Determination of total phenolic content
Total soluble phenolics in the leaf aqueous extract of S. rebaudiana
were determined with Folin-Ciocalteau reagent according
to the method using gallic acid as a standard phenolic compound
(Slinkard and Singleton, 1977). 1.0 ml of extract solution containing
1.0 g extract in a volumetric flask was diluted with 46 ml of
distilled water. 1.0 ml of Folin-Ciocalteau reagent was added and
mixed thoroughly. Three minutes later, 3.0 ml of 2% sodium carbonate
was added and the mixture was allowed to stand for 3 h with
intermittent shaking. The absorbance of the blue color that developed
was measured at 760 nm. The concentration of total phenolic
content was expressed as mg/g of dry extracts. The concentration of
total phenolic content in the extract was determined as g of gallic
acid equivalent using an equation obtained from the standard gallic
acid graph: