The content of flavanols, including EGCg, in a cup of tea varies
remarkably. It depends on steeping time and temperature. For
the extremes examined here (3 min at 70 C and 7 min at
100 C), the concentration of EGCg is 3.1-fold higher in the infusion
prepared at the higher temperature and for the longer time. The
factors for the other flavanols are similar. Further, lowering pH of
the infusion increases the concentration of flavanols. An increase
of 20% (EGCg) is found for pH 3.0, which is the pH of a cup of tea
with 10 mL of lemon juice.
Two reaction mechanisms are proposed, that are inversely affected
by steeping time and temperature: the diffusion of flavanols
from the leaf into the aqueous phase and their structural alteration.
In the case of EGC oxidation seems to be prevalent, since ascorbic
acid inhibits degradation. The other flavanols are rather affected by
other reactions that cannot be slowed down by ascorbic acid, but
by low pH. Since EGC starts to degrade after a few minutes, the
other flavanols may also degrade after more than 7 min.
Additionally, low pH facilitates extraction by structural degradation
of the leaf. The range of concentrations might become even
wider than observed here, if different teas were considered.
This should be taken into account, when the intake of tea flavanols
is estimated in epidemiological studies. For those, who want
to maximise their flavanol consumption from tea, the message is
clear: add lemon juice (while steeping), use boiling water and let
it steep for a while.