The Chemistry of Tea
If you were impressed by the fact that a cup of coffee contains around 1,500 chemicals , you will not be surprised to find out that a cup of tea contains almost 2,000 chemical compounds! Many of these chemicals possess antioxidant properties and health benefits, just like in coffee. However, the chemical structure of these antioxidant compounds differ between coffee and tea, just like the chemicals in green and black tea are different, even though they come from the exact same plant. Several people have asked me about the medicinal and chemical properties of tea. After all, the rest of the world that doesn’t drink coffee, most probably drinks tea. It turns out that there is also an exciting world of chemical reactions going on during tea processing.
This drink has been consumed for thousands of years and this long safety record makes the compounds produced in the extraction process attractive for drug discovery. There have been many research endeavors claiming green tea to be helpful for atherosclerosis, LDL cholesterol, cancer, inflammatory bowel disease, diabetes, liver disease, weight loss, neurodegenerative diseases and even halitosis. However, well designed controlled studies are still needed for an absolute scientific confirmation in vivo.
White tea, green tea, Oolong tea and black tea come from the exact same plant: Camelia sinensis, however the flavor and chemical content develops by a “chemical oxidation” during tea processing. This oxidation is stopped by applying heat because heat inactivates the enzymes. Enzymes and small molecules (such as flavonoids) occupy many specific compartments inside the living cells.
The process of oxidation starts immediately upon “plucking” the leaves from the tree (for details on tea processing check Wikipedia). White and Green tea do not undergo oxidation because heat is applied soon after picking. White tea is made from young leaves and buds and green tea requires more mature tea leaves. The leaves are then dried and are ready about 1-2 days after harvesting. Apart from caffeine, the most important components in white and green tea are called flavonoids which give live plants the various colors that attract insects and birds for pollination and also act as antioxidants to protect them from stress. The vast majority of health studies have been performed of green tea.
Check out the flavonoids in the figure below. Their structure is very similar and by looking at them it almost feels like one is playing the game “spotting the difference”. Flavonoids are characterized by 3 rings labeled as A, B and C and depending on small changes in the rings (colored in red), they can be sub-classified into flavones, flavanols, flavanones, catechins, anthocyanidins and isoflavanoids. All these compounds are also found in many fruits (such as blueberries) and vegetables (such as broccoli) and their consumption appears to promote good health.
The Oolong tea is a “semi-oxidized tea” because it is allowed to oxidize for short period before applying heat. The whole process from withering to drying takes 2 to 3 days.
Finally the black tea is allowed to oxidize 100%. The main difference is that the leaves are cut and bruised disrupting the cell structures so all the leaf juices (containing flavonoids) and enzymes (like polyphenol oxidase) mix up allowing a complete oxidation. The oxidation process takes up to three hours under a high humidity and a optimal temperature for enzyme function. Once the process is complete, the broken leaves are dried and shaped for packing.
The figure below shows an example of oxidation of catechins (a flavonoid alsho shown above) to theaflavin and thearubigin by the enzyme polyphenol oxidase. You can see that now the structures really start looking different. Thearubigin is what gives black tea its characteristic color and taste.
So, in summary, white and green tea contain a mixture of all kinds of flavonoids and is the closest you will get to nature, so I can appreciate why many people prefer to drink green tea. Black tea contains very few flavonoids and mostly theaflavins and thearubigins. Caffeine is present in both. Although the chemical composition of green and black tea is different, research by the U.S. Department of Agriculture has measured and suggested that levels of antioxidants in green and black tea do not differ greatly, as green tea has an oxygen radical absorbance capacity (ORAC) of 1253 and black tea an ORAC of 1128 (measured in μmolTE/100g). So both kinds of tea have the capability to exert antioxidant effect by scavenging free radicals in the body.
A question of recent debate is whether or not the addition of milk, as widely practiced in the UK, modifies the biological activities of tea. A study published in the European Heart Journal (2007) concluded that milk may counteract the health effects on vascular function, perhaps due to the fact that flavonoids in tea bind to proteins present in milk (eg. Caseine) affecting their antioxidant capacity.
However, other studies indicated that although the plasma catechin concentration is reduced in tea with milk compared to black tea, it did not affect the beneficial antioxidant activity. It is worthy to note that there have been other studies failing to establish an effect of milk on antioxidant properties of tea. Therefore, better-designed experiments are needed to confirm the findings.
There is so much more to learn about tea, so don’t hesitate to go read about its origin and history. No wonder tea is perceived as one of Britain’s and India’s cultural beverages and has been incorporated in so many cultures, such as tea ceremonies in China and Japan. All we can do is appreciate the medicinal wonders of another gift from mother nature.
*Please note that this blog has been about teas proceeding from the plant Camelia sinenis. Teas made from fruits and other plants are called “herbal teas”. There are also many mixed teas like the “Earl grey” which apart from tea plant contains oil extracted from the rind of the bergamot orange.
This article was originally post at "Think Science Now" and was written by: Maria Angelica Linton, Principal Scientist
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