Curing

Curing tobacco is an art. It consists in promoting important physical and chemical changes to the leaves harvested in the fields, followed by a gradual and slow water extraction process. The entire process takes place inside the curing facilities, called curing barns. The curing plan starts at transplanting. The number of plants or the size of the cultivated areas is determined by the capacity of the curing facilities of every farmer. Growers know that, for a specific curing barn, there is a specific number of plants. In other words, 30 thousand plants for a 450-stick curing barn. As a stick holds 130 leaves, on average, a full barn holds around 59 thousand leaves. It means that two leaves should be picked per plant at every harvest.

Of course, there are different harvesting models, and they have altered the traditional concept. Nowadays, field-based (area) harvest is frequently mentioned, and there are other concepts around, but nothing will change the capacity of the curing facilities in terms of number of leaves. For early maturity cultivars, the number of plants per curing unit should be adjusted to this situation.

The concept is applied to conventional curing barns. Some adaptations, like forced air, can increase the water extraction capacity and, therefore, increase the curing barn load.

Another facet to be observed is the characteristic of every curing barn. To provide good curing, the barn should be equipped with an efficient heating and ventilation system, both for air admission and exhaustion.

As a rule, in conventional barns, the admission fans, also called vents, occupy an area one third smaller than the exhaust fans. This is because the air that is taken into the barn is denser and its temperature is lower than the internal temperature of the barn. When it gets warm, this air expands, increasing in volume and tries to find its way out through the upper portion of the barn. Warm air always tends to rise. While rising, this air comes across a layer of moisture, consisting of the water released by the leaves. The mass of air’s temperature is reduced in contact with the moisture, which it takes out of the barn. Once the curing barn exhaustion process has started, the air rises inside the barn, carries the moisture upwards and out of the barn. The air admitted by the lower fans gets warmer and permeates through the leaves, regardless of temperature losses through contact with the moisture on its way up. This is the principle that guides tobacco curing in conventional barns.

Besides the fans, the barns should have no air outlets above the layer of tobacco, nor should they have lateral openings or cracks in the walls or right under the roof. To check if the barn is tightly sealed, the only thing to do this is to go into it, before loading it with tobacco. It should be completely dark, with no light coming in. Where light enters, air enters too. It is recommended to seal cracks and holes on walls tightly, with mud or cement. To make the curing barn more efficient for curing purposes and more economical in terms of wood consumption, it is suggested to plaster the outer side of the walls to prevent rains and winds from cooling its structure. The installation of the flues inside the barn also requires care.Soot free flues and clean chimneys contribute greatly towards effective barn functioning.

To maximize the curing results, everything starts with a good tobacco field, followed by a good harvest, that is to say, picking ripe and uniform leaves and, finally, a curing barn in proper conditions. Even if tobacco is harvested in poorer than desired conditions, there is always a chance to minimize the losses with some additional cares at curing, respecting the number of hours required at every stage, as a reduction in the curing time will inevitably lead to losses in quality.

The barns should be equipped with wet and dry bulb thermometers. The difference in temperature between the bulbs shows the relative humidity level inside the barn. When it is too low, tobacco will dehydrate and dry before the ideal time. When it is very hot, the temperature should be lowered until moisture levels stabilize according to curing table figures. All the companies hand out curing tables indicating ideal temperatures for a good curing process. If the temperature/moisture relation is not in accordance with the curing table, inevitable quality losses are more than likely to happen.

Tobacco curing must mandatorily go through all stages, from yellowing to stem drying, complying with the number of hours for each stage, thus maximizing leaf quality, as described below:
 
CURING STAGES

1 – Yellowing stage:

This stage is also called termination of the maturation in a controlled environment, as it is the time when the main desirable physical and chemical changes take place. During this stage, the leaf cells continue to be alive.

Tobacco curing normally starts at normal ambient temperature. If it is below 90°F, the temperature of the barn has to be raised to this. This temperature should be kept for approximately 12 hours and then gradually raised, 2°F per hour on average, to a maximum of 100°F.

