Green tea is tea made solely with the leaves of Camellia sinensis that have undergone minimal oxidation during processing. Green tea originates from China and has become associated with many cultures in Asia from Japan to the Middle East. Recently, it has become more widespread in the West, where black tea is traditionally consumed. Many varieties of green tea have been created in countries where it is grown. These varieties can differ substantially due to variable growing conditions, processing, and harvesting time.
Over the last few decades green tea has been subjected to many scientific and medical studies to determine the extent of its long-purported health benefits, with some evidence suggesting that regular green tea drinkers have lower chances of heart disease and developing certain types of cancer. Although there’s no scientific evidence that plain green tea can produce weight loss, a green tea extract rich in polyphenols and caffeine has been shown to be useful for “obesity management”, since it induces thermogenesis and stimulates fat oxidation.
While commonly called brewing, the process of making a cup of tea is actually steeping. Generally, 2 grams of tea per 100ml of water, or about one teaspoon of green tea per 5 ounce cup (150ml), should be used. With very high quality teas like gyokuro, more than this amount of leaf is used, and the leaf is steeped multiple times for short durations.
Green tea steeping time and temperature varies with individual teas. The hottest steeping temperatures are 180°F to 190°F (81°C to 87°C) water and the longest steeping times 2 to 3 minutes. The coolest brewing temperatures are 140°F to 160°F (61°C to 69°C) and the shortest times about 30 seconds. In general, lower quality green teas are steeped hotter and longer, while higher quality teas are steeped cooler and shorter. Steeping green tea too hot or too long will result in a bitter, astringent brew, regardless of the initial quality. It is thought that excessively hot water results in tannin chemical release, which is especially problematic in green teas as they have higher contents of these. High quality green teas can be and usually are steeped multiple times; 2 or 3 steepings is typical. The steeping technique also plays a very important role to avoid the tea developing an overcooked taste. Preferably, the container in which the tea is steeped or teapot should also be warmed beforehand so that the tea does not immediately cool down. It is common practice for tea leaf to be left in the cup or pot and for hot water to be added as the tea is drunk until the flavor degrades.
Green tea contains salubrious polyphenols, particularly catechins, the most abundant of which is epigallocatechin gallate. Green tea also contains carotenoids, tocopherols, ascorbic acid (vitamin C), minerals such as chromium, manganese, selenium or zinc, and certain phytochemical compounds. It is a more potent antioxidant than black tea, although black tea has substances which green tea does not such as theaflavin.
In vitro, animal, preliminary observational, and clinical human studies suggest that green tea can reduce the risk of cardiovascular disease, dental cavities, kidney stones, and cancer, while improving bone density and cognitive function. However, the human studies are inconsistent.
Green tea consumption is associated with reduced heart disease in epidemiological studies. Animal studies have found that it can reduce cholesterol. However, several small, brief human trials found that tea consumption did not reduce cholesterol in humans. In 2003 a randomized clinical trial found that a green tea extract with added theaflavin from black tea reduced cholesterol.
A study performed at Birmingham (UK) University, showed that average fat oxidation rates were 17% higher after ingestion of green tea extract than after ingestion of a placebo. Similarly the contribution of fat oxidation to total energy expenditure was also significantly higher by a similar percentage following ingestion of green tea extract. This implies that ingestion of green tea extract can not only increase fat oxidation during moderately intensive exercise but also improve insulin sensitivity and glucose tolerance in healthy young men.
A recent study looked at the effects of short term green tea consumption on a group of students between the ages of 19–37. Participants were asked not to alter their diet and to drink 4 cups of green tea per day for 14 days. The results showed that short term consumption of commercial green tea reduces systolic and diastolic Blood Pressure, fasting total cholesterol, body fat and body weight. These results suggest a role for green tea in decreasing established potential cardiovascular risk factors. This study also suggests that reductions may be more pronounced in the overweight population where a significant proportion are obese and have a high risk of cardiovascular disease.
In a study performed at the Israel Institute of Technology, it was shown that the main antioxidant polyphenol of green tea extract, EGCG, when fed to mice induced with Parkinson’s and Alzheimer’s disease, helped to protect brain cells from dying, as well as ‘rescuing’ already damaged neurons in the brain, a phenomenon called neurorescue or neurorestoration. The findings of the study, led by Dr. Silvia Mandell, were presented at the Fourth International Scientific Symposium on Tea and Human Health in Washington D.C., in 2007. Resulting tests underway in China, under the auspices of the Michael J. Fox Foundation, are being held on early Parkinson’s patients.
A study performed at the National institute of Chemistry in Ljubljana, Slovenia, demonstrated that EGCG from green tea inhibits an essential bacterial enzyme gyrase by binding to the ATP binding site of the B subunit. This activity probably contributes to the antimicrobial activity of green tea extract and may be responsible for the effectiveness of green tea in oral hygiene.
In a recent case-control study of the eating habits of 2,018 women, consumption of mushrooms and green tea was linked to a 90% lower occurrence of breast cancer.
A recent study on rats at the University of Hong Kong, published in the February issue of Journal of Agricultural and Food Chemistry, found that the catechins in green tea were absorbed by the lens, retina and other parts of the eye. The absorbed catechins reduced oxidative stress in the eye for up to 20 hours, suggesting that green tea may be effective in preventing glaucoma and other diseases of the eye.
Tea consumption had its origin in China more than 4000 years ago. Green tea has been used as both a beverage and a method of traditional medicine in most of Asia, including China, Japan, Vietnam, Korea, India and Thailand, to help everything from controlling bleeding and helping heal wounds to regulating body temperature, blood sugar and promoting digestion.
The Kissa Yojoki (Book of Tea), written by Zen priest Eisai in 1191, describes how drinking green tea can have a positive effect on the five vital organs, especially the heart. The book discusses tea’s medicinal qualities, which include easing the effects of alcohol, acting as a stimulant, curing blotchiness, quenching thirst, eliminating indigestion, curing beriberi disease, preventing fatigue, and improving urinary and brain function. Part One also explains the shapes of tea plants, tea flowers, and tea leaves, and covers how to grow tea plants and process tea leaves. In Part Two, the book discusses the specific dosage and method required for individual physical ailments.
Unless specifically decaffeinated, green tea contains caffeine. Normal green tea itself may contain more caffeine than coffee (by dry weight–for caffeine per serving size, see below), but the length of infusion with hot water and the number of times the leaves are reused can greatly alter caffeine intake. Using a given amount of green tea leaves steeped in 100 mL of water, experiments have shown that after the first 5 minutes of brewing, the tea contains 32 mg caffeine. But if the same leaves are then used for a second and then a third five minute brew, the caffeine drops to 12 mg and then 4 mg, respectively.
While coffee and tea are both sources of caffeine, the amounts of caffeine in any single serving of these beverages varies significantly. An average serving of coffee contains the most caffeine, the same serving size of tea provides 1/2 to 1/3 as much. One of the more confusing aspects of caffeine content is the fact that coffee contains less caffeine than tea when measured in its dry form. The caffeine content of a prepared cup of coffee is significantly higher than the caffeine content of a prepared cup of tea.
Green teas contain two caffeine metabolites (caffeine-like substances): theophylline, which is a stronger stimulant than caffeine, and theobromine, which is slightly weaker than caffeine.
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