Thearubigins of black tea : manufacturing-based studies
Thearubigins are polyphenolic oligmers which contribute to the quality of black tea: it
is necessary to learn more of their origin and structure in order to understand their
function, and to utilise opportunities to influence their development during processing
to meet market demand.
Tea samples were manufactured under controlled conditions, by methods parallel to
commercial production in Malawi. Leaf handling, withering, fermentation and drying
were manipulated. Conditions selected could be used in current commercial tea
factories with only minor modifications. Non-volatile water-soluble components of
black tea were analysed in an attempt to identify the critical points in the process.
Solvent partition, adsorption chromatography or caffeine precipitation followed by
size..exclusion HPLC were used to further separate the thearubigins and estimate
molecular mass. Reverse-phase HPLC was also used; both methods were monitored
by UV-VIS spectroscopy. Eluate was collected and portions challenged with chemical
probes to identify functional groups within the oligmers. Theafulvin was shown to be
heterogeneous in both chemical composition and mass.
Prolonged fennentation in the presence of air promoted the development of theafulvin,
caffeine-precipitable thearubigin, and hump unresolved by reverse-phase HPLC. This
is the first evidence that theafulvin is a product of fermentation rather than a plant
Leaf handling and fermentation conditions have a greater impact on liquor colour and
perceived quality than withering or well-controlled drying. Modified dryer operating
conditions preserved product composition and quality. Opportunities to manipulate
product composition to meet market demand were identified.