Tobacco Leaf Composition: Alkaloids and Polyphenols Change with Growing Region and Maturity

Source from:  separationsnews.com  06/05/2014

The tobacco plant remains an important contributor to the economies of more than one hundred countries, despite concerted moves to limit smoking around the world. And the geography influences the flavour of the tobacco leaves that end up in cigarettes, cigars and snuff, along with the variety of the plant, the growing and cultivation conditions, the maturity of the plant at harvest time, and the postharvest treatment processes.

The chemical components of tobacco have been studied extensively in order to discover which compounds contribute to the flavour and which might be harmful to smokers but there are two factors which have been overlooked to date, according to a team of Chinese researchers. Gang Xu, Yan Zhou and colleagues from the Chengdu Institute of Biology, the Kunming Institute of Botany, Heilongtan, and the Jiangsu Province Academy of Traditional Chinese Medicine, Nanjing, wanted to examine the effects of the growing region and maturation rates.

To accomplish this, they harvested leaves of the same cultivar, Nicotiana tabacum k326, that had been grown in three regions of China, Zhengzhou, Zunyi and Jiangchuan. They were picked 40, 60, 85 and 100 days after transplantation, corresponding to the rosette, squaring, physiological maturation and harvest maturation stages.

Alkaloids and polyphenols

The research team decided to target eight compounds in particular to see how their quantities varied in the different leaves. They comprised five alkaloids (nicotine, nornicotine, neonicotine, anatabine, myosmine), two polyphenols (rutin, quercetin) and the coumarin scopoletin. They also looked at the occurrences of other metabolites, using an HPLC/MS method.

A leaf extract was injected onto a column containing monomeric octadecylsilica as the stationary phase which is suitable for compounds of low molecular mass. As they eluted, the compounds were monitored by a photodiode array detector operating at 254 or 340 nm before being directed to a quadrupole-time-of-flight mass spectrometer. This is a high-resolution instrument that allows the exact molecular masses to be determined, markedly improving the confidence of identification.

The detection limits of the eight components were 0.2-1.5 ng and the spike recoveries were all around 100%, demonstrating the good performance of the method.

The fat-soluble compounds in the leaves, which are also regarded as important, were analysed by GC/MS. More than 100 of these were detected in total and their abundances fell from the rosette stage to the harvest maturation stage. Certain components were more abundant in tobaccos from one particular region.

Flavour differentiation

Of the eight compounds measured by HPLC-UV/MS, nicotine and rutin were the most abundant alkaloid and polyphenol, respectively, in all of the tobacco samples, so they were selected as representatives of both groups.

The nicotine content varied widely, from 0.66 to 25.40 mg/g tobacco over the various growing regimes. Going from the rosette through to the final stage it began at a medium level before dropping, then maximising, and falling sharply at harvest maturation.

The contents of rutin varied in different ways, depending on the growing region. For instance, in the Zunyi samples, the levels were more or less constant in the first three growth stages, before rising sharply at harvest maturation. Conversely, the levels for the Zhengzhou tobacco were highest for the physiological maturation stage at 726 µg/g and roughly the same but far lower for the other three stages at 159-186 µg/g.

This new method devised for alkaloids and polyphenols clearly shows how their contents vary during the growing season and can help growers and tobacco producers to tailor the plants to particular requirements. Alkaloid levels could be used to assess maturity and polyphenols levels will give an indication of the expected flavour. Enditem