Black Tea Research Papers - Academia.edu (original) (raw)

An initial investigation into the quality of teas available in Australian supermarkets was conducted. Samples of both green and black leaf tea and teabags were randomly collected from supermarkets, including Australian teas. Moisture, water extraction, total polyphenols (PPs), theaflavin, thearubigen and theabrownin were measured in the samples. The aim was to provide an overall profile of the quality of teas available in Australia.
Phenolic compounds constitute 50 % to 70 % of tea water extracts and are thus considered the main quality parameters for teas. Theaflavin (TF), thearubigin (TR) and theabrownin (TB) are the main polyphenols that determine the quality of black tea. These compounds were measured in 56 samples of leaf teas and teabags available in Australian supermarkets. The variability in TF, ranging from 0.29 % to 1.25 % for black teabags and 0.32 % to 1.10 % for black leaf tea indicates a quality difference among the teas studied. Low TF suggests the teas were over-fermented and/or stored for long periods.
The solubility of TR and TB from teabags ranged from 82 % to 92 % indicating that the permeability of teabags was variable. Total polyphenols in black tea leaf were between 14 % and 20 %, with an average of 17 %. This was slightly lower than the total polyphenols detected in black teabags, which ranged from 13 % to 27 % with an average of 18 %. Total polyphenols in the green teas studied ranged from 15 % to 34 % with an average of 23 %. This shows quality differences in both green and black teas. The solubility of total polyphenols from teabags could be a useful quality index of the filter paper used for the teabags. Results from this chemical analysis suggest that phenolic compounds could be used for the quality control of both Australian grown and imported teas.
The aim of the next part of the study was to investigate in detail flavonoids and other polyphenols in Australian grown and made tea. Phenolic compounds in fresh tea leaves collected from a tea farm in North Queensland were analysed. This field study was conducted on different commercial harvests from April 2000 to May 2001, using both hand plucking and mechanical harvesting to collect the fresh tea leaves in order to determine seasonal variations. The analysis of Australian made tea was conducted on the samples collected off the processing line of a black tea processing factory in North Queensland, at three month interval from April 2000 to January 2001, so the process could be evaluated in terms of its effects on tea flavonoids. Prior to this study, no published research has been conducted on the flavonoids and other polyphenols of Australian tea.
Fresh tea leaves, consisting of one apical bud and two adjunct leaves, were hand plucked just before each mechanical harvest during the sampling period. The samples were packed in dry ice and delivered to the laboratory by overnight transport and stored in a freezer at –80 ˚C before analysis.
The phenolic compounds were extracted with methanol using a method developed and optimised for this study, and were analysed using an HPLC with photodiode array detection. Four catechins, six catechin gallates, five flavonoid glycosides, and seven phenolic acids were identified and quantified.
The main flavonoids found in the fresh tea leaves were epigallocatechin gallate (EGCG), epicatechin gallate (ECG) and epigallocatechin (EGC).
The major findings of the analysis of the fresh tea leaves from the field are that there were lower levels of catechins gallates in the tea leaves harvested in the cooler months of July to September 2000 (EGCG, 92.94 mg/g; ECG, 33.41 mg/g; total catechin gallates, 132.61 mg/g), and higher levels in the tea leaves harvested during the warmer months of November 2000 to February 2001 (EGCG, 112.37 mg/g; ECG, 37.13 mg/g; total catechin gallates, 159.34 mg/g). For the catechin levels in the harvested tea samples, higher and constant levels were found in those harvested in the cooler months (EGC, 50.50 mg/g; total catechins 89.67 mg/g) and lower levels were found in those harvested in the warmer months (EGC, 44.86 mg/g; total catechins, 79.26 mg/g). On comparing the hand plucked samples with the mechanically harvested tea leaves, it was found that the levels of catechins and catechin
gallates were found to be lower in the mechanically harvested leaves probably due to more mature leaves being in these samples than in the hand plucked ones.
The samples from the processing line were collected during three sessions of black tea processing in April, July, and October 2000, and January 2001. The in-line samples were collected at each of the main processing steps from the initial field green leaves to the final black tea. The samples were also packed in dry ice and delivered to the laboratory by overnight transport and stored in a freezer at –80 ˚C before analysis. The phenolic compounds in these samples were extracted using methanol and aqueous methanol, and analysed using a HPLC with photodiode array detection. Catechins, catechin gallates and theaflavins were the main flavonoids quantified for the in-line samples.
Analysis of the results for the in-line tea samples shows that the main decreases in the individual and total catechins and catechin gallates due to oxidation occurred at the early stages of the fermentation period. Correspondingly, formation of individual and the total theaflavins occurred at the early stages of the fermentation period, suggesting the individual theaflavins are oxidation products of the catechins, catechin gallates and other phenolic acids such as gallic acid. The only seasonal variation that can be explained is that higher levels of the catechin gallates in the fresh leaves in January 2001 corresponded with higher levels of the oxidation products, theaflavins, in the resulting black tea.
However, other seasonal variations in the formation of theaflavins and reduction in the levels of the catechins and their gallates showed no regular patterns throughout the black tea processing. This could be due to factors associated with the tea processing.
It can be concluded that tea leaves harvested in the warmer months, such as January, when processed, would contain the highest level of theaflavins, and produce a correspondingly higher quality black tea.
The ratio (ECG+EGCG)/EGC could be used to measure seasonal variation in the levels of flavonoids in green tea leaves in the field and thus for monitoring the best time of the year to harvest tea leaves to produce quality black tea. As the ratio is similar in both mechanically harvested and hand plucked green tea leaves, the more easily obtained mechanically harvested leaves can be analysed for the ratio, which can be used as a quality index for the processed black tea.
In the part of the study above, the theaflavin levels peaked well before the designated end of fermentation indicating that the tea was overfermented. This highlighted the need to develop an online method of determining optimum fermentation, ie an on-line method for measuring theaflavin content during fermentation.
The first step involved assessing the methods using in other countries for measuring theaflavin content. Most of these methods are based on extracting the theaflavins and thearubigens and measuring by spectophotometric analysis. For these trials the dried tea samples collected during fermentation and after final drying were analysed for theaflavins, thearubigins, soluble solids, and total colour. The methods were assessed for their ease of use and adaptability for on-line use as well as their ability to distinguish between levels of theaflavins. None of the methods were simple enough to fit the criteria for adaptation to the factory situation. Whilst the measurements of theaflavins is relatively simple, the need to extract the theaflavins make the tests more complex requiring space, equipment and skilled technical support not available in the factory situation.
Next the feasibility of using colour as measure of theaflavin content was investigated. The drawback found here was that as the theaflavin content peaked during fermentation, total colour and thearubigens continued to rise. So it was not possible to separate the contribution of theaflavins to colour using these methods. However some recent work suggested that the colour contribution of theaflavins could be separated from thearubigen contribution using a colour meter such as the Hunter Lab or Minolta colour meter. These instruments measure 4 different components of colour and it appears that theaflavins can be positively correlated with some of these. Also the Minolta colour meter is a hand held instrument making it ideal for on-line use. More information is first required on thearubigen fractions before futher work can progress on this. Some preliminary analytical gel permeation chromatography work has been undertaken.