Pages 380–383
Abstract
Research summary supplied by the authors
Global longan production in 2023 reached 4.2 million tonnes, with China contributing 60%. The seed, about 17% of fruit mass, is mostly discarded or burned, wasting biomass and polluting soil and air. Converting seed polysaccharides into tanning agents offers a waste-minimising, chrome-free alternative. Native polysaccharides are poorly soluble and show limited collagen affinity; fragments of 500–3000 Da crosslink optimally, while smaller or larger molecules act as fillers. Controlled depolymerisation and modification are therefore essential. In this study, longan seeds were treated with á-amylase-assisted ultrasonic hot-water extraction to yield a high-purity polysaccharide (LSP) rich in vicinal diols. Compared with the non-enzymatic control, the enzymatic route increased yields of total sugars, polyphenols, saponins and reducing ends. LSP was selectively oxidised with NaIO4 at varying stoichiometries to produce dialdehyde longan seed polysaccharide (DLSP). Moderate oxidant levels cleaved glycosidic bonds and converted C2-C3 diols into aldehyde groups; excess periodate over-oxidised aldehydes to carboxylates, decreasing active carbonyl content. The resulting lowmolecular-weight DLSP, bearing high densities of aldehyde and hydroxyl groups, rapidly penetrated collagen fibres and formed stable cross-links. Leather tanned with the optimised DLSP exhibited a shrinkage temperature of 76.4 °C, tensile strength of 17.03 MPa, elongation at break of 90.59% and tear strength of 86.54 N/mm, outperforming formaldehyde- and glutaraldehyde-tanned controls. This work demonstrates that á-amylase-coupled ultrasonic extraction efficiently releases polysaccharides, polyphenols and saponins from longan seeds; selective periodate oxidation converts these extracts into a novel, biodegradable, chromium-free tanning agent, providing a sustainable alternative for the leather industry and valorising agro-waste.