Green tea was originated from China and has been consumed for thousands of years. It was initially used as a natural remedy and in religious ceremonies before it became a beverage. Heavy green tea drinkers like the monks and Japanese have been known to live longer than average people. It is believe that regular consumption of tea is good because it helps to relax one’s mind, decrease his need for sleep, and lightens his weight by reducing fat in the body system. These observations have been gradually proven by scientists. Today, several billion cups of green tea are consumed daily by people around the world. It has become one of the most widely consumed drinks and popular even among younger generations who are health conscious. Although the majority of drinkers are Asians, it is gaining popularity in the West as more people hear how beneficial it can be to human health. Many people who used to drink coffee or black tea with milk are now joining the trend of drinking green tea. Those who do not enjoy drinking green tea but want its health benefits can take green tea extracts in the form of supplements.
- Aims for the dissertation
A lot of studies and research have been done on green tea extracts and their effects on health the past few decades. While most scientists confirmed the goodness of green tea, some experts expressed concerns about the potential health risks of consuming too much green tea extracts. The objective of this paper is to look at the benefits and drawbacks of consuming green tea. It also attempts to find out whether green tea is really so good and as safe as so many people claim or think it to be.
There are now many books about green tea, websites designed to inform and update visitors on green tea, as well as thousands of scientific articles, research studies and media reports on the health benefits and potential risks. This paper was based on information and data from secondary source, mainly literature review of books, journal articles searched from the Internet database and newspaper reports. Updates from government and official websites were also used.
- Overall structure
This dissertation looks at the history of green tea, its composition, applications and uses. It then discusses the health benefits and potential risks of the green tea extracts, as well as the controversy about some of the health claims.
There are different varieties of the tea plants and as many as 500 existing species of tea grown in around 50 different countries, with China having the most species than the others. Most tea specialists prefer to cultivate the species Camellia sinensis which almost all green tea comes from. Green teas are made exclusively from the young leaves and buds of the Camellia sinensis plant. This plant is a slow-growing evergreen perennial tree of the genus Camellia, which survives in tropical or sub-tropical rainforest climates and thrives at altitudes of 2,100 meters above sea-level. The two parent strains were originated in Darjeeling, Assam and China. In order to harvest them more easily and conveniently, the trees are kept short as shrubs. There is no uniform grading for green tea. Good quality green tea consists of a leaf and a bud. The eight criteria to determine good green tea are the appearance, shape, colour, completeness, aroma, liquor, flavour and wet leaf.
- Green tea processing
Teas are classified into three major types, depending on how they are processed: green, black and oolong. Green teas are made from unfermented leaves. They are steamed, roasted, or pan-fried almost immediately after being picked. Since there is no time for them to ferment, no chemical change occurred. This is why it tends to be lighter in color and have the delicate ‘green’ character. Oolong teas are produced when the fresh leaves are subjected to a partial fermentation stage before drying. Black teas undergo post-harvest fermentation stage before drying and steaming. As a result of the fermentation and oxidation, many of the components in the tea leaves that are beneficial for health are destroyed during the production of black tea. Green tea is considered better than black tea or oolong tea for health reasons because it does not go through fermentation or oxidation during its production. It is the purest form of all teas.
In China, green tea leaves are allowed to dry naturally before they are roasted or pan-fried. Such treatments prevent fermentation and soften the leaves which will then be rolled and twisted to remove additional moisture. The Japanese use a steaming method before rolling, twisting and drying the leaves. China’s green tea is known to be the most delicate of all with a sweet and mild grassy taste. Some of the best Chinese green tea like fine Dragonwell could result in five or more infusions. The best green teas are those picked during early spring around the time of the Qing Ming Festival. Modern tea masters could list 500 or more green teas which could be plucked and processed slightly differently to give slightly different taste.
