Camellia sinensis is a shrub native to
East, South and Southeast Asia. It is now cultivated across the world in
tropical and sub-tropical regions. In 2012 over 4.8 million tonnes of tea was
produced (1). The plants are harvested by hand every few weeks with only the
bud and first 2-3 leaves removed.
Tea drinking originated in China where, according
to legend, the Emporer Shen Nung discovered the drink in 2737 BC (this date
seems rather precise to me!) Leaves from Camellia sinensis are
processed in different ways to produce different types of tea. To make black
tea the leaves are allowed to oxidise during the drying process resulting in
the darkening of the tea leaves as the chlorophyll breaks down and tannins are
released. Green tea is made from tea leaves that have undergone minimal
oxidation during drying. White tea, which is actually green in colour, is
processed in a similar way to green tea but only the youngest leaves and buds
are used to make it.
Like coffee, tea contains caffeine. There is some
confusion regarding the caffeine content of tea compared with coffee. While at
university I was told by a professor that tea contains just as much caffeine as
coffee, but sedatives in tea counteract some of the stimulant effects of the
caffeine.
Fresh tea leaves do have a higher caffeine
content than coffee beans (2-3). However, since tea and coffee are made
differently this doesn’t mean a higher amount of caffeine is present in a cup
of tea. A study published in 2008 used a range of commercial varieties of tea
to examine caffeine content (4). They found that 170 ml servings of tea
contained between 14-61 mg of caffeine, which is typically lower than a cup of
coffee (4). It is often said that most of the caffeine is released from tea
leaves within the first 10-20 seconds of brewing. However, the scientists found
that caffeine content of the tea increased much more slowly over the 5 minute
brewing period.
The lack of a caffeine “ buzz” from tea drinking
is due to the presence of an amino acid called L-theanine. It modulates the
stimulant effects of caffeine by promoting relaxation but not drowsiness (5).
The combination of L-theanine and caffeine seems to be able to improve performance
in cognitive tasks.
Alongside a lower caffeine content, tea is also
thought to be a healthier drink than coffee due to the presence of antioxidant
compounds (catechins and polyphenols). Green tea contains higher amounts of the
catechins whereas black tea contains more polyphenols (products of oxidation of
catechins) (5). The most well-known class of polyphenols in tea are the
tannins, responsible for the bitter taste and dark colour of tea.
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