Does harvesting age matter? Changes in structure and rheology of a shear-thickening polysaccharide from Cyathea medullaris as a function of age

Abstract

A shear-thickening polysaccharide from the New Zealand Black tree fern (Cyathea medullaris, commonly known as mamaku) extracted from different age fronds (stage 1: young, stage 2: fully grown and stage 3: old) was characterised in terms of structure and rheological properties. Constituent sugar analysis and ¹H and ¹³C NMR revealed a repeating backbone of -4)-β-D-GlcpA-(1 → 2)-α-D-Manp-(1→, for all mamaku polysaccharide (MP) samples from different age fronds without any alterations in molecular structure. However, the molecular weight (M_w) was reduced with increasing age, from ~4.1 × 10⁶ to ~2.1 × 10⁶ Da from stage 1 to stage 3, respectively. This decrease in M_w (and size) consequently reduced the shear viscosity (η_{s-Stage 1} > η_{s-Stage 2} > η_{s-Stage 3}). However, the extent of shear-thickening and uniaxial extensional viscosity of MP stage 2 was greater than MP stage 1, which was attributed to a greater intermolecular interaction occurring in the former. Shear-thickening behaviour was not observed in MP stage 3.

Keywords: Extensional rheology; Mamaku polysaccharide; Molecular structure; Shear rheology; Shear-thickening.