Enzymatic polymerisation in situ of depolymerised mimosa tannin applied to stabilisation of collagen

Vegetable tannins used in tanning of hides and skins are limited to surface reactions by their large molecular weights. The molecular weight reduces penetration into the skin and lowers the thermal stability or tanning effect. Investigation into the utilisation of small phenolic compounds such as catechin to improve penetration with subsequent in situ enzymecatalysed polymerisation may provide a novel and alternative tanning agent. In this research, catechin was oxidised by enzymatic catalysis using laccase, with the polymerisation confirmed by FT-IR and UHPLC. Tanning experiments were undertaken to measure the effect of laccase-catalysed polymerisation of catechin in the thermal stabilisation of collagen, by monitoring the change in shrinkage temperature between the treated and untreated sample of hide powder (ΔTs). This study demonstrates that the stabilisation of collagen (ΔTs) is increased with the use of in situ enzyme-catalysed polymerisation. Depolymerisation of condensed tannins is presented as an alternative source of low molecular weight phenolics to be applied in the stabilisation of collagen. In this research, mimosa tannin from the Black Wattle tree (Acacia mearnsii) was used in the depolymerisation process. Preliminary experiments on depolymerisation were undertaken using the methods laccase-mediator system and L-cysteine in mild acidic medium, both showing unsatisfactory results. Acid-catalysed depolymerisation followed by nucleophile addition is a common analytical method for determining the degree of polymerisation of proanthocyanidins. The acid-catalysed depolymerisation method was scaled-up, with the addition of pyrogallol as a nucleophile trapping agent, and powdered depolymerised mimosa was obtained. Data show an increase in the monomeric content and decrease in percentage of condensed tannins for the acid-catalysed process, indicating the potential of the depolymerisation of mimosa tannin in obtaining environmentally friendly sources of low molecular weight phenolic compounds for use in large scale/industrial applications. The depolymerised product was applied in the stabilisation of hide powder and goat skin. Using hide powder, data show that the laccase-assisted polymerisation of the depolymerised mimosa reached higher values of ΔTs in comparison with the conventional process employing unmodified mimosa. Using goat skin, the new process achieved similar values of ΔTs as the conventional process. In order to obtain similar results for goat skin as obtained with hide powder, an optimisation may be carried out. Therefore, data obtained demonstrates the potential of the new route in vegetable tanning of leather.