Tea

Bioactivity and Therapeutic Potential

Yong-Su Zhen

• Publisher: CRC Press
• Year: 2002
• eBook ISBN: 9781134481354
• Topic: Biological Sciences
• Subtopic: Biology
• Index: Biological Sciences

The tea plant, Camellia sinensis, is of particular importance to humans, and the consumption of tea has a long history of over 2000 years. Currently, tea is one of the most popular beverages worldwide. In recent years the subject of tea has attracted a great deal of attention. As well as the use of tea in traditional medicine, modern biochemical re

1. TEA AND HEALTH—AN OVERVIEW

MIAO-LAN CHEN

Chinese Academy of Medical Sciences and Peking Union Medical College, 9 Dong Dan San Tiao, Beijing 100730, China

Tea is one of the most popular beverages in the world. Drinking a cup of tea for pleasure or in times of stress is a part of daily life for millions of people all over the world. It is estimated that one-half of the population in the world consumes tea. The production and trade of tea has become an important business for centuries. In 1990, the tea growing area in the world reached 2.45 million hectares and total output reached 2.51 million tons. World tea consumption has increased steadily. China, for example, has experienced an increase in tea consumption of 6.0% annually for the period 1961–1984 and is expected to maintain the momentum at a rate of 10% to the year 2000. In the United Kingdom, annual tea consumption is expected to increase 1.4% annually until the year 2000. In terms of annual tea consumption per capita, Ireland has the highest value at 3.07 kg (triennial average over the period 1986–1988), followed by Iraq (2.95 kg), Qatar (2.91 kg), UK (2.84 kg) and Turkey (2.73 kg) for the same period (Robinson and Owuor, 1992; Chen and Yu, 1994). The daily consumption of tea is approximately 3 billion cups all over the world. About 80% of consumers prefer black tea and the rest consume green tea and semi-fermented oolong tea. Green tea is preferred in China, Japan, and Middle East countries, the oolong tea is mainly consumed in the eastern part of China and in Japan. In Great Britain, tea is drunk by more than 80% of the population. The average intake for those tea drinkers considering tea as a healthy drink was estimated to be 0.8 litre per day (Marks, 1992). Tea is effective for quenching thirst. However, the ability of tea to quench thirst is not the main reason for its popularity as a beverage. This relies much more on its sensory properties, customs, prices, availability and apparent health benefits. Because of the high popularity of tea, the relationship between tea and health has come as one of the most attractive topics in biomedical sciences.

1. HISTORY OF TEA CONSUMPTION

The discovery and use of tea has a long history. It may originate during the “Shen-Nong” era of ancient China, around 5000 to 6000 years ago (Chen, 1994). The probable center of origin of tea is in southwest China. The tea plant, Camellia sinensis, has been known to and cultivated by Chinese people for a very long time. The word “cha” meaning tea in Chinese is the first word that was coined for tea in the world (Yao, 1992). Originally, tea was used as medicine for various illnesses in China. The first literary mentions of tea all agree that its taste was remarkably bitter. At that time it was also called “bitter tea”. Bitterness of tea provides an important clue permitting the assumption that early drinkers decocted freshly picked tea leaves. The development from tea as “medicine” to tea as a “drink” began in late Zhou Dynasty (1124 B.C.–222 B.C.) and tea drinking gradually rose in popularity in Qin Dynasty (221–206 B.C.) (Yao and Chen, 1995). Since then, tea has been recognized both as a beverage and, in some occasions, as a medicine. In Qin Dynasty, great changes took place in the way tea was perceived. Medicinal tea had become tonic tea. The difference between medicines and tonics is that the former cured disorders whereas the latter kept one fit. Chinese medicine had always stressed prevention; therefore, doctors recommended tea to healthy persons to keep them that way. Certainly, the market of tea for healthy people wanting to maintain good health was far greater than that solely for curing illnesses. This change in attitude toward tea resulted in a great rise in popularity (Evans, 1992a).

