Green tea is made from the leaves from Camellia sinensis that have undergone minimal oxidation during processing. Green tea originates in China, but it has become associated with many cultures throughout Asia. Green tea has recently become more widespread in the West, where black tea has been the traditionally consumed tea. Green tea has become the raw material for extracts which are used in various beverages, health foods, dietary supplements, and cosmetic items. Many varieties of green tea have been created in the countries where it is grown. These varieties can differ substantially due to variable growing conditions, horticulture, production 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 may have a lower risk of developing heart disease and certain types of cancer. Although green tea does not raise the metabolic rate enough to produce immediate weight loss, a green tea extract containing polyphenols and caffeine has been shown to induce thermogenesis and stimulate fat oxidation, boosting the metabolic rate 4% without increasing the heart rate.
The mean content of flavonoids in a cup of green tea is higher than that in the same volume of other food and drink items that are traditionally considered of health contributing nature, including fresh fruits, vegetable juices or wine. Flavonoids are a group of phytochemicals present in most plant products that are responsible for health effects such as anti-oxidative and anticarcinogenic functions. However, the content of flavonoids may vary dramatically amongst different tea products
Research and health effects
Green tea contains a variety of enzymes, amino acids, carbohydrates, lipids, sterols, polyphenols, carotenoids, tocopherols, vitamins, caffeine and related compounds, phytochemicals and dietary minerals. Numerous claims have been made for the health benefits of green tea based on chemical composition, in vitro and animal studies, though results in humans have been inconsistent and few clear benefits for humans have been demonstrated. There is also evidence suggesting consuming large volumes of green tea, and in particularly green tea extracts, may cause oxidative stress and liver toxicity.
A 2012 systematic review concluded the evidence that green tea can prevent cancer “is inadequate and inconclusive” but with some evidence for a reduction in certain types of cancer (breast, prostate, ovarian and endometrial). Green tea may lower blood low-density lipoprotein and total cholesterol levels, though the studies were of short duration and it is not known if these effects result in fewer deaths and evidence does not support green tea reducing coronary artery disease risk. Several randomized controlled trials suggest green tea can reduce body fat by a small amount for a short time, though it is not certain if the reduction would be meaningful for most people. One study has found that green tea may actually damage DNA. Green tea is forbidden for people with Multiple Myeloma(MM) if they use the drug Bortezomib (Velcade)or similar
Brewing and serving
Green tea leaves steeping in a gaiwan
Steeping is the process of making a cup of tea; it is also referred to as brewing. In general, two grams of tea per 100 ml of water, or about one teaspoon of green tea per five ounce cup, 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 different tea. The hottest steeping temperatures are 81 to 87 °C (178 to 189 °F) water and the longest steeping times two to three minutes. The coolest brewing temperatures are 61 to 69 °C (142 to 156 °F) 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[by whom?] 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; two or three steepings is typical. The steeping technique also plays a very important role in avoiding the tea developing an overcooked taste. 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.