Air Curing Burley Tobacco

1. Air Curing Burley Tobacco

2. What Is Tobacco Curing? Curing includes all processes that turn the burley tobacco from this: into this: Cured picture The most obvious physical and biochemical process is drying

3. Chemical Conversions Burley leaves respond to the curing conditions imposed upon them by undergoing substantial chemical conversions which are indicated by changes in color and texture.

4. Two Phases of Curing Green - Yellow • In the first phase, green or greenish tobacco turns to yellow. • This is the most rapid phase and occurs immediately after harvest. • Tobacco left to field wilt may pass this phase before housing. • In this phase chlorophyll degrades allowing yellow pigments to appear .

5. Problems During Green – Yellow Conversions • Any process that damages cell integrity may allow chlorophyll to leak and stain tobacco. Damage can be due to sunburn, mechanical damage or cool, dry air. • This can produce a crude green color after curing. These stains are very sensitive to light and will degrade rapidly if exposed to sunlight. Sunburned tobacco must be left in the field so green will degrade.

6. Second Phase of CuringYellow – Brown • The second phase represents a loss of pigments and chemical changes. This stage includes the remainder of the curing process and leads to a brown color due to oxidation of polyphenols and the polymerization to brown products. • This phase is much slower or should be. Drying rate must match enzymatic rate for good cure.

7. Problems DuringYellow – Brown Conversions • Curing that is too rapid will not allow sufficient time for conversion of yellow pigment which is the most obvious sign. • However other characteristics such as leaf texture and tobacco taste will be affected by incomplete conversion of undesirable products in the leaf.

8. Spongy Leaf Cells Aid Curing • Tobacco is a material that has considerable intercellular pore space. Spongy leaf cells help the tobacco exchanges moisture with the air that surrounds it until the moisture content of the leaf is in equilibrium with the relative humidity of the surrounding air. • For example, dry air (low humidity) will allow the moisture in the tobacco to diffuse relatively faster than would moist (high humidity) air with all other factors remaining the same. • Freshly cut tobacco is approximately 85% moisture and will dry to about 15% moisture by the end of the curing process.

9. Environmental Factors Three environmental factors affect the curing of burley tobacco; Temperature, Relative Humidity and Air Flow. The range of these variables that produce good quality burley tobacco are: English Metric

10. Importance of Relative Humidity The narrowness of the relative humidity range makes it the most critical variable and thus the most likely to deviate outside the norm.

11. Lack of Control Our only control of the curing process is control of the drying rate. This was done primarily in conventional barns by opening the ventilators to promote drying and closing the ventilators to retard drying.

12. Improper Curing ConditionsTemperature • Low temperature (10o C or Below) during the initial stages of curing results in green leaves regardless of the relative humidity and air flow. • The chemical conversions are too slow because of the low temperature. • The higher the drying rate, the greener the cured leaf.

13. Improper Curing ConditionsRelative Humidity/Temperature Interaction • Low humidity, moderate temperature results in greenish or mottled leaf. RH temp = • Low humidity, high temperature ( 24 degrees C and above) causes pie-bald (yellow) leaves. RH temp = • High humidity, moderate to high temperature causes houseburn which results in a dark leaf with excessive loss of dry weight. RH temp =

14. Factor Interaction • Drying increases as airflow increases for a constant relative humidity and temperature. • Drying = airflow (RH -, temp -) • Drying increases as relative humidity decreases for a constant temperature and airflow rate. • Drying = RH (temp -, airflow -) • Drying increases as temperature increases for a constant relative humidity and airflow. • Drying = temp (RH -, airflow -)

15. Optimum Curing Management • Tip #1. Harvest the crop at optimum maturity

16. Optimum Curing Management • Tip #2. Stagger spacing to avoid green tips in contact with drier flyings of the rail below

17. Optimum Curing Management • Tip #3. Provide proper spacing on the tier rail. • 23cm to 36 cm in older barns. • 18 cm to 20 cm in the newer 3-tier barns. • 10 to 20 cm in curing structures.

18. Optimum Curing Management • Tip #4. Never hang freshly harvested tobacco under partially cured tobacco • Cured tobacco may pulls moisture out of fresh tobacco too quickly causing quick cure. • Fresh tobacco may cause too high relative humidity causing houseburn or rot in cured tobacco

19. Preventing Houseburn Speedup Cure • When confronted with humid, houseburning weather, we want tobacco to dry as much as possible. • Keep ventilators open. • Fans are sometimes used to help circulate the air to prevent stagnation. • Heat can also be added that helps to lower the relative humidity.

20. Curing Structure Material Clear plastic works well on the sides, but is not suited for the top. Bleaching of the tobacco will occur.

21. Black Plastic Material Black plastic is best for most situations, but in areas of intense sun, heat buildup may be a problem. White plastic can help to reflect some solar energy.

22. Ability to Control Moisture Loss A structure without some control leaves tobacco to the mercy of the weather.