Air Curing Burley Tobacco

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Air Curing Burley Tobacco
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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
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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.
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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 .

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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.

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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.

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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.

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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.

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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
Metric
English
Temperature 65-95 F 18-35 C
Relative Humidity 65 70 65 70
Air Velocity (minimum) 15 ft/min 4.6m/min
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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.
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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.
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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.

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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

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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 -)

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Optimum Curing Management

• Tip 1. Harvest the crop at optimum maturity

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Optimum Curing Management

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

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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.

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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

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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.

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Curing Structure Material
Clear plastic works well on the sides, but is not
suited for the top. Bleaching of the tobacco
will occur.
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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.
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Ability to Control Moisture Loss
A structure without some control leaves tobacco
to the mercy of the weather.