barn is ±9 kg of wood to produce a kg of cured tobacco.
Airflow inside a typical conventional up draft flue curing barn.
In this barn, the heat is introduced to cure the tobacco leaf from the top vent which is directly linked to the heat exchanger system .The advantage of this barn is that the curing air is introduced from the top of the barn. As the hot air descends, it becomes denser and, therefore, is likely to move faster aided by its own weight. There are also limited chances of barn fires from leaking heat exchangers because the heat system is isolated from the curing room. However, this barn requires an airtight and insulated ceiling to prevent heat losses through the roof top.
Principle of operation of conventional down draft barn.
These are a condensed version of the ordinary fan barn and the difference is that leaves are held in frames (racks) instead of string or clips. In principle, a fan cycles air from the burner through the barn and subsequently over the leaves that are evenly spaced within the barn .Heat is transferred to the leaves and the return air is recycled into the burner. The barn can cure up to 2ha ± 1ha depending on the size of the leaf. The energy efficiency of the barn is ±1.5 kg of coal peas to produce a kg of cured tobacco.
In principle, hot air from a single heat source is supplied to tobacco at the midrib drying stage and is passed through consecutive earlier curing stages. As it passes through the tobacco it evaporates moisture, cooling and becoming more humid as it progresses, which establishes suitable temperature and humidity gradients. The barn can cure up to 120 ha of tobacco. The energy efficiency of the barn is ±1 kg of coal nuts to produce a kg of cured tobacco. The designs of the barn depend on the curing capacity and local conditions.
This consists of barns connected in series and supplied by a single heat source. Separate hot air ducts are connected to any one of the barns (compartment) that is at mid rib drying stage. The hot air is passed through each barn in turn before being exhausted from the coldest coloring barn .Every 24 hours the inlet hot air is diverted to the next barn. The first is cooled, conditioned, offloaded, refilled and reconnected to the airflow at the end of the series to become the last one in line. The energy efficiency of the barn is ±1.5 kg of coal nuts to produce a kg of cured tobacco.
The rocket barn is an improved up-draft conventional barn designed such that an increase in barn temperatures increases the air suction effect of the double exhaust chimney, and the induced draft extracts barn moisture, and sucks dry outside air into the barn. When ambient air enters the barn, its temperature is quickly raised and it becomes less dense. This makes the air move at a faster rate, extracting moisture from the leaves before exhausting it through the large diameter chimney. The enhancement of the rocket barn with a double exhaust chimney increases the air draft from natural to assisted air circulation. Since the drying process is a function of three factors, namely air temperature, humidity and air speed, the rocket barn design effectively uses these three factors in their appropriate proportions. The energy efficiency of the barn is ±4 kg of wood to produce a kg of cured tobacco.
The barn was designed for beginners and low income small scale tobacco growers. It is a 4 m × 4 m × 4.5 m structure with a capacity of up to 360 clips (0.5 ha). Temperatures of 70˚C are achievable and a turnaround time of 6-7 days can be attained. A plastic barn requires about 2000 farm bricks, 37 tree poles (105 – 125 mm thick) and 80 m2 of 250 μm black polythene plastic are required making the barn inexpensive to construct. Usually the barn has 3 tiers up by 5 tiers wide. The barn can cure up to 0.6 ha. The energy efficiency of the barn is ±4.5 kg of wood to produce a kg of cured tobacco.
Tobacco is a deep- rooted solanceous crop and, therefore, requires deeply ploughed soil to allow easy uptake of water and nutrients by the roots
A well -prepared land will also help to control weed pressure by burying weed seeds deeply where they cannot germinate.
A well -prepared soil profile is 40 to 50 centimetres deep, is free from large clods of compacted soil, and has well decomposed organic matter within the soil. The pH of the soil should be adjusted to the ideal range of 5.3- 6.3 using Dolomitic lime or Gypsum.
1. Early land preparation
Early ploughing around late February or early March is recommended and has the following advantages:
• Costs are lowered
• Greater uniformity and depth of land preparation
• Better quality of tilth
• Greater moisture conservation
• Early and uniform decomposition of organic matter thus making Nitrogen available at planting.
2. Late land Preparation.
Late land preparation is for heavier textured soils where the farmer needs to limit the natural nitrogen level in the soil or in tight rotations where tobacco is planted after a winter crop. This method of land preparation is generally more expensive than early land preparation as the farmer has to do more tractor work to achieve the same result. Land preparation commences late in the season, usually after July when the soils are considerably drier. Resultantly, organic materials not completely decomposed due to limited micro organism processes, which in turn leads to the delayed release of nutrients. Lime or Gypsum applied at this time will also not be utilized as effectively as in an early prepared land.
• The land will need to be irrigated heavily to enable a good quality ploughing. If irrigation is available the land preparation process is the same as an early ploughing system.
• If irrigation is not available, the land will need to first be ripped or cross -ripped to enable the disc harrow and plough to achieve an acceptable depth and tilth of soil preparation.
• Once the irrigation or ripping operations are complete, the discing and ploughing operations are done in the same manner as in the early land preparation method.
A well prepared land will make the rest of the tobacco operations such as ridging, planting and weeding much simpler and more efficient leading to better yields and quality at reduced cost.