Tea drinking continued to spread widely and rapidly during the Han Dynasty (206 B.C–220 A.D.) and the effects of tea was documented. As listed in the Materia Medica (Ben Cao Jing), tea as a medicine acts as “an antidote to herbal poisons, as a cure for swelling and abscesses in the head and as a sleep inhibitor.” The famous Han surgeon Hua Tuo summed up the medical viewpoint of that period in his dissertation, “To drink bitter tea constantly makes one think better.” The word “constantly” by Hua Tuo is significant because it shows that people by the end of the dynasty were already drinking tea all day long, evidently as a tonic of longevity. There were records stating that “tea sobers one after drinking alcohol.” In the era of Three Kingdoms (220–280 A.D.), people utilized tea to offset the effects of inebriation caused by the exaggerating consumption of alcohol. Tea had become a popular remedy for drunkenness and its ill effects. There were also records indicating that “drinking tea induces sleeplessness” and “tea keeps one awake.” People of that era knew that drinking too much tea and too frequently may result in chronic sleeplessness (Evans, 1992b).

In Tang Dynasty (618–906 A.D.), tea production including cultivation, harvesting and processing had developed rapidly and the popularity of tea drinking reached a new level. Green tea was first invented as steamed green tea cake instead of directly cooking the tea leaves for drinking brew. Although tea was still considered a healthy drink and often qualified as “invigorating”, it had now become a prestigious social drink, the so-called national drink of China. There were meticulous and luxurious tea banquets, tea parties and tea competitions held in the Imperial Palace, in the circle of high-ranking officials and social elite. Tea was also widely consumed by the average person. There existed a variety of tea specialities. Even different classes of Tang society, rich and poor, urban and peasant, as well as ethnic minorities drank tea differently. The book The Classic of Tea written by Lu Yu of the Tang Dynasty is the first monograph on tea in the world. In the book Lu Yu presented a comprehensive description and discussion on tea including production, preparation, quality assertion etc. Lu Yu stated that “tea growing wild is superior, garden tea takes second place.” In general, Tang tea cultivators planted hill-tea on shady land along mountain slopes subject to cloud cover, exposed to fogs or frequent mists. The time for harvesting is important. Tea leaves were picked “in March-April prior to the spring rains” when new shoots had appeared and the leaves were young and tender. Tea leaves picked thereafter are less desirable, because tea leaves grow faster after rains and are consequently larger. Having good quality water is important in making a tea drink. A good cup of tea depends upon the quality of the water as much as the quality of tea. Lu Yu listed selected tea-water sources and chose mountain spring water over all others. People in Tang Dynasty paid very much attention to the preparation of tea drinks. As Lu Yu warned, “Tea improperly prepared can cause sickness.” He mentioned: “The first cup of tea should have a haunting flavor, strange and lasting. When you drink tea, sip only, otherwise you will dissipate the flavor. Moderation is the very essence of tea.” A Tang poet described how pleasant and joyful people felt on tea physically and mentally. In the so-called “The seven cups of tea”, he wrote that tea could moisten the lip and throat, break loneliness, and penetrate one’s barren entrails; moreover, could call one up to the realm of immortals and the feeling to ride a sweet breeze and waft away (Evans, 1992c).

Tea continued to gain popularity in China after Tang Dynasty. Teahouses first appeared in Song Dynasty (960–1279 A.D.) and quickly spread throughout the country. Teahouses were known as places where one could relax and have a good time. Black tea which the Chinese called “red tea” was manufactured and consumed in Ming dynasty (1368–1644 A.D.). Most of the manufactured black tea was exported and the majority of Chinese remained consuming green tea. Drinking of tea was considered beneficial to health. In the book Tea Manual (Cha Pu) written in Ming Dynasty, the author concluded that “Drinking genuine tea helps quench the thirst, aids digestion, checks phlegm, wards off drowsiness, dispels boredom and dissolves greasy foods.”

In Japan the first tea was brought from China in the early 9th century. China started supplying Russia with small quantities of tea toward the end of the 17th century, and the trade was first carried overland by caravans. The first tea to reach Europe went by way of the Dutch who brought the first consignments to Holland in the early part of the 17th century. The early supplies of tea entering England were brought over from Holland. In London the first tea was served to the public in 1657. By the mid 1750s tea houses and tea gardens were appearing in and around London. Tea was soon to become the national drink in the British Isles (Weatherstone, 1992). An author in the late 18th century described the difference in the way of tea drinking between Chinese and the Europeans. He mentioned that Chinese drank tea without sugar; however, almost everyone in Europe added sugar to tea. Since then, great changes have taken place, and the difference, at least in some regions, seems to be less prominent in the present time.

