Join Date: Nov 2017
Found a good publication from the University of Georgia titled, "Harvesting and Curing Flue-Cured Tobacco." Some very good information in here that I think is directly applicable to us. There's also a very helpful chart outlining what the temp and humidity needs to be, with wet and dry bulb thermometer readings, at all steps of the process. Seems you start around 95* and high humidity (95%+) to begin with, then slowly increase temperature in steps while allowing the humidity to reduce, finishing the coloring around 135* and then progressing to drying (colorset) which finishes after a period at 165*.
Actually, in the beginning while still working the bugs out, when I first attempted to leave the barrel going overnight at about 120*, the water drip clogged some time in the night up due to some debris in the tank, causing the temperature to rise and humidity to drop. By the next morning the temp had climbed to 160-something degrees and the barrel dried out. In the process the buds turned completely brown, a deep rich caramel type color. They had burnt smell also which I think was due to the cherry chips (laying on top of the lava rocks) getting too hot and igniting, which actually smoked the buds. It seemed the buds were ruined but they actually turned out to be some really great smoke! So it seems that accidental failure turns out to be more the way it's supposed to be done. When done properly, with better process control, the results should be similar without any burnt smell at all.
Here's the text:
Historically, tobacco curing has been considered an art. Since the use of bulk barns, growers have much more control over the processes. Management skills include understanding the principles of controlling airflow, temperature and humidity in a controlled environment.
Purpose of Curing -- Curing develops and preserves the potential quality, flavor and aroma of tobacco. Once the tobacco is in the curing barn, make a concerted effort to bring the tobacco to a brilliant color (lemon orange). Once you have the desired color, dry the leaf to preserve that color.
Color is important. It indicates that certain chemical changes have taken place, and it is used as an index of leaf quality. Industry representatives estimate that 75 percent of the market value of the leaf is based on the color. The objectives are to maintain life in the leaf until biological processes are completed (yellowing phase). During this phase starch is converted to sugar. Next, stop bio*chemical activities by removing leaf moisture (leaf drying). Finally preserve the leaf by drying the stem.
Closely monitor tobacco throughout the curing process for temperature, humidity and color. Look through observation ports regularly to check the wet-bulb, dry-bulb thermometers and color changes taking place. Open the loading doors carefully because this may release too much moisture and harm the curing process, especially during yellowing. For updraft barns, place the dry-bulb thermometer under the tobacco near an observation port so you won't need to open the door. Place it in between racks or on top of the tobacco for a more accurate indication of wet-bulb temperature. Reverse location of thermometers when using down-draft barns.
Temperature Advance Schedule -- There is considerable variance in advancing the temperature depending on the condition of the tobacco. A wet-bulb, dry-bulb temperature schedule (Figure 1) is effective with mature, good quality tobacco.
Humidity -- During the yellowing and leaf-drying phases, humidity control is essential. The relative humidity drops as the curing advances. Control the humidity by adjusting the fresh air exchange rate with the vent system. By controlling the humidity, the coloring time may be extended or shortened to get the most desirable color. If the tobacco is drying too fast (drying before yellowing), close the vents. On the other hand, opening the vents will speed drying.
Remember a couple of points about air and humidity: (1) Air at higher temperature has more drying potential at the same relative humidity. (2) At a constant relative humidity, 105°F air will hold twice as much water as air at a temperature of 85°F.
Using Wet-Bulb, Dry-Bulb Thermometers -- The wet-bulb thermometer measures the temperature of the leaf during the early stages of cure; the dry-bulb thermometer measures air temperature. Most growers use the dry-bulb thermometer, but you also need a wet-bulb thermometer. Since the humidity as well as the dry-bulb temperature must be controlled, a wet-bulb thermometer indicates when adjustments in the vents are necessary. Don't use thermo*stats as thermometers. They may not be calibrated to sense the
same temperatures as thermometers. You can buy wet-bulb thermometers at a local fuel supply dealer, or you can make one at a fraction of the cost. A homemade wet-bulb thermometer designed especially for bulk tobacco barns can be used.
The relationship between the wet-bulb and the dry-bulb temperatures determines the relative humidity within the barn. The closer the wet-bulb temperature is to the dry-bulb temperature, the higher the relative humidity. The relative humidity within the barn determines the leaf's rate of drying. The lower the humidity, the faster the leaf dries; the higher the humidity, the slower the leaf dries. Maintaining the proper wet-bulb temperature not only results in the best possible cures but also minimizes the amount of fuel needed to cure the tobacco.
