TSNA Task Force Group

By Anna Wiernek

On the meeting at F.W. Rickard Seed In Winchester the 18^th of April I was asked to write some comments on the factors influencing TSNA accumulation during storage. The question was, if the observed increase of TSNA-content during storage of baled tobacco is due to microbial activity or other mechanisms.

It is common knowledge that TSNA are produced during curing and are formed  in the reaction of alkaloids with nitrite-derived nitrosating species such as NO₂, N₂O₃ and N₂O₄. The source of nitrite during the air-curing process is microbial. The time for microorganisms to produce nitrite is by the end of yellowing when the cells in tobacco leaves loose their integrity and the cellular content is leaking into the intercellular space. Nutrition is then made accessible to microorganisms and the water content (water activity) in the tobacco is high enough for bacteria to grow. Nitrite may accumulate as a result of nitrate reduction by bacteria and TSNA are formed by chemical reactions between nitrite (source of nitrosating species) and alkaloids. When the tobacco dries out during the curing process the water content (water activity) decreases dramatically. At the end of curing the water content (water activity) in tobacco is too low, < 40% (< 0.8) for bacteria to grow and contribute to further production of nitrite.

In various air-curing experiments it has been shown that a substantial nitrite production during air-curing does not necessarily lead to high levels of TSNA in cured tobacco. The reaction between alkaloids and nitrite is not very effective at pH 6, which is the pH of the cured tobacco. However, besides nitrosation of alkaloids the fate of nitrite is not well known. Possible reactions of the excessive nitrite could involve nitrosation of other compounds in tobacco or/and loss as gaseous nitrogen oxides. So if the curing process generated nitrite, the freshly air-cured tobacco is still rich in unstable nitroso-compounds. These compounds will participate in continuous nitrosation of alkaloids during storage and lead to further TSNA accumulation. The reactions involved are chemical reactions  such as transnitrosation and gas phase nitrosation. These reactions may occur under a relatively dry conditions where microorganisms are unable to grow and contribute to nitrite formation.

In order to avoid TSNA accumulation during storage of cured tobacco the most important is to have a good curing process with as low nitrite formation as possible. The tobacco in the bales should not be packed too tightly in order to permit residual nitrosating species to be ventilated into the atmosphere instead of reacting with alkaloids. The moisture content in the bales should be as low as possible in order to prevent any chemical reactions and microbial growth.