Curing

All Burley cultivars released by ProfiGen, including the hybrids licensed by the University of North Carolina – USA and the University of Kentucky – USA, are produced from basic seed selected for low conversion of nicotine to nornicotine. The basic seed is from plants with a maximum conversion rate of three percent (3%), in compliance with the protocol developed by the University of Kentucky – USA.

Curing

Burley Tobacco Curing

To cure tobacco is more than just drying the leaves. Curing has to do with a large number of chemical and physical transformations the leaves go through after harvest. The ambient in which the curing process takes place determines the nature of these transformations and has a substantial impact on quality and prices fetched by cured leaf. The curing ambient is almost entirely focused on the temperature, relative humidity and the exchange of air and ventilation. The ideal environment for curing Burley tobacco requires temperatures from 18°F to 32°F, and relative air humidity ranging from 65 to 70%.

For most crops, the key to a successful curing is to keep to the desired relative humidity in the curing structure, with sufficient ventilation to prevent a condition of air stagnation. In many crops, the natural conditions provide an acceptable condition, without much need for management and handling practices. But in atypical conditions, like rainy or dry spells, the ability to control the moisture inside the shed or curing structure is critical for the production of high quality Burley. Ventilation control is the key to managing the curing environment. Stagnated and humid air is responsible for leaf scalding or barn rot. On the other hand, excessively low relative air humidity rates could lead to fast leaf drying, causing undesirable color and smoke characteristics. In addition, recent research into leaf chemistry suggests that high humidity curing conditions increases the nitrosamine contents (TSNAS) in cured Burley leaf.

Curing stages:

Curing gives continuity to the ripening process, and mainly involves the transformation of nutrients and the reduction of the moisture levels. The curing process comprises three stages:

The first is the yellowing stage. During this stage, the leaves gradually degrade from green to yellow, while the midribs continue green. The yellowing stage normally takes from one to three weeks, depending on the degree of ripeness, climatic conditions, spacing between tiers inside the barn, pre-wilting in the field or not, and on the type of curing structure. If the yellowing process occurs too fast (as a result of long spells of low relative humidity levels, particularly if followed by excessive ventilation), an undesired color setting will occur. It is usually a light, variegated or mottled color (normally referred to as K tobacco) if the temperatures are high, and green color if the temperatures are low. If the yellowing stage happens very slowly (due to high humidity, particularly under poor ventilation conditions, stagnated air), leaf scalding and barn rot might occur. Scalding reduces the weight and quality of the cured leaves and could cause high levels of TSNAs.

The second stage consists in drying the leaves. During this period, the leaf gradually turns from yellow to a darker color (brown, chestnut brown or reddish brown). Stem drying is the third and final stage. During this stage, the stems wilt and lose most of their moisture. Once the midribs have dried, the curing process has finished, and the stripping can begin. The curing process starts gradually from the lower or bottom leaves to the tips and this is why there is an overlapping of the three phases of curing, from the lower leaves to the tips.

Under normal conditions, the curing process lasts from 4 to 8 weeks.

Management of the curing environment:

The ability in managing the moisture levels and air circulation inside the barn or curing structure is critical for the production of Burley tobacco with good physical and chemical quality in years with unfavorable climatic conditions.

Ventilation control:

Regardless of the type of barn or curing structure, ventilation control is the basis and essential factor for managing the curing environment. The control over the movement of the air affects both temperature and humidity, which control the curing process. In conventional barns, as a rule, ventilators and doors should remain open by day and shut in late afternoon or early evening.

However, if the leaf is curing too fast because of dry climatic conditions (relative air humidity below 65%), the barn should remain closed by day and open by night. On the other hand, if the leaves are curing very slowly due to high humidity levels (excessive humidity, prolonged rain spells), the barn should remain open to promote ventilation.

Stagnated humid air is a bigger problem than circulating, fresh moist air. In some extreme cases, artificial ventilators and supplementary heat are needed if leaf scalding or barn rot is to be prevented. Barn loading density, positioning in accordance with prevailing winds, ventilators and other openings in the structure, as well as its size, have an influence on air movements. The air moves through the leaves, humidity evaporates and this cools down the tobacco leaves. At the same time, ventilation boosts the curing process.

To maximize the ventilation potential, at least 30% of the barn’s lateral surface should remain open, or have openings for ventilation. In field curing structures, ventilation will depend on the way the lateral curtains are handled.

Low temperatures and air circulation:

Under normal planting conditions, low temperatures pose no problem to Burley curing. Nevertheless, the following are tips and considerations for curing under low temperatures, and using artificial means of managing the environmental conditions. Low temperatures reduce relative air humidity without adversely affecting the color of the leaves. Excessive heat might lower the relative humidity considerably, resulting in fast curing and undesirable color.

Supplementary heat in Burley barns should be generated by heaters that produce hot air (the ones that are powered by propane, GLP, natural gas, etc.), and never through direct fire (open flames) that produce smoke and leaves impregnated with fumes. The smoke produced by the heat source should be released outside the curing barn. Even with supplementary heating ventilation is required to let the moisture out of the barn.

Otherwise, condensation is created contradicting the goal of supplementary heating. The heaters have to be adjusted so that the temperature in the lower tier does not exceed 30°C. Air circulators are just another way to control the moisture levels within the structure during prolonged rain spells or excessive humidity. To be effective, the air has to move through the tobacco leaves, instead of passing around them. The correct location of the vents and the manner tobacco is positioned or hung in the barn, are critical for the effectiveness of this ventilation.

Loading density and lateral curtains:

In conventional Burley barns, spacing between sticks normally varies from 15cm to 30 cm, depending on the spacing between the racks and the degree of ventilation inside the barn. Research has shown that Burley tobacco can be hung in higher densities in curing structures with a low profile, with open laterals, without any further risk of scalding or barn rot. Higher densities mean lower barn costs per unit of cured tobacco. In field-curing structures, the curing environment is mainly controlled by loading density (spacing between sticks or plants) and by lateral curtain handling. In such types of structures, there should be smaller spacing between the sticks, compared to conventional curing facilities. Spacing ranging from 9 to 12 cm normally functions properly, depending on the size of the tobacco, wilting intensity of the stalks and climatic conditions. The structure should be covered with plastic tarpaulin/canvas right after loading. However, if the leaves are humid or wet, they must be left to dry before the structure is covered.

In general, the extremities of any field-curing structure should have openings on the upper portion (between the stalks and the plastic cover), in order to allow the air to circulate and allow the humid air out of the facility. While the humidity of the leaves evaporates, the air inside of the barn gets saturated with moisture. If this moisture is not removed from the barn through air circulation, this evaporation process will stop. The lateral covers should normally remain up or open during the yellowing and leaf drying stage, and then lowered or closed in order to complete the curing process. An exception to this rule occurs in prolonged high temperature or low relative humidity spells, lasting for days or weeks. Under these conditions, the lateral covers should remain lowered during the yellowing and leaf drying stage, in order to slow down the curing process and minimize the undesired variegated and mottled colors. Once the lateral covers have been lowered, close monitoring of both stalks and stems is needed in order to detect possible scalding/houseburn conditions, which require open curtains. This management care is particularly important for large field-curing structures that are three or more tiers wide.

Types of barns or curing facilities:

The use of low-profile structures with good curing management, seems to result in cured burley that is darker and redder than burley cured in conventional barns. This has been observed in research work and also in experiments carried out by growers. The industries have widely accepted the Burley tobaccos cured in such structures.

Low-profile barns, with lateral opening, are good for curing, but not for storing unstripped cured tobacco. The tobacco should be removed (stripped or unloaded) from the field-curing structure in the shortest time possible, right after the curing process has terminated. Timely removal will minimize damages from leaf crumbling, excessively humid leaf tips, wind-induced risks and losses. Direct sunshine impairs post-curing colors, and therefore, tobacco should not be exposed to direct sunshine either inside the barn or afterwards.

Loading and curing practices should be specific for every curing facility and geared towards quality and yield preservation, remembering every structure and every crop is different. All structures are, in a way, unique in their curing characteristics and require specific management practices. Field-curing structures normally require more management, but also allow for better curing ambient management compared to most traditional barns. 

A frequently asked question is: “What is the best barn or structure?” The answer is: no structure or barn is necessarily the best. The fact that field-curing structures covered with plastic tarpaulin/canvas cost less, as a group in general, does not mean that a grower’s entire crop should be cured in such type of structure; they are precarious structures for storing tobacco over long periods because of climate-induced risks. If the growers are unable to unload the tobacco in accordance with its curing process, than a structure with a good roof, and maybe with lateral protection, either partial or total, would be more appropriate than any field-curing structure covered with a plastic canvas for storing the cured tobacco not yet stripped from the stalk. For example, a conventional barn with a metal covered, gable-roof, and low profile barn, would be an ideal choice for protection against climatic factors. A tall conventional barn, with one or two racks/tiers, with a higher-than-normal density (to take some labor and cost advantages of the low-profile structure system) offers excellent protection against climatic conditions for cured tobacco ready to be stripped from the  stalks and graded later.

Lots of farmers might draw the conclusion that it is best to use both systems, low cost structures that offer minimum protection against climatic conditions and more firmly built structures, but more expensive to construct and operate. They might conclude that this system offers labor and time flexibility, whilst helping with managing the climatic risks inherent to Burley. In some cases, to reach an intermediate solution in terms of cost efficiency to gain flexibility, improve time efficiency and reduce risks might be justified as excellent management practice, so as to preserve or even increase the net profits within and uncertain production environment.