Transcript Air Curing Burley Tobacco

Air Curing Burley Tobacco
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
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.
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 .
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 Curing
Yellow – 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:
English
Metric
Temperature
Relative Humidity
Air Velocity (minimum)
65-95 F
65 – 70%
15 ft/min
18-35 C
65 – 70%
4.6m/min
Importance of Relative Humidity
MOISTURE CONTENT, % (db)
The narrowness of the
relative humidity range
makes it the most
critical variable and
thus the most likely to
deviate outside the
norm.
100
10
65 o F
80 o F
100 o F
120 o F
140 o F
1
30 40 50 60 70 80 90 100
RELATIVE HUMIDITY, %
FIGURE 3. Eqilibtium moisture content as
a function of temperature and relative
humidity from combined data of Jeffrey
(1941) and Loclair, et al. (1975)
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 Conditions
Temperature

Low temperature (10o C or Below) during the
initial stages of curing results in green leaves
regardless of the relative humidity and air flow.
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The chemical conversions are too slow because of
the low temperature.
The higher the drying rate, the greener the cured
leaf.
Improper Curing Conditions
Relative Humidity/Temperature
Interaction

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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 =
Factor Interaction

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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 -)
Optimum Curing Management

Tip #1. Harvest the crop at optimum maturity
Optimum Curing Management

Tip #2. Stagger
spacing to avoid
green tips in contact
with drier flyings of
the rail below
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.
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
Preventing Houseburn
Speedup Cure
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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.
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 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.
Ability to Control Moisture Loss
A structure without some control leaves tobacco to the
mercy of the weather.