In China, from Tang Dynasty to Qing Dynasty (618–1911 A.D.), there had existed a great number of books contributed to tea. Those included mainly 3 categories, namely, books on herbal medicines, tea manuals and general historic publications. As summarized by Lin (1992), tea was reported to exhibit 24 kinds of physiological and therapeutic effects, such as causing less sleep, calming down, clearing sight, relieving headache, dispelling thirst, dissipating fever, detoxification, helping digestion, reducing obesity, diuresis, as a pectoral for chest diseases, invigorating, strengthening teeth, and more. In addition to the probable applications as medicine, tea used as a daily beverage has made great contributions to human health in at least two major aspects (Zhu, 1992). Firstly, tea drinking changes the habit of how people consume water. In ancient times, when people felt thirsty they would simply drink natural, unprocessed water that might contain pathogenic microbes. Since the adoption of tea drinking, people had used boiling water to make tea infusion. In fact this practice helped people avoid a variety of infectious diseases. Secondly, tea appears to be a good substitute for alcoholic beverages. Those people who very much enjoyed tea drinking might avoid alcohol over-consumption that causes severe damage to the human body.

2. COMPOSITION OF TEA AND THE ACTIVE CONSTITUENTS

For a better understanding of the bioactivity of tea and its physiological and pharmacological effects, it is essential to scrutinize the chemical composition of tea and its bioactive constituents. There exist volatile and non-volatile compounds. Generally speaking, the tea aroma is mainly dependent on the volatile compounds it contains, while the color and the taste of tea are mainly dependent on the non-volatile compounds.

2.1. Chemical Composition of Tea Flush

Tea flush is generally a reference to young shoots of tea that consist of the terminal bud and two adjacent leaves. In fresh tea flush there exist a wide variety of nonvolatile compounds: polyphenols, flavonols and flavonol glycosides, flavones, phenolic acids and depsides, amino acids, chlorophyll and other pigments, carbohydrates, organic acids, caffeine and other alkaloids, minerals, vitamins, and enzymes. The chemical composition of the tea leaves depends upon leaf age, the clone being examined, soil and climate conditions, and agronomic practices.

The total polyphenols in tea flush ranges from 20% to 35%. Tea polyphenols include mainly six groups of compounds. Among them, the flavonols (mainly the catechins) are the most important group and occupy 60–80% of the total amount of polyphenols. The catechins have been widely, and intensively investigated for their bioactivity and utilization. Four major catechins, namely (-)-epigallocatechin-3-gallate (EGCG), (-)-epigallocatechin (EGC), (-)-epicatechin-3-gallate (ECG), and (-)-epicatechin (EC), constitute around 90% of the total catechin fraction; and (+)-catechin (C) and (+)-gallocatechin (GC) are present about 6% of the fraction. There are some minor catechins that constitute less than 2% of the total catechins. The catechins that are water-soluble, colorless compound contribute to astringency and bitterness in green tea.

There are three major flavonol aglycones in the fresh leaf, kaempferol, quercetin and myricetin. These substances occur both as free flavonols and as flavonol glycosides. The glycosidic group may be glucose, rhamnose, galactose, arabinose, or rutinose. These compounds are considered to contribute to bitterness and astringency in green tea (McDowell & Taylor, 1993).

Amino acids constitute around 4% in tea flush. The most abundant amino acid is theanine (5-N-ethylglutamine) which is apparently unique to tea and found at a level of 2% dry weight (50% of free amino acid fraction). The precursors in the biosynthesis of theanine in tea plant have been identified as glutamic acid and ethylamine. The site of theanine biosynthesis is the root and from there it is transferred to younger leaves; thus the roots have the highest concentration in the tea plant.

