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Thursday, March 1, 2007
Green Tea IOverview
A GREEN TEA OVERVIEW
Green tea is the palest in color, ranging from light green to light yellow in color. Green tea is not oxidized; the leaves are steamed or baked immediately after being plucked. They are then rolled and dried allowing the leaves to remain green in color. Green tea is made from both new buds as well as young leaves – “pluckers” take ‘two leaves and a bud’.
Green tea, which is unfermented tea, remains the most popular tea in Asian countries such as China and Japan. In fact, up until the eighteenth century, it was also the most popular form of tea in Britain as well. Imports of green tea into the United States outpaced black tea until about 1915. Much of the tea dumped into the Boston harbor during the Boston Tea Party, was in fact green tea. Thus it is unlikely that the harbor turned to a copper color!
About ninety percent of the world's green tea is produced in China. While most of the world's black tea comes from countries such as Sri Lanka (Ceylon), India, Kenya, Indonesia and Argentina, these countries produce little, if any, green tea. Several hundred varieties of green tea are made in China alone. Some of the most popular include Gunpowder, Hyson, Imperial Green, and Gyokuro (Japan). A cup of green tea is generally much lighter than other teas. While Asian cultures have believed for centuries that green tea has properties beneficial to human health, modern science is just now discovering that this may be true.
Manufacture
Differences between green, black, and oolong teas occur during the processing of the tea leaves.
Plucking
All types are normally hand plucked. Pluckers take only the bud and two leaves, as only young leaves produce good quality tea. Machine plucked tea is usually inferior.
Withering
The next step in manufacturing is withering. Tea leaves are laid out and allowed to wilt for several hours. Leaves are laid out in bamboo trays or in withering beds indoors. Withering reduces the moisture content in the leaves and prepares them for the next step. Withering can be omitted during the production of green tea, but is crucial in black tea manufacture.
Steaming
This process differentiates green tea from other types. After withering, green tea leaves are immediately steamed, baked, or pan heated. This crucial step in green tea manufacture prevents the oxidation (fermentation) of the leaves so that they remain green.
Rolling
Green tea is very often hand rolled, but mechanical rollers are used as well. The rolling process gives the leaves their appearance. During green tea manufacture, leaves are usually steamed rolled and steamed alternatively. Tightly rolled leaves are an indication of good quality tea.
Firing
Green tea leaves are given a final firing to dry the leaves and ensure no oxidation of the leaves occurs. The firing of leaves is done in ovens.
Useful Of Green Tea
Green tea has been extensively studied in people, animals, and laboratory experiments. Results from these studies suggest that green tea may be useful for the following health conditions:
Atherosclerosis
Population-based studies indicate that the antioxidant properties of green tea may help prevent atherosclerosis, particularly coronary artery disease. (Population-based studies refers to studies that follow large groups of people over time and/or studies that are comparing groups of people living in different cultures or with different dietary habits, etc.)
High cholesterol
Green tea has demonstrated an ability to lower total cholesterol and raise HDL ("good") cholesterol in both animals and people. One population-based study found that men who drink green tea are more likely to have lower total cholesterol thank those who do not drink green tea. Results from one animal study suggest that polyphenols in green tea may block the intestinal absorption of cholesterol and promote its excretion from the body.
Cancer
The cancer-protective effects of green tea have been reported in several population-based studies. For example, cancer rates tend to be low in countries such as Japan where green tea is regularly consumed. However, it is not possible to determine from these population-based studies whether green tea actually prevents cancer in people. Emerging animal and clinical studies are beginning to suggest that substances in green tea known as polyphenols may play an important role in the prevention of cancer. These substances act as powerful antioxidants. Researchers also believe that polyphenols help kill cancerous cells and stop its progression.
Bladder cancer
Only a few studies have examined the relationship between bladder cancer and green tea consumption. In one study that compared people with and without bladder cancer, researchers found that women who drank black tea and powdered green tea were less likely to develop bladder cancer. A follow-up study by the same group of researchers revealed that bladder cancer patients (particularly men) who drank green tea had a substantially better 5-year survival rate than those who did not.
Breast cancer
Studies in animals and test tubes suggest that polyphenols in green tea inhibit the growth of breast cancer cells. In one study of 472 women with various stages of breast cancer, researchers found that women who consumed the most green tea experienced the least spread of cancer (particularly premenopausal women with early stages of breast cancer). They also found that women with early stages of the disease who drank at least 5 cups of tea every day before being diagnosed with cancer were less likely to suffer recurrences of the disease after completion of treatment. However, women with late stages of breast cancer experienced little or no improvement from drinking green tea.
