Methods for Quantification of Tannins and Other Polyphenols in Syzygium cumini (L.) Bark for Potential Use in Leather Tanning Colouration

Abstract

Most tanneries still use chromium as a tanning agent, specifically Cr(III), because of the their hydrothermal stability and excellent physical properties. However, the use of these synthetic dyes generates waste that is hazardous to the environment, especially the oxyanions of chromate and some Cr(VI) salts. Due to this, there is a growing interest in natural dyes that produce environment-friendly products, hence the search for vegetable tannins. This study attempts to determine whether the tanning parameters of Syzygium cumini (L.) (S. cumini) bark are sufficient to add to the list of the known vegetable tanning sources. To assess this, a methanolic extract was analysed for the total polyphenol content (TPC), tannin content, and tannin-to-non-tannin ratio. For the measurement of TPC, a colourimetric method utilising Folin-Ciocalteu reagent and pyrogallic acid reference solution was employed. Quantification was done using UV-Vis at 760 nm, and this revealed a TPC of 48.6%. Also, the method validation was tested and proven suitable, with an acceptable linearity from 12.81 – 27.45 μg/mL, and a LOD of 0.44 μg/mL. Moreover, the tanning strength based on the tannins/non-tannin ratio was determined via the total soluble content (TSC) and Stiasny number of the extract, with the assay showing an acceptable tanning strength of 3.21 for S. cumini (L.), with high non-tannin content. These results indicate the potential viability of using the S. cumini (L.) bark extracts for tanning purposes, hence useful as a viable vegetable tanning material for the potential production of chromium-free leathers from sustainable resources.