Extraction and structural characterization of hydrolyzable tannins from Coriaria nepalensis leaves

Tannins are important natural products that are widely used in industrial production. In this study, Coriaria nepalensis was identified as a novel source of tannins, which were obtained from the leaves using the aqueous two-phase extraction method. The optimal extraction process conditions were established through single-factor and response surface optimization experiments. Using an aqueous two-phase extraction solvent established with 21% (w/w) ammonium sulfate and 25% (w/w) ethanol, the extraction yield of tannins reached up to 15.07% under·an extraction temperature of 55℃, extraction time of 44 min, and liquid–solid ratio of 31 mL·g−1. The range of the relative molecular weight of the tannins was 433–1375 Da, as determined via gel permeation chromatography. Fourier transform infrared spectroscopy identified the extracted tannins as hydrolyzable tannins with a carbonyl group peak at 1710 cm−1. The phenolic hydroxyl signals of the tannin structure were detected using 1H NMR at around 9.36 ppm. The carbon signals of the polyol structure were detected via 13C NMR at 61.63–75.62 ppm and those of the ester carbonyl group in the tannin structure were detected at 165.94–168.14 ppm, which further confirmed the structural composition of C. nepalensis tannins as hydrolyzable tannins. The results of tanning experiments showed that the tannin extracts had excellent leather tanning properties. Thus, C. nepalensis as a novel source of vegetable tannins exhibiting potential industrial applications.