Paul E. Sumner
When to Harvest
Container Loading
Guide for Bulk Curing
There are several points you should keep in mind for a quality cured leaf. First, the decision must be made whether to cleanup primings or leave them in the field. If you elect to harvest primings, harvest when the leaves appear greenish-yellow in color. Lower stalk tobacco contains more moisture on a percentage basis than the upper stalk tobacco. Therefore, the primings should be harvested when the leaf is dry. One way to ensure an acceptable leaf for sale is to hand-harvest the lower leaves. Hand harvesting causes less damage to the tobacco and there is less foreign material (such as sand and sucker) in the cured tobacco. Properly hand-harvested tobacco may require very little market preparation labor compared with machine-harvested primings that often contain so much sand, that shaking and removal of the foreign matter are required before you can get a reasonable price.
Care should be taken when harvesting drought stressed tobacco. The middle and butts of the tobacco leaf will appear to be ripe according to color. But actually, the leaf has not fully matured. Let the leaves stay in the field rather than trying to color them in the barn. But if this type of tobacco is harvested remember to maintain moisture levels in the curing barn high and keep dry bulb temperatures just above (3-5 degrees F) outside temperatures. Once the leaf has yellowed then raise temperature fast enough to fix color and start drying the leaf.
Since the introduction of the bulk curing barns, there have been problems with properly loading the bulk containers. It is difficult to get labor to load containers uniformly. When packing containers, make sure there are no air tunnels or packed lumps of tobacco. Spread tobacco evenly over the entire container as it is being filled. Lumps or wads of tobacco cause tight spots and the tobacco will not cure properly. Fill the corners and edges of bulk containers first and pack slightly more then the center. Unless there is uniform airflow to all leaves in the container, there will be leaves or pockets of leaves that do not cure.
The type and condition of the tobacco determines to a large extent how tight the tobacco can be packed in bulk containers. Primings and lugs, if harvested, should not be packed at all. Just enough tobacco to fill the containers will result in the best cures. Good quality, upper stalk, dry tobacco can be packed to a density of 15 lb/cu. ft. and get good results. Remember that the density of tobacco may increase from morning to afternoon. Tobacco is usually turgid (swollen with moisture) in the morning, but it may be completely wilted in the afternoon. Do not pack wilted tobacco as tightly as turgid tobacco. Containers should fit snugly together so air does not pass between them. A board or other materials should be used to block air movement between the doors and the outside container.
Curing develops and preserves the potential quality, flavor and aroma of tobacco. Once the tobacco is in the curing barn, a concerted effort to bring the tobacco to a brilliant color (lemon orange) should be made. Once the desired color is achieved, dry out the leaf to preserve that color. Color is important. It indicates the degree of chemical changes that have taken place, and is used as an index of leaf quality. It is estimated that 75 percent of the market value of the leaf is based on the color. Closely monitor tobacco throughout the curing process to check on temperature, humidity, and condition of tobacco. Look through observation ports at periodic intervals to check the wet-bulb, dry-bulb thermometer and the color changes taking place. Place the wet-bulb, dry-bulb thermometer under the tobacco near an observation port so it will not be necessary to open the doors.
Close air intake dampers before heater is turned on. Turn heater on and raise temperature to yellowing range gradually. Temperature should not be raised more than 5 degrees F at any one jump. Allow about 30 minutes between temperature rises to provide time for curing air to become humid.
Yellow at a dry-bulb temperature of 95 degrees to 105 degrees F and wet-bulb of 93 degrees to 97 degrees F. To maintain high humidity and prevent color setting before it is desired, dampers should be almost closed. Maximum drying short of color setting is desirable; however, for fuel economy and for the best cure. Dampers should, therefore be "cracked" open to the maximum extent short of color setting, especially when using boxes. Venting or moisture removal during yellowing will aid air movement through the boxes during leaf drying. Tobacco that is sappy or high in moisture requires considerably more moisture removal before color setting than does droughty or low moisture tobacco.
Raise dry bulb temperature slowly, 1 degree per hour to 118 degrees F then 2 degrees per hour to 135 degrees F. Leaf drying is the most critical period of the cure. Time is required for moisture removal to keep up with temperature increases. If tobacco gets too hot, water scalding or sweating will occur.
Ventilation of outside air into the barn should increase during this stage of curing to maintain proper wet-bulb temperature. When the curing temperature is raised above 118 degrees F, the dampers should be open enough to hold wet-bulb down to 100 degrees F. The more the dampers are opened, the lower will be the wet-bulb temperature. Keep dampers open enough to hold wet-bulb temperature of 100 degrees to 105 degrees F throughout leaf drying (100 degrees F for the first 24 hours and 105 degrees F for the final stages). For diseased or extra thin tobacco, a lower wet bulb (90 degrees to 100 degrees F) may produce a brighter cure.
Wet-bulb temperature is the same as the leaf temperature until the leaf has dried. The leaf cell breaks down and browning or scalding occurs at a leaf temperature of 113 degrees F. Thus, the wet-bulb temperature should never be allowed to exceed 110oF until the leaf lamina is completely dry.
Raise temperature gradually for stem drying. After leaf is essentially dry, temperature should be raised gradually (2 degrees to 3 degrees F per hour) to 165 degrees F for stem drying without sponging or scalding.
Close dampers gradually during stem drying. Maintain damper opening sufficient to hold wet-bulb temperature down to 110 degrees F during the first 12 to 18 hours of stem drying. Dampers are usually closed completely about the time the leaf is completely dry and the temperature has reached 165 degrees F.
Do not exceed 165 degrees F during stem drying. Sugars caramelize and leaves turn red at excessively high temperatures. The following temperature schedule (dry-bulb, wet-bulb) should prove effective with normal, good quality tobacco. The time can change according to the condition of the tobacco when it begins the cure. Factors which affect time required for curing in certain phases are maturity of tobacco, stalk position of the leaf, the use of ripening agents and weather condition at harvest.
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