Wednesday, April 11, 2007

The Flavor of Tea: The Polyphenol Connection


The enjoyment of drinking tea is a complex phenomenon. On a specifically physiological level, what we enjoy most about drinking a particular tea is the result of the interaction of various biochemicals that are present in the tea. These, in the right proportions of quantity and quality, give the resultant pleasant flavour -- just as, in a good symphony, the different musical pitches are combined in right proportion.

In an earlier essay I discussed the presence and importance of polyphenols in tea. The partial oxidation of polyphenols in the tea-leaf is a vital aspect of the production of Darjeeling and oolong teas. Indeed it has a decisive effect on what the final flavour of these teas will be. In this connection, it is worth examining just how -- and to what extent -- polyphenols are (or should be) brought to a process of oxidation.

The Physiology of Aroma

Flavor sensed by nose is called "Aroma." Aroma is perceived by two different mechanisms. It can either be sensed nasally (via smelling the tea infusion through the nose) or retronasally. Retronasal perception occurs when the tea liquor is either present in the mouth or has been swallowed, and aromatic volatile compounds drift upward into the nasal passage.

Polyphenols in Darjeeling & oolong Teas

Polyphenols in the tea brew give astringency/ briskness/ body and strength to the tea brew, and these characteristics tend to overlap or shadow the flavor. Hence:
• A lower level of polyphenols in the brewed tea yields more/better flavor.
• The lower the percentage of polyphenols in the tea shoot, the lower the percentage of polyphenols in the brewed tea. Hence the better will be the flavour of the liquor. For this reason the C. sinensis cultivar, having lower polyphenol levels than the C. assamica cultivar, in general leads to better flavour.
• In oolong processing, only a limited number of cells are bruised before inactivation of the enzymes. Hence there is more limited oxidation of polyphenols in the course of the oolong process, which in turn yields its excellent flavor.
• Substantial amounts of polyphenols are polymerized during the processing of the tea, to the extent that they become insoluble. This yields better flavor, for the same reason as detailed above.
Less bruising of the leaf cells during processing yields better flavour, for the same reason as detailed above.
• Clones having lower levels of polyphenol oxidase (PPO) activity give more flavor.

Shading Tea During Growth

• Tea that is shade-grown prior to plucking yields more theanine and less polyphenols, besides small size. And less polyphenol content in such leaves is one of the major factor to give better flavor in oolong teas and Darjeeling teas which are processed from such leaves. as is known to all. Shading is used in Japan for the production of some of its best green teas, but this practice also improves the quality of oolong teas, as is clear from the following information.

The effects of different levels of shade (0, 40, 60, & 80%) & different durations (5 & 10 days) were tested in Taiwan to determing how these influenced variations in the quality of oolong tea. The best results were obtained with 60% shading, maintained for 10 days prior to plucking; this yielded smaller and more tender green leaves than the unshaded control. The tea made from such leaves was low in polyphenols, high in caffeine, & had a good appearance. It is concluded that shading treatments could be used to improve the quality of summer tea generally.

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