Effects of baking and aging on the changes of phenolic and volatile compounds in the preparation of old Tieguanyin oolong teas

Abstract

A series of old oolong teas was prepared by baking fresh Tieguanyin oolong teas at 120–140 °C for 72 h, followed by a long-term storage of 5, 10 and 20 years. To monitor the chemical changes of the tea preparation, phenolic and volatile compounds in a fresh or its newly baked tea were analyzed and compared with those in the three aged teas. The contents of phenolic compounds in the teas were apparently altered by the baking, but remained relatively stable thereafter in the aging process. The contents of major catechin derivatives and flavonol glycosides presumably responsible for astringency and bitterness of teas were reduced while those of gallic acid and gallocatechin gallate, putatively originated from the decomposition and isomerization of (−)-epigallocatechin gallate, were elevated after baking. In contrast, linear and branched hydrocarbons, the abundant volatile compounds in the fresh tea, were relatively stable in the baking process, but nearly diminished during the long-term storage. Relatively minor contents of substituted benzaldehydes and pyrrole derivatives probably rendering the unique flavor of old oolong teas were generated after the baking and aging processes.

Introduction

Tea processed in different products, such as black tea, oolong tea, green tea, and scented tea, is one of the most consumed beverages worldwide. In the past few decades, oolong tea has been the most favorite choice for Taiwanese due to its special taste and flavor. In the preparation of oolong tea, young green shoots are freshly harvested and allowed to undergoing a semi-fermentation process, where the term ‘fermentation’ refers to natural browning reactions induced by oxidative enzymes in the cells of tea leaves (Haslam, 2003). The fermentation degree of oolong tea mainly depends on the demand of customers, and is mostly in the range of 20–80%. Tieguanyin is a premium variety of oolong tea traditionally prepared with a relatively high degree of fermentation (50–80%). The versatile processes of Tieguanyin preparation were originally developed in Anxi county of Fujian province, China around 300 years ago. In comparison with other oolong tea varieties, Tieguanyin is famous for complex volatiles, partly resulted from its high degree of fermentation during preparation (Schuh & Schieberle, 2006).

Traditionally, old oolong tea is named for those oolong teas that have been stored for more than five years (Lee, Chen, Liao, Tzen, & Chang, 2008). Empirically, the longer oolong tea is stored and further oxidized gradually, the better it is in terms of taste and beneficial effects to human health. Moreover, oolong teas with higher degrees of fermentation are practically found to have a better chance to be converted into superior old oolong teas. Besides the fermentation process, long-term storage (aging) and professional baking in a specialized oven at various desired temperatures are generally regarded as two major factors for the quality control of old oolong teas. Two different types of old oolong teas have been developed in Taiwan; one requires periodical baking refinement at least annually while the other relies on a fierce baking treatment in the beginning without further drying refinement in the following long-term storage. So far, there is no direct scientific evidence showing which process is a better protocol for the preparation of superior old oolong teas.

In our previous studies on old oolong teas prepared by periodical baking refinement, four distinct characteristics were observed by liquid chromatography tandem mass spectrometry (LC/MS/MS) and gas chromatography/mass spectrometry (GC/MS) analyses (Dou et al., 2007, Dou et al., 2008, Kuo et al., 2011, Lee, Chen, Liao, Tzen and Chang, 2008, Lee, Dou, et al., 2008). The first was the substantial accumulation of gallic acid (GA) in old oolong teas presumably released from (−)-epigallocatechin gallate (EGCG) during the baking process. The second was the unique occurrence of three flavonols, myricetin, quercetin and kaempferol that were presumably resulted from deglycosylation of their glycoside derivatives in fresh oolong teas after repeated baking. The third was the significant degradation of long straight-chains of alcohols and acids that were rich in the fresh oolong teas. The fourth was the formation of pyrrole derivatives, such as 2-acetylpyrrole and 2-formylpyrrole, which putatively contributed to the unique flavor of old oolong teas.

In the present study, we aimed to analyze the chemical conversion of old oolong tea prepared by a fierce baking treatment in the beginning without further drying refinement in the following long-term storage. Tieguanyin with a high degree of fermentation (80%) was selected to prepare a series of old oolong teas by baking at 120–140 °C for 72 h, followed by a long-term storage of 5, 10 and 20 years. Differences of chemical constituents in these Tieguanyin teas at different stages, i.e., before and after baking or different storage periods after baking, were used to evaluate the effects of baking and aging on the old tea preparation. Chemical conversion in the old Tieguanyin teas was also compared with that in old oolong teas prepared by periodical baking refinement.