Solvent extraction conditions for the analysis of condensed tannins from grape skin and seeds

The total tannin and individual condensed tannin concentrations in the fruit skin and seeds of “Campbell Early” grapes were analyzed by ultrasound-assisted extraction using various solvents (ethanol, methanol, and acetone, both pure and diluted to 70% with and without 0.5% HCl). The total tannin contents of the grape skin and seeds depended on the extraction-solvent conditions and varied between 0.01 and 2.92 mg/g for the skin, and 8.82 and 14.72 mg/g for the seeds. The most effective solvent for extracting total tannins from both the grape skin and seeds was found to be 70% acetone, which facilitated the detection of high concentrations of tannin monomers and dimers. Monomer tannins (catechin and gallocatechin) concentrations in grape seeds were detected at high levels using both 70% acetone and 70% methanol as the extraction solvents; however, dimer tannins (procyanidin B1 and procyanidin B2) were detected at high levels in grape seeds using 70% methanol. Hence, 70% methanol was effective for measuring the concentrations of condensed tannins in grape seeds. Since we are currently analyzing individual tannins, research into finding the analysis conditions most appropriate for individual situations is still required.