Detection and cross-linking evaluation of secoiridoid tannins activated with endogenous enzymes

Abstract

Environmental and health concerns related to mineral and synthetic tanning agents in leather production are prompting a shift toward sustainable, plant-based alternatives. One group of plant-based tanning agents are the secoiridoids from plants of the Oleaceae family (olive, privet). When their leaves are damaged, secoiridoids are enzymatically deglycosylated to produce aglycones containing aldehyde groups, which are able to cross-link collagen covalently. However, a reliable method to quantify these active aldehyde compounds has been lacking, hindering the application of such tannins. Here, secoiridoid aglycones with aldehyde groups in Olea europaea and Ligustrum vulgare leaf extracts were measured using HPLC-DAD after derivation with 2,4-dinitrophenylhydrazine. Low-temperature extractions (≤40 °C) yielded high aldehyde content (up to 17 mg/g extract), attributed to enzymatic activity, while high-temperature extractions (≥60 °C) preserved the inactive, glycosylated precursors but contained negligible aldehydes due to enzyme denaturation. The cross-linking potential of the extracts was quantified by testing the denaturation temperature and proportion of covalently bound lysine groups of the hide powder treated with endogenously activated Oleaceae extracts. A strong linear relationship was found between aldehyde content and tanning capacity. The most essential aldehyde-containing substances in the extracts as well as the key parameters for efficient extraction were identified. In conclusion, we introduce a new method to accurately measure aldehyde content and assess the cross-linking potential of Oleaceae plant extracts, and expect to support the development of safer, plant-based tannins for the leather industry.

Keywords

Oleaceae / Olive / Privet / Cross-linking / Secoiridoid

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Funding

Bundesministerium für Wirtschaft und Medien, Energie und Technologie(49MF210154)

RIGHTS & PERMISSIONS

The Author(s)