Recent studies of the volatile compounds in tea (original) (raw)
Abstract
Tea aroma is one of the most important factors affecting the character and quality of tea. Recent advances in methods and instruments for separating and identifying volatile compounds have led to intensive investigations of volatile compounds in tea. These studies have resulted in a number of insightful and useful discoveries. Here we summarize the recent investigations into tea volatile compounds: the volatile compounds in tea products; the metabolic pathways of volatile formation in tea plants and the glycosidically-bound volatile compounds in tea; and the techniques used for studying such compounds. Finally, we discuss practical applications for the improvement of aroma and flavor quality in teas.
Highlights
► Different manufacturing processes affect the formation of tea volatiles. ► Hydrolysis of glycosidically-bound volatiles are involved in manufacturing processes. ► Tea has a variety of volatile compounds derived from diverse metabolic pathways. ► Techniques differ for the collection of endogenous and emitted volatiles in tea. ► Abiotic and biotic stresses can influence the formation of tea volatiles.
Introduction
The tea plant, commonly grown for beverage production, is of great economic importance. According to the Food and Agriculture Organization of the United Nations, world tea production is 3.2
million tons per annum, and India (27.4%), China (24.7%), and Sri Lanka (9.8%) are the main producers. Tea extracts are used as flavoring in sweets, bakery products, and in bottled tea beverages.
Tea infusions are among most consumed beverages around the world. Teas can be divided into three major categories: unfermented green teas, semi-fermented oolong teas, and fully-fermented black teas, which the latter one accounts approximately 80% of the total world tea production, respectively (Flaten, 2002). Around 300 kinds of teas have been described, of which only a very small number are known and consumed only in non-producing tea countries. For example black tea, green tea, and oolong tea are consumed around the world, whereas slightly fermented white tea, post-fermented yellow tea, and dark tea are almost exclusively used in China.
Tea quality is important for its market value and is defined by color, freshness, strength, and aroma. While phenolic compounds are responsible for the color and the taste, volatile compounds are fundamental for tea odor and aroma. Therefore, it is not surprising that volatile compounds in tea have been investigated since the 1930s. Since that period the number of reports evaluating tea quality has steadily increased. To date, hundreds of volatiles have been described in black tea, with fewer numbers in oolong and green tea, due to the lesser degree of fermentation when producing these teas. More than 600 volatiles have been identified in fully fermented black tea, and 41 of these have been identified as important contributors to the black tea aroma (Schuh & Schieberle, 2006). In unfermented green tea fewer volatiles can be found, and among these 200 volatiles, about 30 contribute to the characteristic green tea aroma (Kumazawa and Masuda, 1999, Kumazawa and Masuda, 2002). Some important tea volatiles, with their odor characteristics, are summarized in Table 1.
An analysis of 56 kinds of green, oolong, and black teas identified five volatiles as key components (_trans_-2-hexenal, benzaldehyde, methyl-5-hepten-2-one, methyl salicylate, and indole) that distinguish unfermented teas from fermented ones. _Trans_-2-hexenal and methyl salicylate may help to classify the semi- and fully-fermented teas (Wang et al., 2008). Moreover, the different kinds of tea show large variations in volatile concentrations and profiles influenced by Camellia variety and origin as well as pre- and postharvest treatments including steaming, firing, heat-rolling, withering, solar-drying, and fermentation (Kawakami & Kobayashi, 2002). Additionally, the tea aroma is influenced by the consumer, for example, by brewing temperature and brewing time.
Recent advances in tea aroma formation and analysis and knowledge of tea volatile components are summarized in this review.
Section snippets
Metabolic pathways of volatile compounds in tea plants
If one considers the economic importance of tea it is not surprising that the metabolic pathways and enzymes involved in the formation of tea volatile compounds have attracted the attention of many researchers. Tea flavor develops by chemical and biochemical transformations during its cultivation, production, and processing. In this section we concentrate on the biosynthetic pathways. The major volatiles are derived from either the terpenoid and shikimate pathways, or by oxidation of fatty
Direct organic solvent extraction
Direct extraction with organic solvents generally obtains a more complete profile of volatile compounds in sample. This method has been applied to investigate endogenous volatile compounds in plant tissues (Bergougnoux et al., 2007, Boatright et al., 2004, Maeda et al., 2010). However, using this method, non-volatile materials such as leaf waxes, pigments, lipids, and plasticizers from laboratory apparatus, are also likely to be extracted and may complicate analysis or affect the analytical
Practical applications for the improvement of flavor quality in teas
Approaches to improving or modifying the natural flavors of teas without the use of food additives can include genetic engineering, traditional breeding, and utilization of stresses. The safety of tea products produced by genetic engineering has not been fully evaluated. Traditional breeding takes a comparatively long period of time, with a low success rate. Therefore, the idea of using stress treatments to produce tea volatiles has recently attracted increased levels of interest. Tea volatile
Concluding remarks and perspectives
This paper provides a comprehensive review of our current knowledge of tea volatiles. The enormous variety of volatiles emitted from fresh and processed tea leaves suggests that there is much more to be learned in this area. Several important questions should be addressed in future studies:
- Methods should be improved for extracting tea volatile mixtures without altering their original compositions.
- The development of real-time techniques, such as the use of novel sensors, will be very helpful for
Acknowledgments
Z. Yang acknowledges the support of the “100 Talents Programme of the Chinese Academy of Sciences” (Y321011001 and 201209) and Major State Basic Research Development Program (2013CB127104). A part of the research aspects done by the authors are supported by a project ‘From Shizuoka to the world: Research and development of next-generation bottled tea drinks and tea extracts’ of Shizuoka Prefecture and Shizuoka City Collaboration of Regional Entities for the Advancement of Technological
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