- History of green tea
Green tea has been used by people as a healthy and medicinal drink for thousands of years. There are many different stories about how it was first discovered. The most popular one from China dated as far back as 2737 B.C. The Chinese legend described how Shen Nong accidentally discovered the soothing taste of the beverage after a leaf dropped from a tea plant into a pot of boiling water while he was in the garden. He became very interested in the infusion and began to study about its various healing properties. Other stories of tea link it with Zen Buddhism. Whichever is true, there is no doubt that tea was originated from China. Anthropologists have reasons to believe that prehistoric humans living in the area of Yunnan chewed on the leaves of tea trees to increase their alertness when hunting. Fresh green tea leaves were gradually used by people for medicinal properties such as to treat depression, digestive and nervous conditions. During the Han dynasty, tea plants were known to be grown by monasteries in Sichuan. Some people started to steam tea leaves and then compressed them into cakes. These tea bricks would be baked and hardened so that they could be prevented from spoiling and be kept for a longer time. By the Tang Dynasty, as tea cultivation improved and trade increased, tea drinking became very popular in the upper class. Tea rules and ceremony were developed during this golden age of tea. Gradually, tea was consumed as a common healthy drink for all levels of society in China and Tibet instead of mainly used as a remedy to treat different health complaints for certain groups of people.
Around A.D. 780, a book dedicated to tea called Ch’a Ching (Tea Classics) written by Lu Yu was published. He described in great details how tea was grown, cultivated and processed. He even wrote about the utensils and proper way of tea consumption. It was Lu Yu who transformed the process of tea drinking into a form of art, which would eventually be passed to Japan by monks who travelled around Asia. As the popularity of drinking tea continued to grow in China, a tea culture began to develop in Zen ceremonies and secular society in Japan, where the tea plant was able to adapt very well. The tradition of drinking green tea involves a wide range of green tea and is still an important part of their society.
By the 12th century, tea plants were already exported to Japan on a large scale. It was until the 14th century when the culture of drinking tea was first introduced to Europe via the Silk Road and soon spread to other parts of the world. During the 1600’s a Chinese ambassador brought tea to Moscow which led to a flourishing tea trade to Russia. The Manchus were in power when China was the most important trading country in the world and tea trade was a monopoly. The Portuguese were the first traders to bring tea to Europe in large scale. China was able to be a sole exporter of tea by keeping the knowledge of tea cultivation technique for a long time. It was the British, eager to learn how to cultivate tea, who uncovered the secret of growing tea outside China in the 1800s. They sent a man to China who disguised himself as a merchant. The seeds he collected in China were brought to India but efforts to cultivate them failed. However, experiments using the Chinese techniques to grow local India tea plant were successful. Since the 19th century, the British began to cultivate tea on plantations in the colonies of India and Sri Lanka. It was only then when black tea, mass-produced and sold in packets, became more popular in the rest of the world.
Nowadays, the tea plant is cultivated in many countries around the world, with China, Japan, India and Sri Lanka being the greatest producers. About 3.1 million tons of dried tea is produced every year, 20% of which is green tea.
Although green tea has been consumed by the Chinese and Japanese for such a long time, its popularity increased only gradually in the West as the health benefits became more widely known. This is because green tea research has been widely conducted only in recent years by scientists. In fact, well-publicized results of research on green tea have been available only since the early 1990s. Green tea is now one of the most popular beverage consumed by people besides water. In some places, brewed tea steeped from carefully harvested green tea leaves of delicately grown tea tree is treated like prized wine or rare coffee. As studies continue to show evidence of its benefits, green tea will continue to be sought after by more and more people everywhere. The use of green tea and its extracts by manufacturers will continue to increase as green tea’s reputation keeps growing.
- Green tea components
While the history of green tea is long and interesting, its chemistry is complex and studied by scientists only quite recently. To understand more about green tea extracts and their effects on health, it is necessary to look at the composition as well as chemical and biochemical properties of green tea.
The composition of green tea leaf is very similar to that of other fresh leaf since green tea, being the most natural form of tea, is made from unfermented leaves from the tea plant. Only a few changes to the enzymes of the leaves occur right after they are plucked from the plant and some new volatile substances are produced when they are dried.
