Air Curing Burley Tobacco

What Is Tobacco Curing?

Curing includes all processes that turn the burley Curing includes all processes that turn the burley tobacco from this: into this:tobacco from this: into this:

Cured picture

The most obvious physical and biochemical process is drying

Chemical Conversions

Burley leaves respond to the curing conditions imposedupon them by undergoing substantial chemical conversions which are indicated by changes in color and texture.

Two Phases of Curing

•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 .

Green - Yellow

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.

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.

Problems During Yellow – 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.

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.

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:

MetricEnglish

TemperatureTemperature 65-95 F65-95 F 18-35 C18-35 C

Relative HumidityRelative Humidity 65 – 70%65 – 70% 65 – 70%65 – 70%

Air Velocity (minimum)Air Velocity (minimum) 15 ft/min15 ft/min 4.6m/min4.6m/min

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.

FIGURE 3. Eqilibtium moisture content as a function of temperature and re lative

humidity from combined data of Jeffrey (1941) and Loclair, e t al . (1975)

1

10

100

30 40 50 60 70 80 90 100

RELATIVE HUMIDITY, %

MO

IST

UR

E C

ON

TE

NT

, % (

db)

65 o F

80 o F

100 o F

120 o F

140 o F

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.

Improper Curing ConditionsTemperature

Low temperature Low temperature ((10o C or Below) during the during the initial stages of curing results in initial stages of curing results in greengreen leaves leaves regardless of the relative humidity and air flow.regardless of the relative humidity and air flow.

The chemical conversions are too slow because of The chemical conversions are too slow because of the low temperature. the low temperature.

The higher the drying rate, the greener the cured leaf.The higher the drying rate, the greener the cured leaf.

Improper Curing ConditionsRelative Humidity/Temperature Interaction

Low humidity, moderate temperature results in greenish or Low humidity, moderate temperature results in greenish or mottled leaf. RH temp = mottled leaf. RH temp =

Low humidity, high temperature ( 24 degrees C and above) Low humidity, high temperature ( 24 degrees C and above) causes pie-bald (yellow) leaves. RH temp = causes pie-bald (yellow) leaves. RH temp =

High humidity, moderate to high temperature causes High humidity, moderate to high temperature causes houseburn which results in a dark leaf with excessive loss houseburn which results in a dark leaf with excessive loss of dry weight. RH temp =of dry weight. RH temp =

Factor Interaction

Drying increases as airflow increases for a constant Drying increases as airflow increases for a constant relative humidity and temperature.relative humidity and temperature.

Drying = airflow (RH -, temp -)Drying = airflow (RH -, temp -) Drying increases as relative humidity decreases for a Drying increases as relative humidity decreases for a

constant temperature and airflow rate.constant temperature and airflow rate. Drying = RH (temp -, airflow -)Drying = RH (temp -, airflow -) Drying increases as temperature increases for a constant Drying increases as temperature increases for a constant

relative humidity and airflow.relative humidity and airflow. Drying = temp (RH -, airflow -)Drying = temp (RH -, airflow -)

Optimum Curing Management

Tip #1. Harvest the crop at optimum maturityTip #1. Harvest the crop at optimum maturity

Optimum Curing Management

Tip #2. Stagger Tip #2. Stagger spacing to avoid spacing to avoid green tips in contact green tips in contact with drier flyings of with drier flyings of the rail belowthe rail below

Optimum Curing Management

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

Optimum Curing Management

Tip #4. Never hang freshly harvested tobacco Tip #4. Never hang freshly harvested tobacco under partially cured tobaccounder partially cured tobacco Cured tobacco may pulls moisture out of Cured tobacco may pulls moisture out of

fresh tobacco too quickly causing quick fresh tobacco too quickly causing quick cure.cure.

Fresh tobacco may cause too high relative Fresh tobacco may cause too high relative humidity causing houseburn or rot in cured humidity causing houseburn or rot in cured tobaccotobacco

Preventing Houseburn

When confronted with When confronted with humid, houseburning humid, houseburning weather, we want tobacco to weather, we want tobacco to dry as much as possible.dry as much as possible.

Keep ventilators open.Keep ventilators open. Fans are sometimes used to Fans are sometimes used to

help circulate the air to help circulate the air to prevent stagnation. prevent stagnation.

Heat can also be added that Heat can also be added that helps to lower the relative helps to lower the relative humidity.humidity.

Speedup Cure

Curing Structure Material

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

Black Plastic MaterialBlack 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.

Ability to Control Moisture LossA structure without some control leaves tobacco to the mercy of the weather.