Enzymatic treatments improve quality of black tea.

Tea is the most widely used stimulant in the world. Variations in the flavor and quality of tea are dependent on the interplay of complex metabolic events that occur in its leaves during processing.

In conventional processing, the tea cut dhools (particles) undergo a series of procedures, including withering, rolling, fermentation and drying. During processing, enzymes--polyphenol oxidase (PPO) and peroxidase--oxidize the polyphenols present in the tea leaves, causing various black tea components to form.

However, the partial disruption of tea leaves during the rolling process reduces the amount of peroxidase and PPO in the native leaves. The cell wall polysaccharides also act as a barrier for enzyme substrate interaction.

To overcome this problem, Taiwanese scientists sprayed the withered tea leaves (Assam TTES 18 cultivar) with 35 ml (1 unit per mL) of a commercial cellulase, one-third of the way into the 90-minute rolling process. Then they sprayed the leaves with 35 ml (1 unit per mL) of peroxidase or PPO at the beginning of the 90-minute fermentation step.

The investigators evaluated several quality parameters, including sensory evaluation scores, total liquor color, total catechins, total polyphenols, theaflavin (TF) and thearubigen (TR). As a result of the cellulase and peroxidase treatments, the TF content in the leaves increased by 11.75 %. The amount of TR grew by 27.42%.

During the cellulase and PPO treatments, the amount of TF increased by 41.9%. The TR content increased by 29.65%. All of the sensory evaluation scores for processing that included the enzymatic treatments had comparable or higher scores than for processing that did not include enzymatic treatments. The researchers determined that the polyphenols present in the tea leaves were oxidatively polymerized to TF and TR by peroxidase and PPO. The enzymatic treatments were effective in enhancing the quality of black tea.

Further information. Chi-Yue Chang or Kuang-Hsien Tsui, Dayeh University, No. 112 Shanjiao Rd., Dacun, Changhua, Taiwan 51591, Republic of China; phone: +886-4-8511888; fax: +886-4-8511666; URL: www.dyu.edu.tw/english.