The buds and leaves of the tea plant contain carbohydrates, proteins, lipids, enzymes and the genetic material necessary for growth and photosynthesis. The main constituents of the leaves are polyphenols, the catechin and catechin-derived antioxidants; caffeine; theobromine; theophylline; and theanine. Other compounds in green tea that affect the human health are fluoride, minerals, vitamins such as B1, B2 and C, and trace elements such as chromium and manganese.
Table 1 – Mean Composition (%) of Green Tea
|Oxidised phenolic compounds***||0|
*Data referred to dry weight of tea leaves.
***Especially thearubigins and theaflavins.
[Sources: Belitz DH, Grosch W(1997), “Qu?mica de los Alimentos.” Zaragoza: Acribia]
The amount of these ingredients differs according to where the green tea is cultivated and age of the leaves. Young leaves and buds contain more caffeine while older leaves have larger amount of tannin (flavonols). Fresh green tea buds and leaves contain 75-80% water while the polyphenol components make up the remaining 20-25% of solid matter. Careful drying could prevent changes to the active ingredients of the green tea leaves whereas fermentation and oxidation that occur when black tea is processed would cause chemical changes. This leads to the major difference in the effects and taste between black tea and green tea.
Green tea extracts are herbal derivatives from green tea leaves which are used or taken orally by people. The extracts can be divided into 4 categories: a) Strong infusions – Green tea leaves are processed by soaking in alcohol solution; b) Soft extracts – the solution made by strong infusion is concentrated to 20 – 25%; c) Dry extracts – the solution from strong infusion is further concentrated to 40 – 50% solids and turned into dehydrated extract powder; d) Partly purified extracts – techniques such as solvent extraction, column chromatography, membrane extraction and separation are used to acquire more purified derivatives of green tea in order to produce supplements like green tea tablets and capsules.
- Tea Polyphenols
The color of green tea is partly due to chlorophyll and partly due to a kind of naturally occurring compound in it, called polyphenols. These compounds are responsible for the pungency and unique flavor of green tea. They are antioxidants which is a type of phytochemical compounds found in most plants, vegetables and fruits as well as coffee, cocoa, wine and tea. Polyphenols are the most biologically active group among the tea components, with antioxidative, antimutagenic and anticarcinogenic effects. Exposure to oxygen during enzymatic process reduces polyphenols levels. Green tea has unaltered polyphenols because unlike oolong or black tea, they do not undergo oxidation. Because of this, green tea has the greatest effect on health among all teas. In green tea, polyphenols are in the form of flavonoids. The main flavonoids present in green tea are the green tea catechins (GTC) which comprise four major derivatives: (-)-epigallocatechin-3-gallate (EGCG) (represents approximately 59% of the total of catechins); (-)-epigallocatechin (EGC) (19% approximately); (-)-epicatechin-3-gallate (ECG) (13.6% approximately); and (-)-epicatechin (EC) (6.4% approximately). Catachins have a carbon structure C6-C3-C6 composed of two aromatic rings.
[Source: V. Jane, et al. (Jan 2003), “Tea Catechins and Polyphenols: Health Effects, Metabolism, and Antioxidant Functions”, Critical Reviews in Food Science and Nutrition, 43(1):89-143]
The catechin content of green tea depends on how the leaves are processed, the geographical location and growing conditions (e.g. soil, climate, agriculture, practices and fertilizers), the type of green tea and the infusion (e.g. brew time, temperature) Green tea polyphenols are important for their ability to halt the damaging effects of oxidation which is a process of molecular DNA damage caused by the formation of toxic molecules called free radicals that develop in the human body. Individual catechins have different antioxidative and health properties.
