Small molecules : where do they go to on tea leaves?

• Main Author: Cummins, Declan
• Published: University of Nottingham 2012
• Subjects: 541; QD450 Physical and theoretical chemistry
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.570385

The aims of this project were to investigate the surface chemistry and morphology of processed tea leaves the techniques of atomic force microscopy (AFM), scanning electron microscopy (SEM) and time of flight secondary ion mass spectrometry (ToF-SIMS). Data from the spectra obtained by ToF-SIMS was also analysed using principal component analysis (PCA). Further experimentation was performed on tea leaves by the addition of diluted samples of aromas, methyl salicylate, trans-2-hexenal and linalool and examining these leaves using depth profiling to discover how far into the leaf the aroma had penetrated and if there was any connection between the chemistry and size of the aromas and how far they penetrated. A general characterisation of the tea leaves was performed in Chapter 3 where layers of waxes of a comparable size were observed on both green and black tea leaves with AFM, as were micro-crystals on black tea and areas showing two distinct types of interaction between the cantilever tip and the surface of a green tea leaf indicating different surface properties. SEM images revealed a visual difference between green and black tea leaves, where the black tea leaves had more debris on the surface and greater changes in topography due to the different processing methods. The presence of lipids and epicuticular waxes were observed on the surface of the tea leaves using the ToF-SIMS. In Chapter 4 the effects of infusion in hot water on the morphology and surface chemistry of the tea leaves are examined. SEM revealed structural damage to the leaves from 30 seconds of infusion and this increased with infusion time, resulting in the formation of holes in the cutin on the adaxial surface of the tea leaf. By examining positive and negative ToF-SIMS spectra and using PCA, a change in surface chemistry could be detected from 15 seconds of infusion. The intensity of C3H5O2+ peaks in the spectra increased as infusion time increased, indicating that the waxy cuticle of the leaf surface had been removed revealing the underlying epidermal cell layer. Peaks associated with octadecenoic and octadecanoic acids were shown to have a reproducible effect on the positioning of the different infusion times within the PCA plots. Though chemical changes can be detected at 15 seconds, the first 30 seconds of infusion were found to be responsible for the majority of the chemical changes on the surface. Taken together these data indicate that the melting of the cutin layer, primarily within the first 30 seconds of infusion may be related to the release of flavour, aroma and constituents such as polyphenols. The penetration into the leaf of aroma molecules was examined in Chapter 5. Diluted solutions of methyl salicylate, linalool and trans-2-hexenal were added to tea leaves and then examined using depth profiling with ToF-SIMS. For the first time a leaf was depth profiled using a C60. The presence of trans-2-hexenal was detected in the palisade mesophyll layer of the leaf as was methyl salicylate, but to a smaller intensity. Methyl salicylate showed partitioning in diffusion across the cuticle with a large intensity in the cuticle and also in the interface between the epidermis and palisade mesophyll layers. Linalool was present on the surface of the leaf and showed partitioning in the cuticle of the tea leaves. As trans-2-hexenal was the smallest and least lipophilic of the three aromas examined it is theorized that the smaller the size of the aroma molecule plays a key role in the penetration of the dehydrated leaves.