Wright, Jonathan P.
(2007)
Tea flavour : delivery and perception.
PhD thesis, University of Nottingham.
Abstract
A novel method was developed, enabling volatile release from mugs of freshly prepared, hot black tea infusions to be monitored online using atmospheric pressure chemical ionization-mass spectrometry (APCI-MS). Given the number of volatile compounds contributing to the aroma of black tea infusions, and the one dimensional nature of the APCI-MS technique, it was necessary to determine which ions present on APCI-MS spectra corresponded to which volatile compounds present in tea headspace.
Assignment of APCI ions to specific compounds was achieved by gas chromatography of black tea infusion headspace with simultaneous electronionization and APCI-MS detectors (GC-EI/APCI-MS). Using this approach, 15 ions were selected for future monitoring, assigned to compounds with varying levels of confidence. Six ions were unequivocally assigned to individual compounds, so monitored and quantified with certainty. In other cases, it was only possible to assign ions to groups of compound, as was the case for isobaric compounds such as heptanal and heptanone (m/z 115), or stereoisomers such as E-2-heptenal and Z-4-heptenal (m/z 113). In some cases, although ions could be assigned to compounds, some unknown impurities were also present. The compounds represented by the 15 ions covered a range of physicochemical properties, sensorial significance, formation mechanisms, and odour properties. The analytical system was shown to yield reproducible data, with confidence variation values generally less than 5 %.
The effect of infusion preparation method on volatile release from black tea infusions was determined using this novel method, where three variables; infusion water temperature, infusion concentration, and infusion duration were investigated. Infusion water temperature and concentration were shown to exhibit the greatest effects, the higher the temperature and concentration, the greater the release into the headspace.
The effect of infusion water temperature was shown to be compound dependent, with differences in release partly explained by differences in physicochemical properties affecting extraction out of the leaf matrix into the aqueous phase. It was suggested that there was very efficient extraction of some compounds such as the more polar, water soluble Strecker aldehydes (2-methyl propanal, 2- and 3-methyl butanal), and less efficient of the more hydrophobic compounds such as β-damascenone and β-ionone. Location of compounds within the leaf matrix, and additional formation of some compounds (e.g. dimethyl sulfide and the Strecker aldehydes) during the infusion process were also thought to play a key role.
The significance of these results for tea consumers was explored based upon the orthonasal aroma discriminability of infusions prepared according to different methods. Utilising a signal detection theory approach, values of d’ were obtained for pairs of infusions, prepared using a range of infusion water temperatures and concentrations. Differences in orthonasal aroma caused by differences in preparation method could to a large extent be detected by consumers. Values of d’ ranged from 0.08 to 3.26, indicating a range in the magnitude of stimulus differences between different pairs. The ratio of the difference in infusion concentration played a key role in discriminability of samples (e.g. 0.25 vs. 0.5 %w/v d’ = 1.34 cf. 1.75 vs. 2.0 %w/v d’ = 0.08), although insufficient data were available to be entirely conclusive. In addition, pairs prepared with higher temperature water (90 vs. 100 °C) appeared more discriminable (d’ = 1.58) than those prepared with lower temperature water (40 vs. 50 °C, d’ = 0.95). These results supported the theory that a d’ of 1.0 reflects a ‘just noticeable difference’ in perception, this value having to be exceeded before differences can be detected.
Given the effect infusion preparation method has on volatile and non-volatile composition of infusions, sensory analysis utilising a quantitative descriptive analysis (QDA) approach was carried out investigating the presence of perceptual interactions between the specific attributes; aroma, bitterness and astringency. Use of a trained panel showed no evidence to support the presence of perceptual interactions between these attributes. This was thought to be due to a combination of factors, including the nature of samples, subjects, and test procedure. Use of consumers revealed a possibility of bitterness suppression caused by black tea aroma, although results were not conclusive.
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