Other compounds found in different plants also have antioxidative action. Catechins found only in green tea however have been proved to be more effective than many well-known antioxidants. Professor Catherine Rice-Evans of the Guy’s Hospital in London carried out tests and determined that green tea catechins have greater effects as antioxidants than the same quantity of Vitamin C and E, or beta-carotene. She also ranked the catechins according to the proportion of their presence in green tea. According to her study, EGCG was the most active of the catechins, responsible for 32% of the antioxidant property of green tea. The order from most antioxidative to least antioxidative are: 1) EGCG 2) EGC 3) ECG and 4) EC.
Caffeine is a central nervous system stimulant belonging to the family of chemicals called methylxanthines. It can be found in more than sixty different plants.  Most of them have been utilized as foods or beverages by people since ancient times. Caffeine is a trimethyl derivative of purine 2,6-diol. It was first discovered in coffee by Runge in 1820 and later isolated from tea by Nakabayashi. Caffeine content is usually 2.5-4.5% in dry green tea leaves. The amount of caffeine in green tea drink is about 1/10 to 1/5 of that in brewed coffee.
[Source: V. Jane, et al. (Jan 2003), “Tea Catechins and Polyphenols: Health Effects, Metabolism, and Antioxidant Functions”, Critical Reviews in Food Science and Nutrition, 43(1):89-143]
Caffeine has a strong effect on the brain and muscles which is why drinker will experience a mental boost shortly after drinking tea. The amount of caffeine varies among different types of tea. Black tea has the greatest amount whereas green tea contains only one-third the caffeine of black tea. It has been shown in many studies that caffeine improves cognitive performance and certain aspect of memory. Besides mental health, caffeine may also helps to enhance one’s emotional health, for example, making the drinker feel energized and motivated to work. Caffeine is known to be a cardiac stimulant and a diuretic. It also stimulates the cerebral cortex, causing excitation in the central nervous system. However, it may have adverse effects on some people and its intake could cause health risks instead of benefits. Because of that, this component is often removed and excluded when green tea is extracted for applications such as health supplements.
Besides polyphenols and caffeine, the two most commonly known components, green tea also actually contains many nutrients, including different kinds of vitamins and inorganic compounds.
Commercial green tea leaves contain about 280mg of Vitamin C (VC, ascorbic acid) per 100g of dried leaves. The content of Vitamin C in green tea can be ten times that in black tea because the vitamin is partly destroyed during fermentation, which green tea does not undergo. Other vitamins found in green tea in different amounts are Vitamin B2, Vitamin D, Vitamin K, and carotenoids.
- Inorganic elements
There are many minerals in green tea such as calcium, iron, copper, sodium, zinc and potassium. Green tea can also be a rich source of selenium if it is grown in a soil rich in this essential mineral. Another important mineral found in green tea is manganese which is used by the body in digesting protein as well as maintaining healthy bones and connective tissue. Green tea also contains a lot of fluoride – the mineral known for fighting dental cavities.
The amounts of aluminum in green tea leaves are higher than any other plants. Fortunately, the tea plant is able to biochemically neutralize the toxicity of aluminum, which exists mainly in a chelate form, which is less absorbable than the ionic form, with less potential to cause adverse effect on health. Experiments on animals and people also confirm that tea catechins can prevent the damage caused by accumulation of aluminum in the bones.
“Some specific inorganic compounds in the tea plant are aluminum, fluorine, and manganese.”
Table 2 Inorganic Elements and their contents in green tea leaves (per 100g dried leaves)
N3.5 – 7.1
Al420 – 3,500
P0.2 – 0.7
As0.20 – 0.42
K1.6 – 2.5
Ba1.3 – 5.1
Ca0.12 – 0.57
Br7.8 – 25.0
Mg0.12 – 0.30
F17 – 260
S0.24 – 0.48
Na20 – 33
Fe100 – 200
Ni1.3 – 5.9
Mn500 – 3,000
Pb2.2 – 6.3
Cu15 – 20
Rb8 – 44
Mo0.4 – 0.7
B20 – 30
Se1.0 – 1.8
[Source: “Standard Tables of Food Composition in Japan”, Resources Council, Science and Technology Agency, Tokyo, 1991]
- Other Components
- Amino Acids
The content of total nitrogen in green tea extracts ranges from 4.5 to 6%. Half of that are free amino acids, which are the building blocks of protein. The amount of amino acids in tea leaves harvested during spring time is larger than that during other seasons. Green tea contains some common amino acids like aspartic acid, glutamic acid, glycine, tyrosine etc. as well as an amino acid that is unique to it: theanine.