Advancing Temperatures during Yellowing -- Advancing the dry-bulb temperature and wet-bulb temperature in relation to each other is a critical feature of curing. When starting a barn close air intake dampers before the heater is turned on. Turn the heater on and raise the temperature to the yellowing range (Figure 1) gradually. Don't raise the temperature more than SOF at anyone jump. Allow about 30 minutes between temperature rises so curing air can become humid.
Yellowing Considerations -- Curing each barn of tobacco as the season progresses requires adjustments in the curing schedule. For example, tobacco grown under varying climatic and field conditions calls for different yellowing schedules with dry-bulb temperatures varying from 9So to 10SoF and wet-bulb of 93° to 97°F.
After you harvest each barn of tobacco, decide the best way to yellow the tobacco. Consider the merits of the tobacco in the barn, such as leaf moisture content, maturity and thickness. Then make a decision at each step about how long to maintain a given temperature and humidity so the tobacco will complete its yellowing process. Failure to do this may result in dry leaf tips with a set green color. Tobacco with a high moisture content requires considerably more moisture removal before color setting than does droughty or low moisture tobacco.
Alter the yellowing schedule throughout the season as the tobacco varies from thin to thick and/or turgid to wilted tobacco with a minimum amount of moisture. For example, if you have immature, wet-weather or drought-grown tobacco, yellow it at the lower yellowing temperatures.
Length of Time Required for Yellowing -- Normal tobacco is yellowed at varying lengths of time, depending on the stalk position. For example, primings and lugs should be completely yellowed in 20 to 30 hours. On the other hand, upstalk tobacco may require 60 or more hours to obtain the desired color. You can improve the quality of certain varieties by extending the yellowing period. Certain varieties may sometimes yellow before starch converts to sugar. When this happens, the result may be pale, slick, immature tobacco.
Other Yellowing Suggestions -- Remove as much moisture aspossible during the yellowing phase of curing. With good tobacco, as much as 20 percent of the moisture can be removed during yellowing. When the yellowing phase is almost completed, the tobacco should show a good yellow color at the leaf tip with slight green-tinged colors running along the main stem and veins to the butt. The leaf tips and edges should begin to tuck and dry to a bright yellow.
When the tobacco throughout the barn reaches the desired color, increase temperature and rate of drying. At the end of yellowing, some wilting should have occurred. Avoid flash temperatures that can dry the leaf before yellowing is completed. This sets an undesirable green color.
Wilting -- Some wilting occurs before the end of yellowing at the 10SoF dry-bulb temperature, but most of the wilting should take place as the dry-bulb temperature advances from 110° to 118°F. The rate of temperature advance from 105*F to 110°F should be 1° to 1.5*F per hour and wet-bulb of 100°F. During the Wilting phase, the tobacco loosens considerably and the air can move through readily.
Do not advance the temperature beyond 118°F dry-bulb temperature until wilting is 100 percent complete.
Leaf Drying -- When the tobacco leaves have reached the desired yellow color and are thoroughly wilted, the leaf must be dried. The drying stage is critical because tobacco is sensitive to temperature change. Impatience to capture a good color often results in advancing the temperature too rapidly and producing a browning or barn scald. If the temperature is advanced too slowly, sponging may occur.
There must be positive control of airflow and temperature during leaf drying to prevent undesirable color in the cured leaf. To prevent sponging, dry the leaf as rapidly as possible, but not so rapidly as to cause scalding.
Wet-Bulb and Dry-Bulb Temperatures -- Maintain the wet-bulb temperature near 100°F during leaf drying. Once the tobacco is dry enough (30-40 percent of the moisture removed) to take dry-bulb readings above 135*F, the wet-bulb temperature is not critical to the quality of the cured leaf. Maintaining a wet-bulb temperature of 110°F or higher, however, tends to conserve fuel.
Stem Drying -- Advance the dry-bulb temperature from 135* to 165*F at a rate of 2° to 3°F per hour. Close dampers gradually during stem drying. Maintain a damper opening to hold wet-bulb temperature down to 110°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°F. Stems should be killed out at a temperature of 165°F. Sugar caramelization will cause tobacco to turn red when the dry-bulb temperature is more than 165*F.