The temperature has to be kept at 100°F, without overlooking the wet bulb and dry bulb relation, according to the curing table, until one third of the leaves of the two first tiers have turned yellow. 
* As oxygen enters the leaf through small openings, called stomata, a continuous exchange of water and oxygen takes place during the yellowing stage, and it speeds up the release of water and carbon dioxide from the leaf. It promotes the conversion of starch and other enzymatic activities.

This stage normally takes 48 to 60 hours. 

2 – Wilting stage:

At this moment, we gradually increase the temperature, about 1°F per hour, until reaching 105°F, by opening the lower vents slightly, to let air in to speed up the “yellowing process”, as well as causing the leaves to wilt. The temperature of 105°F is maintained until one third of the leaves of the two first racks have turned yellow. After this, the temperature can be raised to 110°F, until the tips of the leaves begin to curl, not forgetting to properly handle the ventilation process so as to keep the balance between the temperature and humidity, according to the curing table. During this stage, about 20% to 30% of the water is removed from the leaf. *From 108°F on, if the moisture content in the leaf continues to be high, a polyphenol oxidation process is triggered, with dark spots called “guinea” or spider web.

*A basic rule consists in opening the upper vents, some hours after opening the lower ones. Allow the mass of air to heat up inside the barn over the first hours. Another precaution consists of preventing excessively cold air from entering the barn, which always occurs when the external temperature is much below the internal temperature.

This stage takes 18 to 24 hours, on average.

3 – Color setting and leaf drying:

The temperature is gradually raised, at a rate of 2°F per hour, without overlooking the wet bulb and dry bulb relation, according to curing table, adjusting the lower and upper vents. This temperature should be maintained until the tobacco on the upper tiers is completely yellow and wilted.

*Normally, there is a difference of 10-15°F between the thermometer and the upper portion of the barn, depending on the height of the barn. Therefore, we talk about keeping the temperature at 120°F on the thermometer on the first rack, so as to keep the temperature in the upper racks at, or below 105°F, where the leaves contain much moisture and are greenish in color.

If the leaf on the upper racks/tiers is completely yellow and wilted, the temperature can be gradually raised at a rate of 2°F per hour, to a maximum of 135°F. This temperature should be maintained until the leaf on the two lower racks/tiers is completely dry and the leaf on the upper racks, completely wilted. Then the temperature can be gradually increased, at a rate of 2°F per hour, to a maximum of 150°F on the thermometer, which should be maintained until the leaf of the upper racks is dry.

*If the leaves are completely yellow, but with excessive moisture, and if the temperature rises above 135°F, “scalding” or complete browning occurs, also known as frogeye, sub-grade K.

4- Stem drying:

Now only the stems still need to be dried, and the temperature is continually increased until it reaches 165°F. During stem curing, it is recommended to shut the lower and upper vents. As there is little moisture to be extracted, the natural outlets between the tiles are sufficient for gradual moisture exhaustion. Slow moisture extraction is economically desirable and improves the appearance and color of the leaves. This stage normally takes 18 to 24 hours.

Summary of average time needed for curing one barn load of tobacco leaves:

• 48 – 60 hours for “yellowing;
• 18 – 24 hours for wilting;
• 48 – 60 hours for leaf drying;
• 24 hours to terminate stem drying.

* In general, all the stages of the curing process cannot be carried out in less than seven days. The first three stages almost take place simultaneously inside the barn. Whilst wilting the leaves in the lower racks, the termination of the yellowing stage in the upper racks. While drying the leaf in the lower racks, the wilting process is going on in the upper racks. Cured tobacco always reveals how the curing process was conducted and, obviously, how the leaf was harvested.

General considerations on curing:

* When the leaves are not harvested under the best conditions, there is always a chance to minimize the damage with some additional care. To respect the number of hours for each stage is of fundamental importance, once any reduction in curing time will inevitably result in quality losses.

* If a thermometer is put in the upper racks of the barn, a significant difference in temperature between the first rack, where the thermometer is normally put, and the upper portion of the barn can be seen. This is as a result of the cooled ascending air, which, on its upward trajectory, comes across and carries the humidity released by the tobacco leaves, towards the upper openings through which it is expelled.

* When leaf sweating starts, it signals excessive humidity inside the barn. Before increasing the temperature the excessive humidity must be released by opening all the vents (even close to the door), while keeping the temperature constant.