Colorectal cancer
Studies on the effects of green tea on colon or rectal cancer have produced conflicting results. Some studies show decreased risk in those who drink the tea, while others show increased risk. Further research is needed before green tea can be recommended for the prevention of colorectal cancer.
Esophageal cancer
Studies in laboratory animals have found that green tea polyphenols inhibit the growth of esophageal cancer cells. However, results of studies in people have been conflicting. For example, one large-scale population-based study found that green tea offered significant protection against the development of esophageal cancer (particularly among women). Another population-based study revealed just the opposite -- green tea consumption was associated with an increased risk of esophageal cancer. In fact, the stronger and hotter the tea, the greater the risk. Given these conflicting findings, further research is needed before green tea can be recommended for the prevention of esophageal cancer.
Lung cancer
While green tea polyphenols have been shown to inhibit the growth of human lung cancer cells in test tubes, few studies have investigated the link between green tea consumption and lung cancer in people and even these studies have been conflicting. One population-based study found that Okinawan tea (similar to green tea but partially fermented) was associated with decreased lung cancer risk, particularly among women. A second study revealed that green tea and black tea significantly increased the risk of lung cancer. As with colon and esopageal cancers, further studies are needed before any conclusions can be drawn about green tea and lung cancer.
Pancreatic cancer
In one large-scale study comparing green tea drinkers with non-drinkers, those who drank the most tea were significantly less likely to develop pancreatic cancer. This was particularly true for women -- those who drank the most green tea were half as likely to develop pancreatic cancer as those who drank less tea. Men who drank the most tea were 37% less likely to develop pancreatic cancer. It is not clear from this population-based study, however, whether green tea is solely responsible for reducing pancreatic cancer risk. Although promising, further studies in animals and people are needed before green tea can be recommended for the prevention of pancreatic cancer.
Prostate cancer
Laboratory studies have found that green tea extracts prevent the growth of prostate cancer cells in test tubes. However, both green and black tea extracts were also found to stimulate genes that cause cells to be less sensitive to chemotherapy drugs. Given this potential interaction, black and green tea (as well as extracts of these teas) should not be taken while receiving chemotherapy.
Skin cancer
The main polyphenol in green tea is epigallocatechin gallate (EGCG). Scientific studies suggest that EGCG and green tea polyphenols have anti-inflammatory and anti-cancer properties that may help prevent the onset and growth of skin tumors.
Stomach cancer
Laboratory studies have found that green tea polyphenols inhibit the growth of stomach cancer cells in test tubes, but studies in people have been less conclusive. In two studies that compared green tea drinkers with non-drinkers, researchers found that people who drank tea were about half as likely to develop stomach cancer and gastritis (inflammation of the stomach) as those who did not drink green tea. However, a recent study including more than 26,000 men and women in Japan found no association between green tea consumption and stomach cancer risk. Some studies even suggest that green tea may increase the risk of stomach cancer.
Further studies are underway to determine whether green tea helps reduce the risk of stomach cancer. Although green tea is considered safe for people at risk for stomach cancer, it is too soon to tell whether green tea reduces the likelihood of developing this disease.
Inflammatory Bowel Disease (IBD)
Green tea may help reduce inflammation associated with Crohn's disease and ulcerative colitis, the two types of IBD. Also, if green tea proves to be helpful for preventing colon cancer, this would be an added benefit for those with IBD because they are at risk for colon cancer.
Diabetes
Green tea has been used traditionally to control blood sugar in the body. Animal studies suggest that green tea may help prevent the development of type 1 diabetes and slow the progression once it has developed. People with type 1 diabetes produce little or no insulin, a hormone that converts glucose (sugar), starches, and other foods into energy needed for daily life. Green tea may help regulate glucose in the body. More research in this area would be helpful.
Liver disease
Population-based studies have shown that men who drink more than 10 cups of green tea per day are less likely to develop disorders of the liver. Green tea also appears to protect the liver from the damaging effects of toxic substances such as alcohol. Animal studies have shown that green tea helps protect against the development of liver tumors in mice.
Results from several animal and human studies suggest that one of the polyphenols present in green tea, known as catechin, may help treat viral hepatitis (inflammation of the liver from a virus). In these studies, catechin was isolated from green tea and used in very high concentrations. It is not clear at this time, whether green tea (which contains a lower concentration of catechins) confers these same benefits to people with hepatitis.
Weight loss
Studies suggest that green tea extract may boost metabolism and help burn fat, but there have been no specific studies of this herb in overweight or obese individuals. Some researchers speculate that substances in green tea known as polyphenols, specifically the catechins, are responsible for the herb's fat-burning effect.
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