Green tea has four basic tastes: sweet, sour, salty and bitter. There is in fact a fifth taste known as unami which is contributed by the amino acid l-theanine. This constituent was only identified in 1949, by a Japanese who discovered that theanine makes up about 50% of the free amino acids present in tea. Young tea buds harvested in early spring contain a lot more theanine than tea leaves harvested later in the year because the theanine are converted into catechins as the leaves mature. Green tea experts rate the unami taste most highly and consider it to be the most important factor in determining the quality of the tea. High grade green tea is more soothing to drink than lower grade ones because the l-theanine can decaffeinates tea naturally, making it taste less bitter. Scientific studies using electroencephalography show that if 8 times more theanine is present than caffeine, the effect of caffeine will be blunted. Studies have also shown that theanine increases production of dopamine in the brain, giving the drinker the sense of alertness while feeling relaxed.
- Aromatic Oils
Aromatic oils play a major role in determining the fragrance of green tea. The oils accumulate as the tea leaf grows and evaporate during and after harvest of the leaf. For green tea, some aromatic oils remain in the final tea product, contributing to the taste of the tea. About 500 different aromatic oils have been identified in tea leaves.
Total carbohydrates in green tea leaves are about 40%, one third of which is cellulosic fiber. Starch is also contained in green tea. Tea leaves harvested in the morning when there is less starch are considered to be better in quality.
Green tea leaves has an average of 4% oil by weight. The seeds of tea also contain oil of around 20-40% by weight. The oil is nondrying and has a solidifying temperature of -5 to + 15?C.>/p>
- Health Benefits
Since the 1990’s, scientists in different countries, particularly Japan, have almost suggested that every system of the body can be benefited by green tea consumption. The polyphenols are known to reduce the risk of cancer before genetic mutations occur by neutralizing free radicals, prevent cardiovascular disease by preventing LDL cholesterol from changes that promote heart disease, and protect the body from various other illnesses. As more people around the world hear about these benefits, it has become increasingly popular for those who want a healthy life style to drink green tea as beverage or take green tea extracts as supplements.
- Antioxidant Effects
Free radicals are highly reactive molecules that could damage the body and lead to many diseases if left alone. They take an electron form another molecule, turning it into another electron-deficient free radical that can take an electron from a third molecule and so on, leading to a chain reaction. The human body has a number of antioxidant molecules that help to defend it against degenerative diseases, but sometimes they may be overwhelmed by the free radicals. Antioxidants are substances that patrol the body and quench free radical reactions.  The antioxidant property of green tea extract is the most basic of all the health benefits of green tea.
Many plant foods provide an abundant source of antioxidant nutrients. Polyphenols in green tea are among the most effective. Gramza Anna et al. examined the antioxidative activity of several biologically active components from plants to find which are safe for people and showing high antioxidant activities if added to food with lipids such as lard. Results show that the 1000ppm green tea ethanol extract inhibited the oxidation process most strongly among samples of green and black tea leaves. It was observed that the antioxidant activity was higher in tea extracts containing high levels of ECG, EC and C.
The US Department of Agriculture (USDA) analysed nearly 400 foods for their EGCG content and published a report in 2007. It showed that regular green tea contains the highest concentration of antioxidants, followed by decaffeinated green tea, flavored green tea and instant or bottled green tea. That USDA study also showed why green tea is the best way to ingest antioxidants. One gram of green tea steeped in 100 millimeters of water yields 127 milligrams of catechins whereas 100 grams of dark chocolate contains only 54 milligrams, blueberries 52 milligrams and black grapes just 22 milligrams.
