Spectrophotometric Determination of Soursop Fruit Tea Polyphenols

Total polyphenol content of soursop fruit tea extract was determined by the Folin–Ciocalteu colorimetric method, using gallic acid as a standard and previously described by Gyesi et al. (2019 (link)) and Dao et al. (2021 (link)). A total of 1 g of soursop fruit tea was weighed, homogenized with 50 mL of ethanol, and filtered to obtain the extract. The extract (0.1 mL) was placed in a dark‐colored test tube and added with 0.5 mL of FCR 10%, and 0.4 mL 7.5% Na₂SO₃. The sample was incubated in the dark for 1 h before absorbance measurement at the wavelength of 765 nm using GENESYS™ 10S UV–Vis spectrometer (Thermo Scientific, USA). TPC is expressed in milligrams of Gallic acid equivalent (GAE) and calculated by Equation (5). $$\mathrm{TPC}=\frac{C_x\times n\times V\times 100}{m\times \left(100-X\right)}\times {10}^{-3},$$ where Cx is the concentration of gallic acid determined from the standard curve (μg/mL): Cx=61.0676×A+1.8598, R = 0.9991; n is the dilution from the original extract; V is the volume of the original extract (mL); X is the sample moisture (%) and m is the weight of sample (g).