Volume 21 (2005): Issue 6 (July 2005)

Several trials the results of which are compiled in this paper, were carried out at the Tobacco Institute of Bergerac (ITB) and in the area nearby from 1996 to 2003. The objective was to study the formation of tobacco-specific nitrosamines (TSNA) in dark air-cured and burley tobaccos during curing and post-curing treatment under the conditions of cultivation, curing and storage commonly applied by the growers in the south-western part of France. For experimental purposes special treatments were performed in certain trials.

The results showed that the main genetic trait involved in the formation of TSNA is the propensity of a variety to convert nicotine to nornicotine (NN). In addition, the ability of a variety to lose water rapidly limits the formation of nitrite and hence also the formation of TSNA. Furthermore, agricultural practices that led to an increase of alkaloid concentrations in the tobacco leaves also led to an increase of TSNA concentration. Priming, a mode of harvest which speeds up the cure, as compared to stalk-cutting, as well as low temperatures during curing, limit the formation of TSNA, but do not yield tobaccos of the best quality. Ventilation in the barn plays a major role, and the leaves cured in well ventilated curing structures, such as plastic sheds, generally contained smaller amounts of TSNA than leaves cured in a conventional curing barn. The results also indicated that the TSNA concentrations may increase after the end of cure, if the cured tobaccos were kept hanging in the barn under humid conditions. The concentration of TSNA may also continue to increase, whereas nitrite concentrations tend to decrease, when the leaves are kept in bales.

It can be concluded that the French climatic conditions with moderate temperatures and low relative humidity at the time of curing, are favourable for the production of air-cured tobaccos with a good quality and low TSNA concentrations (1.5-3.5 µg/g), provided that the variety has low NN content, the nitrogen fertilization is moderate, the curing is performed in a well ventilated environment, the tobacco is taken down and stripped as soon as it is cured, and the bales are stored as briefly as possible before the leaves are threshed and stabilized.

The present review deals with studies performed during several consecutive years on the effect of air-curing on tobacco-specific nitrosamine (TSNA) formation and quality of tobacco. Temperature, relative humidity, water content and water activity data were collected during curing of dark tobacco in traditional air-curing barns and bulk-curing barns of different sizes, and chemical analysis of the cured tobacco were performed.

Particulate- and gas-phase polycyclic aromatic hydrocarbons (PAHs) were determined in the mainstream smoke (MSS) of 59 manufactured cigarette brands (commercially available brands of unknown tobacco and blend type) with variable ‘tar’ yields and physical/technological characteristics. Depending on the existence/absence of filter, the ‘tar’ yield indicated on the packet, and the cigarette length and diameter, the examined cigarette brands were classified into 15 groups: non filter (NF), high (H), medium (M), light (L), super light (SL), ultra light (UL), one-tar yields (O), 100 mm long cigarettes (H-100, L-100, SL-100, UL-100, O-100), and slim cigarettes (SL-SLIM, UL-SLIM, O-SLIM). Cigarettes were smoked in a reference smoking machine equipped with glass fibre filters for collection of PAHs bound to total particulate matter (TPM), and polyurethane foam plugs (PUF) for collection of gas-phase PAHs. The relationships of gas- and particulate-phase concentrations of PAHs (ng/cig) with the contents of typical MSS components, such as TPM, ‘tar’, nicotine and carbon monoxide were investigated. In addition, the phase partitioning of PAHs in MSS was evaluated in relation to the technological characteristics of cigarettes.

Investigations were made with the new burley variety B 2/93, bred in the Tobacco Institute Prilep, on alluvial soil type in the producing region of Ohrid and Struga. Four rates of nitrogen fertilizer (55, 100, 150, and 200 kg ha^-1) and a constant rate of phosphorus and potassium were applied in the investigation.

Based on the results obtained, it can be stated that fertilization and irrigation have a significant influence on the anatomic structure of burley tobacco leaf. The best developed anatomic structure, with harmonically increased palisade and spongy parenchyma, was found in the variant fertilized with 150 kg ha^-1 N. The cells of the parenchyma are not dense, and the spongy cells are few and irregularly distributed, forming large intercellular spaces between them. As a result of the interactive effect of irrigation and fertilization, this treatment is distinguished by a thick and soft leaf lamina with excellent adsorptive power, high filling capacity and, consequently, better quality.

The combustion mechanism of a shredded tobacco bed during puffing has been investigated. To evaluate changes in the burning rate of the shredded tobacco bed in the region close to the paper char-line, an experimental study was carried out on the reverse combustion of the shredded tobacco bed packed in a furnace. Measurements of the temperature in the shredded tobacco bed were conducted to calculate the combustion propagation rate. The combustion propagation rate decreased with an increase in the tobacco shred width, the tobacco packing density, and with a decrease in the air flow velocity. To investigate differences in the combustion propagation rate, the oxygen transfer coefficient of the shredded tobacco bed was evaluated using the effective diameter of the tobacco shred and the effective surface area of the shredded tobacco bed. The combustion propagation rate of the shredded tobacco bed increased with the oxygen transfer coefficient of the shredded tobacco bed. Furthermore, the weight loss of the cigarette during puffing was evaluated. The weight loss of the cigarette during puffing showed an increase with an increase in the oxygen transfer coefficient of the shredded tobacco bed.

To minimize the effect of permeability variations of tipping and plug wrap paper on the degree of filter ventilation and consequently on the smoke yields, a mathematical model for the flow through a layered structure of perforated tipping paper and porous plug wrap paper was derived. The output of the model was compared to experimental data and a very good agreement was found.

A statistical investigation revealed that measured permeability values of tipping and plug wrap papers are normally distributed. Furthermore, it was shown that the statistical distribution of the total permeability of the layered structure cannot be distinguished from a normal distribution at reasonable levels of statistical significance.

Based on these investigations and on the mathematical model, expressions for the expected value and the coefficient of variation of the total permeability were derived. It was shown that in spite of the non-linear model the difference between the exact expected total permeability and a simple estimate calculated from the expected permeabilities of tipping and plug wrap is sufficiently small, such that for all practical purposes this estimate can be used.

The coefficient of variation of the total permeability was then minimized. The solution of this optimisation problem delivered practically applicable design rules for the permeabilities of tipping and plug wrap papers. It was shown that the minimal coefficient of variation of the total permeability is lower than the coefficient of variation of the permeability of tipping and plug wrap paper. Typically an optimal design can be achieved by choosing the tipping paper permeability between 10% and 50% higher than the total permeability. A simple example demonstrated the usefulness of this approach.