2.5. Pectin Characterization

The degree of esterification and the anhydrouronic acid content were determined by titration relating the methoxyl content with the equivalent weight. 0.50 g of watermelon rind pectin was dissolved into ethanol/water solution (1 : 20 v/v); 5 drops of phenol red indicator were added, and the sample was titrated with 0.1 N sodium hydroxide (V₁, mL) until the indicator changed. Then, 25 mL of 0.25 N NaOH was added, and the sample was heated and stirred vigorously. Five drops of phenol red and 25 mL of 0.25 N HCl were added again, and it was titrated with 0.1 N NaOH (V₂, mL) until the color changes from yellow to faint pink endpoint [43]⁠. The MeO and the DE were then calculated according to Equation (2) [22, 31]⁠ and Equation (3) [11, 23, 44]⁠, respectively.

Equation (4) was used to calculate the anhydrouronic acid content [22]⁠.

where 176 is the molecular weight of anhydrouronic acid expressed as mg/meq and V₁ and V₂ were the volumes used for first and second titration, respectively.