BIOCHEMISTRY OF TEA FERMENTATION: THE ROLE OF CAROTENES IN BLACK TEA AROMA FORMATION

Abstract

ABSTRACT— The carotenoid compounds present in fresh tea leaf were quantitatively extracted and separated by thin layer chromatography. Neoxanthin. Neoxanthin, violaxanthin, lutein, and β‐carotene were identified and estimated by spectrophotometry. Quantitative studies showed that these carotenoid compounds decreased from about 0.053% (dry weight basis) in the fresh tea leaf to about 0.030% in the fermented (3 hr fermentation) leaf to about 0.026% in the fired black tea. This analytical study was followed by an investigation to determine how, and to what, these carotenoid compounds are altered during the black tea manufacturing process. The fate of β‐carotene in tea fermentation was first studied in model systems. The model consisted of a crude soluble enzymes preparation extracted from fresh tea leaves, tea flavanols, and β‐carotene. The model system was buffered at pH 5.4 and incubated for 30 min at 30°C. The volatile compounds formed in this system were studied by gas chromatographic analysis of headspace volatiles. It was found that β‐ionone (identified by GLC retention time and mass spectrometry) was formed in this model tea fermentation system as a result of tea flavanol oxidation and drying of the reaction mixture after oxidation. Further, it was found that all three basic reaction mixture ingredients, i.e., active tea enzymes preparation, tea flavanol, and β‐carotene were necessary for the production of β‐ionone. While β‐ionone was the major volatile product formed from β‐carotene in these systems, experiments utilizing 15,15′.14C‐β‐carotene showed that several unidentified compounds also were formed. The formation of β‐ionone from β‐carotene during black tea manufacture was confirmed in separate experiments. Organoleptic and GLC headspace analyses showed that black teas made in the usual way except that β‐carotene was added at the leaf maceration stage (stage which initiates fermentation) contained more β‐ionone than control black teas. The results obtained with β‐carotene have been generalized on paper for all of the carotenoid compounds known to be present in fresh tea leaf. The results of this work indicate that many of the important black tea aroma constituents are probably formed during the tea conversion process by oxidative degradation of the carotenoid compounds present in the system. The possible mechanism for these transformations is discussed.