Free sugars constitute 3–5% of the dry weight of tea flush. It consists of glucose, fructose, sucrose, raffinose and stachyose. The monosaccharides and disaccharides contribute to the sweet taste of tea infusion. The polysaccharides present in tea flush can be separated into hemicellulose, cellulose and other extractable polysaccharide fraction. Some investigations have demonstrated that polysaccharides extracted from manufactured tea showed decreasing effect on blood-glucose level.

Caffeine is the major purine alkaloid present in tea. The content of caffeine in tea flush is approximately 2–5% (dry weight basis). Theobromine and theophylline are found in very small quantities. Traces of other alkaloids, e.g. xanthine, hypoxanthine and tetramethyluric acid, have also been reported.

Many volatile compounds, collectively known as the aroma complex, have been detected in tea. The aroma in tea can be broadly classified into primary or secondary products. The primary products are biosynthesized by the tea plant and are present in the fresh green leaf, whilst the secondary products are produced during tea manufacture (Sanderson & Graham, 1973). Some of the aroma compounds, which have been identified in fresh tea leaves, are mostly alcohols including Z-2-penten-1-ol, n-hexanol, Z-3-hexen-1-ol, E-2-hexen-1-ol, linalool plus its oxides, nerol, geraniol, benzylalcohol, 2-phenylethanol, and nerolidol (Saijo & Takeo, 1973). The aroma complex of tea varies with the country of origin. Slight changes in climate factors can result in noticeable changes in the composition of the aroma complex. Notably, teas grown at higher altitudes tend to have higher concentrations of aroma compounds and superior flavor, as measured by the flavor index (Owuor et al. 1990). Growing tea in a shaded environment may change the aroma composition and improves the flavour index. The aroma complex also varies with season and these variations appear to be larger under temperate or sub-tropical climates (Gianturco et al. 1974).

2.2. Chemical Composition of Made Tea

A series of changes occur in the process of manufacturing. There are three basic types of tea manufacture, resulting in the production of green, semi-fermented, and black teas. They differ mainly in the degree of fermentation. Green tea undergoes little or no fermentation and black tea is produced as a result of a full fermentation. Semi-fermented tea (oolong tea) is a product of partial fermentation. The major steps for the manufacturing of green tea include spreading-out, fixing, rolling and drying. For black tea manufacture the major steps include withering, rolling, fermentation and drying.

The total content of polyphenols decreases to some degree during green tea processing. In comparison with fresh tea leaves, polyphenol contents of manufactured green tea generally decrease by around 15%. Polyphenols undergo marked changes during black tea processing. As a result of enzymatic oxidation of the catechins by polyphenol oxidase, two groups of polyphenol compounds, theaflavins and thearubigins, are formed which are thought to be unique to black tea. The enzyme oxidizes the catechins to their respective o-quinones, which rapidly react with each other and other compounds to form theaflavins and thearubigins. Theaflavins account for between 0.3 and 1.8% of the dry weight of black tea and between 1 and 6% of the solids in tea liquor....

Table of contents

• COVER PAGE
• TITLE PAGE
• COPYRIGHT PAGE
• PREFACE TO THE SERIES
• PREFACE
• CONTRIBUTORS
• 1. TEA AND HEALTH—AN OVERVIEW
• 2. BOTANICAL CLASSIFICATION OF TEA PLANTS
• 3. GREEN TEA, BLACK TEA AND SEMI-FERMENTED TEA
• 4. THE CHEMISTRY OF TEA NON-VOLATILES
• 5. THE CHEMISTRY OF TEA VOLATILES
• 6. BIOCHEMICAL AND CELLULAR BASES FOR TEA ACTIVITY
• 7. PHARMACOLOGICAL EFFECT OF CAFFEINE AND RELATED PURINE ALKALOIDS
• 8. THE EFFECTS OF TEA ON THE CARDIOVASCULAR SYSTEM
• 9. ANTIMICROBIAL ACTIVITY OF TEA PRODUCTS
• 10. ANTICARCINOGENIC ACTIVITY OF TEA
• 11. ANTITUMOR ACTIVITY OF TEA PRODUCTS
• 12. THERAPEUTIC USES OF TEA IN TRADITIONAL CHINESE MEDICINE
• 13. AGRONOMY AND COMMERCIAL PRODUCTION OF TEA