Since tea contains higher levels of antioxidants than many fruits and vegetables, green tea consumption can protect the cells in the human body from damage caused by free radicals. Flavonoids, act as antioxidants, through four possible mechanisms: “1) as reducing agents, disarming free radicals. 2) by donating hydrogen molecules to prevent the formation of free radicals. 3) by quenching singlet oxygen that would otherwise act as a free radical in the body. 4) by binding with metals that could otherwise initiate the creation of free radicals.”
Among the four polyphenols in green tea, EGCG has been found to be particularly effective as an antioxidant. Researchers from Rutgers University concluded that “the strong antioxidant activities of green tea are mainly due to the higher content of EGCG” after comparing the antioxidant effect of various polyphenols in green tea and oolong tea.  Studies of the effects of tea consumption in people confirm evident that green tea is the most effective scavenger of free radicals among the different types of tea. In one study, five adults each drank two cups of green tea while five others drank the same amount of black tea. Both green and black teas improve the antioxidant capability of the blood, but green tea was found to be six times more powerful. Scientists also found that fresh green tea extracts is a better scavenger of singlet oxygen than stale green tea extract. Green tea polyphenols are also effective in quenching other free radicals such as hydrogen peroxide and superoxide.
- Diabetes Mellitus
Diabetes mellitus is known to affect the structure and function of myocardium, causing increased collagen in the heart and reducing cardiac function. Babu and his team from India found that the antioxidant enzymes of diabetic rats are not sufficient and efficient enough to reduce the oxidative stress caused by hyperglycemia. In 2006, their investigation showed that green tea treatment is effective in controlling the antioxidant system in the heart and aorta by alleviating lipid peroxidation. In 2007, their studies suggested that administration of green tea extract may improve myocardial collagen changes in diabetic rats. They believe that the antioxidant, antihyperglycemic, and hypolipidemic effects of green tea catechins may be responsible and concluded green tea may provide a therapeutic option for the treatment of cardiovascular complications in diabetes.
The majority of the world’s 33 million HIV cases were infected through heterosexual sex with 96% of new infections occurring in developing countries. Therefore ways to fight the spread of HIV in poor countries are extremely necessary. Previously, scientists have carried out lab tests and reported that EGCG may prevent HIV from binding to the T-cells in the immune systems protecting them from HIV’s damage. Although they knew that EGCG inhibited HIV in test tubes, they did not know if the findings would be useful beyond the lab.  Just recently, researchers in Germany have found a practical way to use EGCG to help prevent the spreading of AIDS. The researchers found that EGCG was capable of neutralizing a protein in sperm that served as a vector for the transmission of the virus that causes AIDS during sex. They say that the use of green tea in vaginal creams could provide a simple and affordable way to reduce cases of HIV infection.
- Neuroprotective Effects
- Parkinson’s Disease
Parkinson’s disease is a progressive, degenerative disorder of the central nervous system resulting from loss of dopamine-producing brain cells. A research team led by Professor Zhao affiliated to the Chinese Academy of Sciences Institute of Biophysics using an animal model of Parkinson’s disease discovered that green tea polyphenols can protect dopamine neurons. He hopes that green tea polyphenols may eventually be developed into a safe drug for Parkinson’s disease in humans.
- Alzheimer’s Disease
In an article published by the Journal of Neuroscience, researchers from University of South Florida reported that EGCG prevented Alzheimer’s-like damage in the brain of mice genetically programmed to develop the neurodegenerative disease process. They work by decreasing production of the Alzheimers’-related protein called beta-amyloid which causes nerve damage and memory loss when accumulated abnormally in the brain. The researchers think that a new generation of dietary supplements containing pure EGCG might be beneficial for treating Altheimer’s disease.
- Huntington’s Disease
Like Alzheimer’s and Parkinson’s, Huntington’s disease is also a type of neurodegenerative disease caused by protein misfolding characterized by jerky and uncontrolled movements. Mutant proteins are accumulated in the brain of sufferers and become toxic to the nerve cells in the brain. A res