* When the temperature is increased too early, tobacco leaves will have two characteristics: good resilience, dark color on the external part of the leaf, lack of resilience in the central part. These leaves have been subjected to high temperatures, while they still have high levels of water.

* In the curing tables, there are indications for temperature advances by virtue of the appearance of the leaf and the thermometers’ wet and dry bulbs. The latter also indicate when and to what extent the vents of the barn should be opened. Everything comes down to a question of adjusting the mass of tobacco to its needs and speed of drying in order to avoid losing quality. Too fast or too slow curing, generate less attractive products. There are quality losses.

* Once, oxygen enters the leaves it speeds up the yellowing process. In the yellowing stage, besides the chemical and physical transformations, what also becomes noticeable is the reduction in the intensity of the green color (chlorophyll), which gradually turns into yellow (Xanthophyll). Both pigments are present in the leaves at harvesting. However, the green color hides the yellow. Chlorophyll is more pronounced in the field because of the photosynthesis process.

Additional procedures for leaf harvested under adverse conditions:

Leaf harvested unripe:

It is not recommended to harvest unripe leaf, but if it is necessary in order to prevent further damage, some special cares should be taken. In this case, yellowing should be achieved with a little higher temperature (103°C to 105°C), with a minor inflow or air. Care should be taken not to allow the green color to set. Whenever the tips of the leaves curl while green, air inflow should be cut to increase moisture levels, whilst not allowing the temperature to fall. Normally, for leaf of this type, yellowing takes longer, but the subsequent stages follow their normal course. 

Leaf harvested wet:

Frequently, there is no choice: either harvest the leaf humid or lose it. Under these circumstances, it is of fundamental importance not to overload the sticks or the barn. At the start of the yellowing stage, maximum air inflow is recommended to wipe the leaf. The temperature should be kept from 90°F to a maximum 100°F.Once the leaf has wiped up, with the thermometer pointing to normal moisture conditions, air inflow should be reduced to the appropriate level, so as to maintain the moisture level in line with the curing table.

In the case of excessive precipitation rates, which cause the leaf to turn turgid, it is very important to maintain the inflow of air until the leaf begins to wilt.

In the case of heavy rains, the leaves tend to be harvested with fungal spots or even to be infected by bacteria. Therefore, every care is required in order to prevent these diseases from proliferating within the curing barn. This is achieved by observing the leaf wilting process, in all racks, before raising the temperature. When the temperature reaches 108°F, fungi proliferation starts within the barn. Obviously, with low humidity, they are likely to cause more damage. Therefore, every care is needed not to exceed critical temperatures over the entire curing process.

Leaf harvested under drought conditions:

This is a quite simple process. The only thing to do is to close the curing barn tightly and add moisture, keeping the temperature stable, waiting for the leaf to release the necessary moisture for the curing process. Even leaf harvested under drought conditions, will have over 85% percent moisture.

From the moment moisture stabilizes, the curing process should be started, always observing the wet and dry bulb thermometer.

These are the leaves that require the strictest compliance with the curing table, and an attentive eye towards the wet bulb thermometer which indicates the moisture level within the barn. In case where moisture levels are very low, which is normal with dehydrated leaf, frequently the entire barn load has to be cured with minimum ventilation.

Leaf harvested overripe:

Leaves that overripe in the field, normally have scorching problems, diseases and dead tissue before harvesting occurs. To cure these leaves, care should be taken to promote the yellowing stage along with the wilting stage, however, without advancing the temperature while the leaves have green streaks. The process starts under normal temperature, for 6 hours, with the ventilator off until the barn is heated, advancing gradually, at a rate of 2°F per hour until achieving from 103°F to 105°F, with minor air inflow.

Even yellow leaves put in the barn can develop green color midribs in cases where leaf drying is accelerated. This occurs because leaves harvested overripe recover their greenish color along the secondary nervures/veins as soon as they are put into the curing barn.

Once the leaves have become yellow and wilted, the normal curing process follows, always observing the internal moisture, taking care not to speed up the curing process by advancing the critical temperature limits for every stage, which is 108°F, when the leaves are yellow and wilted in the first racks/tiers, and 135°F, when the leaves are completely dry in the lower racks.