centrifuged at 20,000
g for 20 min at 30 C and the supernatant
was collected into a 50 mL volumetric
flask. One milliliter of the
extract was
filtered through a membrane
filter (0.45 mm, Macherey
Nagel, Germany) and a 20 mL sample was used for HPLC analysis of
organic acids (citric, malic, succinic, tartaric and oxalic).