Identification of aluminium forms in tea leaves by 27Al NMR

Abstract

To identify forms of accumulated aluminium in tea leaves, 27Al NMR spectroscopy was applied to the analysis of intact tea leaves and model Al complexes such as Al-catechin, Al-F, Al-phenolic acid and Al-organic acid. Although mature or old leaves contained more than two forms of Al, both buds and young leaves give virtually one signal. The chemical shifts for the dominant peaks of intact leaves were from 16 to 20 ppm, which is in good accordance with the peak for the Al-catechin complex. The chemical shifts for Al complexes formed with chlorogenic, gallic, citric, malic and oxalic acid were from 14 to 21 ppm. On the basis of the NMR results and leaf constitution, we concluded that most of the Al was bound to catechins, while some portion was bound to phenolic and organic acids. However, peaks for Al-F at ca − 1 ppm were only detectable in one spectrum of mature leaves.